Mycotoxin Production in Fusarium According to Contemporary Species Concepts

Fusarium is one of the most important genera of plant-pathogenic fungi in the world and arguably the world's most important mycotoxin-producing genus. Fusarium species produce a staggering array of toxic metabolites that contribute to plant disease and mycotoxicoses in humans and other animals. A thorough understanding of the mycotoxin potential of individual species is crucial for assessing the toxicological risks associated with Fusarium diseases. There are thousands of reports of mycotoxin production by various species, and there have been numerous attempts to summarize them. These efforts have been complicated by competing classification systems based on morphology, sexual compatibility, and phylogenetic relationships. The current depth of knowledge of Fusarium genomes and mycotoxin biosynthetic pathways provides insights into how mycotoxin production is distributedamong species and multispecies lineages (species complexes) in the genus as well as opportunities to clarify and predict mycotoxin risks connected with known and newly described species. Here, we summarize mycotoxin production in the genus Fusarium and how mycotoxin risk aligns with current phylogenetic species concepts.

Phylogenomic Analysis of a 55.1-kb 19-Gene Dataset Resolves a Monophyletic Fusarium that Includes the Fusarium solani Species Complex

Scientific communication is facilitated by a data-driven, scientifically sound taxonomy that considers the end-user's needs and established successful practice. In 2013, the Fusarium community voiced near unanimous support for a concept of Fusarium that represented a clade comprising all agriculturally and clinically important Fusarium species, including the F. solani species complex (FSSC). Subsequently, this concept was challenged in 2015 by one research group who proposed dividing the genus Fusarium into seven genera, including the FSSC described as members of the genus Neocosmospora, with subsequent justification in 2018 based on claims that the 2013 concept of Fusarium is polyphyletic. Here, we test this claim and provide a phylogeny based on exonic nucleotide sequences of 19 orthologous protein-coding genes that strongly support the monophyly of Fusarium including the FSSC. We reassert the practical and scientific argument in support of a genus Fusarium that includes the FSSC and several other basal lineages, consistent with the longstanding use of this name among plant pathologists, medical mycologists, quarantine officials, regulatory agencies, students, and researchers with a stake in its taxonomy. In recognition of this monophyly, 40 species described as genus Neocosmospora were recombined in genus Fusarium, and nine others were renamed Fusarium. Here the global Fusarium community voices strong support for the inclusion of the FSSC in Fusarium, as it remains the best scientific, nomenclatural, and practical taxonomic option available.

Effects of Soil Conditions on Root Rot of Soybean Caused by Fusarium graminearum

Fusarium graminearum is an important soybean pathogen that causes seedling disease, root rot, and pre- and postemergence damping-off. However, effects of soil conditions on the disease are not well understood. The objective of this greenhouse study was to determine the impacts of soil texture, pH, and soil moisture on seedling root rot symptoms and detrimental effects on seedling development caused by F. graminearum. F. graminearum-infested millet was added (10%, vol/vol) to soil with four different textures (sand, loamy sand, sandy loam, and loam). Soil moisture was maintained at saturation, field capacity or permanent wilting point at soil pH levels of 6 or 8. Seedlings were evaluated 4 weeks after planting for root rot, root length, root and shoot dry weights, leaf area, and F. graminearum colonization (by qPCR). There was a significant interaction between soil moisture and soil texture for root rot assessed visually (P < 0.0001). Highest severity (67%) and amount of F. graminearum DNA were observed at pH 6 and permanent wilting point in sandy loam soils. Pot saturation resulted in the lowest levels of disease in sandy loam and loam soils (11.6 and 10.8%, respectively). Reductions in seedling growth parameters, including root length, foliar area, shoot and root dry weights, and root tips, relative to the noninfested control, were significantly greater in sandy loam soils. In contrast, there were no significant growth reductions in sand. This study showed that levels of root rot increased under moisture-limiting conditions, producing detrimental effects on plant development.

Occurrence in Seeds and Potential Seed Transmission of Xanthomonas vasicola pv. vasculorum in Maize in the United States

This paper reports original evidence regarding the potential role of seed transmission of Xanthomonas vasicola pv. vasculorum in the epidemiology of bacterial leaf streak (BLS) in maize. We evaluated the occurrence of the pathogen on seeds from diseased fields and its subsequent transmission to seedlings. In 2016 and 2017, X. vasicola pv. vasculorum was detected by TaqMan PCR from 22 of 41 maize seed lots harvested from naturally infected fields in Colorado, Nebraska, and Iowa. However, many of the PCR-positive samples did not yield culturable X. vasicola pv. vasculorum colonies. The highest levels of seed contamination were detected in dent maize and popcorn from NE and CO. Seed transmission was evaluated in greenhouse grow-outs from eight seed lots, totaling more than 14,000 plants. Putative seed transmission events from naturally contaminated seed lots, estimated from PCR results, occurred at a frequency between 0.1 and 0.5% in 10-seedling pooled samples and at a frequency of 2.7% from individual plant assays. However, no seedling symptoms were observed during these assays and live X. vasicola pv. vasculorum colonies were not recovered from PCR-positive seedlings. In contrast, seed transmission was readily demonstrated from artificially contaminated seed lots, including typical symptoms and recovery of live bacteria. Seed transmission consistently occurred from seeds soaked in bacterial suspensions with concentrations of ≥10⁶ CFU/ml, suggesting that a threshold population of the bacterium is necessary for the development of BLS symptoms and recovery of live bacteria. The low bacterial populations on naturally contaminated seeds apparently were not sufficient to result in diseased seedlings.

Effects of Temperature and pH on Fusarium oxysporum and Soybean Seedling Disease

Fusarium oxysporum (Fo) is an important pathogen that reduces soybean yield by causing seedling disease and root rot. This study assessed the effects of pH and temperature on Fo fungal growth and seedling disease. In an in vitro assay, 14 Fo isolates collected from symptomatic soybean roots across Iowa in 2007 were grown on artificial culture media at five pH levels (4, 5, 6, 7, and 8) and incubated at four temperatures (15, 20, 25, or 30°C). In a rolled-towel assay, soybean seeds from Fo-susceptible cultivar Jack were inoculated with a suspension of a pathogenic or a nonpathogenic Fo isolate; both isolates were previously designated for their relative aggressiveness in causing root rot at 25°C. The seeds were placed in rolled germination paper, and the rolls were incubated in all combinations of buffer solutions at four pH levels (4, 5, 6, and 7), and four temperatures (15, 20, 25, or 30°C). There was a significant interaction between temperature and pH (P < 0.05) for in vitro radial growth and root rot severity. Isolates showed the most in vitro radial growth after incubation at pH 6 and 25°C. For the rolled-towel assay, the pathogenic isolate caused the most severe root rot at pH 6 and 30°C. Gaussian regression analysis estimates for optimal conditions were pH 6.3 at 27.1°C for maximal fungal growth and pH 5.9 at 30°C for maximal root rot severity. These results indicate that optimal pH and temperature conditions are similar for Fo growth and disease in soybean seedlings and suggest that Fo may be a more important seedling pathogen when soybeans are planted under warm conditions in moderately acidic soils.

Isolate-Cultivar Interactions, In Vitro Growth, and Fungicide Sensitivity of Fusarium oxysporum Isolates Causing Seedling Disease on Soybean

Fusarium oxysporum is frequently associated with soybean root rot in the United States. Information about pathogenicity and other phenotypic characteristics of F. oxysporum populations is limited. The objective of the research described herein was to assess phenotypic characteristics of F. oxysporum isolates from soybean, including the interaction between isolates and soybean cultivars, fungal growth characteristics in culture, and sensitivity to fungicides commonly used as seed treatment products. The pathogenicity of 14 isolates was evaluated in rolled-towel and Petri-dish assays using 11 soybean cultivars. In the rolled-towel assay, seed were inoculated with a conidial suspension and disease severity was observed. In the Petri-dish assay, F. oxysporum isolates were grown on 2% water agar and seed were placed on the F. oxysporum colony to observe the symptoms that developed. Cultivars differed in susceptibility to F. oxysporum, and significant (P = 0.0140) isolate-cultivar interactions were observed. F. oxysporum isolates differed in radial growth on potato dextrose agar at 25°C. Pyraclostrobin and trifloxystrobin reduced conidial germination with average 50% effective concentration (EC50) of 0.15 and 0.20 µg active ingredient (a.i.)/ml, respectively. Ipconazole reduced fungal growth with average EC50 of 0.23 µg a.i./ml, whereas fludioxonil was ineffective. Our results illustrate soybean F. oxysporum isolate variability and the potential for their management through cultivar selection or seed treatment.

Pathogenicity of Fumonisin-producing and Nonproducing Strains of Aspergillus Species in Section Nigri to Maize Ears and Seedlings

Species of Aspergillus section Nigri are commonly associated with maize kernels, and some strains can produce fumonisin mycotoxins. However, there is little information about the extent to which these fungi contribute to fumonisin contamination in grain, the damage they cause to maize ears, or their effects on maize seed germination and seedling health. We compared fumonisin-producing and nonproducing strains of A. niger, A. welwitschiae, A. phoenicis, A. tubingensis, and A. carbonarius from the United States and Italy in laboratory and field studies to assess their ability to contribute to fumonisin contamination, to cause maize ear rot, and to affect seed germination and seedling growth. In laboratory experiments, some strains of each Aspergillus species reduced germination or seedling growth, but there was high variability among strains within species. There were no consistent differences between fumonisin-producing and nonproducing strains. In field studies in Iowa and Illinois, strains were variable in their ability to cause ear rot symptoms, but this was independent of the ability of the Aspergillus strains to produce fumonisins. Contamination of grain with fumonisins was not consistently increased by inoculation with Aspergillus strains compared with the control, and was much greater in F. verticillioides-inoculated treatments than in Aspergillus-inoculated treatments. However, the ratio of the FB analogs FB₂ and FB₁ was altered by inoculation with some Aspergillus strains, indicating that FB₂ production by Aspergillus strains occurred in the field. These results demonstrate the pathogenic capabilities of strains of Aspergillus in section Nigri, but suggest that their effects on maize ears and seedlings are not related to their ability to produce fumonisins, and that fumonisin contamination of grain caused by Aspergillus spp. is not as significant as that caused by Fusarium spp.

Roles of Genotype-Determined Mycotoxins in Maize Seedling Blight Caused by Fusarium graminearum

Fusarium graminearum is an important causal agent of maize seedling blight. The species includes several chemotypes that produce various forms of deoxynivalenol (DON) and nivalenol (NIV). To understand the effects and roles of F. graminearum mycotoxins on maize seedling blight occurring at Zhang Ye of Gansu, China, 23 isolates of F. graminearum were collected and characterized. A PCR assay showed all 23 isolates belonged to the 15-acetyldeoxynivalenol (15-ADON) genotype. This was also confirmed by production of both DON and 15-ADON in either rice culture medium or maize seedling roots, detected by high performance liquid chromatography and mass spectrometry. In maize seedling roots, 15-ADON dominated at 6 days post inoculation (dpi) and DON was the main mycotoxin at 12 dpi. The biomass of F. graminearum doubled from 6 to 12 dpi, and was positively correlated with virulence of the isolates. Both mycotoxins affected maize root vitality, but 15-ADON had a greater effect than DON. ALDH9 and MDH, two dehydrogenase synthesis genes in maize, showed a lower relative expression in 15-ADON treatments than in DON treatments. It indicated that both mycotoxins affected seed germination and root development, with 15-ADON being more destructive. Under scanning electron microscopy and transmission electron microscopy, root hair formation and development were delayed by DON, but completely inhibited by 15-ADON. 15-ADON caused cell shrinkage, loose cellular structure, and widened intercellular spaces; it also destroyed organelles and caused plasmolysis, and eventually ruptured cell membranes causing cell death. DON did not affect cell morphology and arrangement, but altered the morphology of organelles, forming concentric membranous bodies and a large amount of irregular lipid droplets. Thus, both mycotoxins contributed to symptom expression of maize seedling blight, but 15-ADON was more destructive than DON.

Seed Treatment Effects on Maize Seedlings Coinfected with Rhizoctonia solani and Pratylenchus penetrans

The roots of maize seedlings typically are attacked by a complex of organisms that includes fungal pathogens and plant-parasitic nematodes but few studies have examined the effects of these organisms in combination. Rhizoctonia solani can be an important component of the seedling disease complex; like other fungi, its effect on the plant may be influenced by the activity of nematodes such as the root-lesion nematode Pratylenchus penetrans. In this study, we assessed the impact of seed treatments, including fungicide-nematicide combinations, on maize seedlings exposed to R. solani and P. penetrans alone or in combination. In growth-chamber and greenhouse experiments, seed treated with various active ingredient combinations were planted in an autoclaved sand-soil mixture with or without inoculum of R. solani. In some treatments, a suspension of P. penetrans adults and juveniles was added to the sand-soil mixture. In the greenhouse experiments, infection by R. solani caused dramatic reductions in root length, volume, surface area, and numbers of root tips and root forks, whereas P. penetrans infestation alone reduced only shoot fresh weight. Statistical interactions between the effects of the two organisms were not significant, although fungal infestation significantly reduced the numbers of nematodes extracted from roots. Seed treatments significantly improved most root development variables, and the combination that included four fungicides, thiamethoxam, and abamectin was the best treatment for most variables. Results were similar in the growth-chamber experiments, where R. solani caused significant reductions in nearly all shoot and root development measurements, and seed treatment with sedaxane, alone or combined with abamectin, consistently provided the best results. R. solani was more damaging to seedlings than P. penetrans, and the combination of the two organisms did not cause more damage than R. solani alone. Seed-treatment active ingredients that specifically targeted R. solani (sedaxane) and P. penetrans (abamectin) had large positive effects on seedling health, causing significant improvements in root and shoot growth and development compared with untreated seedlings exposed to these pathogens.

Association of Effector Six6 with Vascular Wilt Symptoms Caused by Fusarium oxysporum on Soybean

The Fusarium oxysporum species complex (FOSC) is a widely distributed group of fungi that includes both pathogenic and nonpathogenic isolates. In a previous study, isolates within the FOSC collected primarily from soybean were assessed for the presence of 12 fungal effector genes. Although none of the assayed genes was significantly associated with wilt symptoms on soybean, the secreted in xylem 6 (Six6) gene was present only in three isolates, which all produced high levels of vascular wilt on soybean. In the current study, a collection of F. oxysporum isolates from soybean roots and F. oxysporum f. sp. phaseoli isolates from common bean was screened for the presence of the Six6 gene. Interestingly, all isolates for which the Six6 amplicon was generated caused wilt symptoms on soybean, and two-thirds of the isolates showed high levels of aggressiveness, indicating a positive association between the presence of the effector gene Six6 and induction of wilt symptoms. The expression profile of the Six6 gene analyzed by quantitative reverse-transcription polymerase chain reaction revealed an enhanced expression for the isolates that caused more severe wilt symptoms on soybean, as established by the greenhouse assay. These findings suggest the suitability of the Six6 gene as a possible locus for pathogenicity-based molecular diagnostics across the various formae speciales.

Association of Putative Fungal Effectors in Fusarium oxysporum with Wilt Symptoms in Soybean

Fungi within the Fusarium oxysporum species complex can cause root rot, seedling blight, and wilt of soybean. Isolates recovered from soybean vary in aggressiveness and also the type of symptoms they produce. The aim of this study was to identify genetic markers to detect aggressive soybean wilt isolates. Eighty isolates collected primarily from soybean were tested in the greenhouse for their ability to produce wilt symptoms using susceptible 'Jack' soybean. The same 80 isolates were assessed for the presence of fungal effector genes Fmk1, Fow1, Pda1, PelA, PelD, Pep1, Prt1, Rho1, Sge1, Six1, Six6, and Snf1. All polymerase chain reaction amplicons were sequenced, phylogenies were inferred, and analysis of molecular variance (AMOVA) was performed for 10 of the 12 genes. High incidence of vascular discoloration of roots or stems was observed with 3 isolates, while moderate to low levels of incidence were observed for 25 isolates. Fungal effector genes Fmk1, Fow1, PelA, Rho1, Sge1, and Snf1 were present in all isolates screened, while Pda1, PelD, Pep1, Prt1, Six1, and Six6 were dispersed among isolates. The Bayesian and AMOVA analyses found that the genes Fmk1, Fow1, Pda1, PelA, Rho1, Sge1, and Snf1 corresponded to previously designated clades based on tef1α and mitochondrial small subunit sequences. None of the genes had a significant association with wilt symptoms on soybean. Interestingly, the Six6 gene was only present in three previously known wilt isolates from soybean, common bean, and tomato; of these, the soybean and common bean isolates produced high levels of vascular wilt in our study.

Seed Treatment Effects on Maize Seedlings Coinfected with Fusarium spp. and Pratylenchus penetrans

Seedling diseases of maize are caused by a complex of organisms, including fungi in the genus Fusarium. Root-lesion nematodes (Pratylenchus spp.) are common in fields where maize is grown, and they are known to interact with Fusarium spp. in several crops. The objectives of this study were to assess the impacts of seed treatment combinations on maize seedlings coinfected with Pratylenchus penetrans and two Fusarium spp. that cause seedling disease symptoms (Fusarium graminearum and F. verticillioides) and to determine whether there were interactions between P. penetrans and the Fusarium spp. Growth-chamber experiments were conducted with fungicide- or nematicide-treated or untreated maize seed planted in a sand-soil mixture infested with inoculum of either F. graminearum or F. verticillioides. A suspension of 4,000 P. penetrans (mixed stages) was added to the pots at the time of planting. After 30 days, shoot length and fresh and dry shoot and root weights were determined. Total root length and fine root length, root volume, numbers of root tips and forks, and root surface area were measured through analysis of digital images of the root systems. After 42 days, P. penetrans nematodes were extracted and quantified from roots and soil. There were significant effects of the treatments on root health with interactions between Fusarium spp. and P. penetrans. F. graminearum caused the greatest reductions in root and shoot growth, and interactions with P. penetrans were more evident for F. verticillioides than for F. graminearum. Image analysis of root system architecture showed that seed treatment significantly improved root system characteristics. Seed treatments containing the nematicide abamectin in combination with fungicides reduced root infection by P. penetrans and provided the healthiest root system when under attack by the Fusarium-Pratylenchus complex.

Transcriptome profiling of soybean (Glycine max) roots challenged with pathogenic and non-pathogenic isolates of Fusarium oxysporum

BACKGROUND

Fusarium oxysporum is one of the most common fungal pathogens causing soybean root rot and seedling blight in U.S.A. In a recent study, significant variation in aggressiveness was observed among isolates of F. oxysporum collected from roots in Iowa, ranging from highly pathogenic to weakly or non-pathogenic isolates.

RESULTS

We used RNA-seq analysis to investigate the molecular aspects of the interactions of a partially resistant soybean genotype with non-pathogenic/pathogenic isolates of F. oxysporum at 72 and 96 h post inoculation (hpi). Markedly different gene expression profiles were observed in response to the two isolates. A peak of highly differentially expressed genes (HDEGs) was triggered at 72 hpi in soybean roots and the number of HDEGs was about eight times higher in response to the pathogenic isolate compared to the non-pathogenic one (1,659 vs. 203 HDEGs, respectively). Furthermore, the magnitude of induction was much greater in response to the pathogenic isolate. This response included a stronger activation of defense-related genes, transcription factors, and genes involved in ethylene biosynthesis, secondary and sugar metabolism.

CONCLUSIONS

The obtained data provide an important insight into the transcriptional responses of soybean-F. oxysporum interactions and illustrate the more drastic changes in the host transcriptome in response to the pathogenic isolate. These results may be useful in the developing new methods of broadening resistance of soybean to F. oxysporum, including the over-expression of key soybean genes.

Real-Time PCR Assay for Detection of Sphacelotheca reiliana Infection in Maize (Zea mays) Seedlings and Evaluation of Seed Treatment Efficacy

Head smut of maize, caused by the fungus Sphacelotheca reiliana, is an economically important disease in all major maize-producing countries. Although seed treatments are commonly used for management purposes, evaluating these treatments for efficacy is both time consuming and inefficient. Therefore, in order to improve the capacity for evaluating seed treatment fungicides, we developed a real-time PCR-based seedling assay for S. reiliana infection. We optimized growth chamber conditions and inoculation methods to achieve infection incidence of 60 to 80% in inoculated, nontreated controls. The effects of five commercially available fungicidal seed treatment formulations on seedling infection incidence were compared by PCR analysis of root and mesocotyl tissues. Tebuconazole, fludioxonil, sedaxane, and Maxim Quattro (fludioxonil+mefenoxam+azoxystrobin+thiabendazole) reduced the incidence of infection (P < 0.05) compared with the control, but no difference was found between the azoxystrobin treatment and the control. All rates tested for both sedaxane and tebuconazole were equally effective for seeds coated with 10⁶ teliospores∙seed^-1. Sedaxane, at a rate of 0.1 mg/kernel, eliminated seedling infection if seeds were infested with a low inoculum concentration (10¹ teliospores∙seed^-1). The assay developed here is a valuable tool not only for the detection of fungal infection at the seedling stage, but also for testing the relative efficacies of seed treatments for reducing incidence of infection.

Transgenic Virus Resistance in Crop-Wild Cucurbita pepo Does Not Prevent Vertical Transmission of Zucchini yellow mosaic virus

Zucchini yellow mosaic virus (ZYMV) is an economically important pathogen of cucurbits that is transmitted both horizontally and vertically. Although ZYMV is seed-transmitted in Cucurbita pepo, the potential for seed transmission in virus-resistant transgenic cultivars is not known. We crossed and backcrossed a transgenic squash cultivar with wild C. pepo, and determined whether seed-to-seedling transmission of ZYMV was possible in seeds harvested from transgenic backcrossed C. pepo. We then compared these transmission rates to those of non-transgenic (backcrossed and wild) C. pepo. The overall seed-to-seedling transmission rate in ZYMV was similar to those found in previous studies (1.37%), with no significant difference between transgenic backcrossed (2.48%) and non-transgenic (1.03%) backcrossed and wild squash. Fewer transgenic backcrossed plants had symptom development (7%) in comparison with all non-transgenic plants (26%) and may be instrumental in preventing yield reduction due to ZYMV. Our study shows that ZYMV is seed transmitted in transgenic backcrossed squash, which may affect the spread of ZYMV via the movement of ZYMV-infected seeds. Deep genome sequencing of the seed-transmitted viral populations revealed that 23% of the variants found in this study were present in other vertically transmitted ZYMV populations, suggesting that these variants may be necessary for seed transmission or are distributed geographically via seeds.

Genotypic and Phenotypic Characterization of Fungi in the Fusarium oxysporum Species Complex from Soybean Roots

Isolates in the Fusarium oxysporum species complex (FOSC) from soybean range from nonpathogenic to aggressive pathogens causing seedling damping-off, wilt, and root rot. The objective of this research was to characterize the genotype and phenotype of isolates within the FOSC recovered predominantly from soybean roots and seedlings. Sequence analyses of the translation elongation factor (tef1α) gene and the mitochondrial small subunit (mtSSU), polymerase chain reaction restriction fragment length polymorphism (PCR-RFLP) analysis of the intergenic spacer (IGS) region, and identification of the mating type loci were conducted for 170 isolates. Vegetative compatibility (VC) tests were conducted for 114 isolates. Isolate aggressiveness was tested using a rolled towel assay for 159 isolates. Phylogenetic analysis of the tef1α and mtSSU and PCR-RFLP analysis of the IGS region separated the FOSC isolates into five clades, including F. commune. Both mating type loci, MAT1-1 or MAT1-2, were present in isolates from all clades. The VC tests were not informative, because most VC groups consisted of a single isolate. Isolate aggressiveness varied within and among clades; isolates in clade 2 were significantly less aggressive (P < 0.0001) when compared with isolates from the other clades and F. commune. The results from this study demonstrate the high levels of genotypic and phenotypic diversity within the FOSC from soybean but further work is needed to identify characteristics associated with pathogenic capabilities.

Fumonisins in conventional and transgenic, insect-resistant maize intended for fuel ethanol production: implications for fermentation efficiency and DDGS co-product quality

Mycotoxins in maize grain intended for ethanol production are enriched in co-product dried distiller's grains and solubles (DDGS) and may be detrimental to yeast in fermentation. This study was conducted to examine the magnitude of fumonisin enrichment in DDGS and to analyze the impacts of insect injury, Fusarium ear rot severity, and fumonisin contamination on final ethanol yield. Samples of naturally-contaminated grain (0 to 35 mg/kg fumonisins) from field trials conducted in 2008-2011 were fermented and DDGS collected and analyzed for fumonisin content. Ethanol yield (determined gravimetrically) was unaffected by fumonisins in the range occurring in this study, and was not correlated with insect injury or Fusarium ear rot severity. Ethanol production was unaffected in fumonisin B1-spiked grain with concentrations from 0 to 37 mg/kg. Bacillus thuringiensis (Bt) maize often has reduced fumonisins due to its protection from insect injury and subsequent fungal infection. DDGS derived from Bt and non-Bt maize averaged 2.04 mg/kg and 8.25 mg/kg fumonisins, respectively. Fumonisins were enriched by 3.0× for 50 out of 57 hybrid × insect infestation treatment combinations; those seven that differed were <3.0 (1.56 to 2.56×). This study supports the industry assumption of three-fold fumonisin enrichment in DDGS, with measurements traceable to individual samples. Under significant insect pest pressures, DDGS derived from Bt maize hybrids were consistently lower in fumonisins than DDGS derived from non-Bt hybrids.

Trichothecene Genotype of Fusarium graminearum Isolates from Soybean (Glycine max) Seedling and Root Diseases in the United States

Fusarium graminearum is an economically important pathogen that causes Fusarium head blight of wheat, barley, and oat, and Gibberella ear and stalk rot of maize. More recently, F. graminearum was reported as a soybean seedling and root pathogen in North America (1,5), causing seed decay, damping-off, and brown to reddish-brown root rot symptoms. Type B trichothecene mycotoxins are commonly produced by F. graminearum, which can be categorized into three trichothecene genotypes; those that produce 3-acetyldeoxynivalenol (3-ADON), 15-acetyldeoxynivalenol (15-ADON), or nivalenol (NIV). The 15-ADON genotype is dominant in populations from small grains and maize in North America (4), but the 3-ADON genotype has recently been found (4). F. graminearum was known as a pathogen of wheat and maize in North America for over a century before it was reported as a soybean pathogen. Therefore, we hypothesized that recent reports on soybean could be associated with the appearance of the 3-ADON genotype. The objective of this research was to determine the trichothecene genotype of F. graminearum isolates from soybean in the United States. Thirty-eight isolates from soybean were evaluated. Twenty-seven isolates came from a 3-year survey for Fusarium root rot from 2007 to 2009 in Iowa. Other isolates (Ahmad Fakhoury, Southern Illinois University, Carbondale) were collected from soybean seedlings during a multi-state survey in 2012, and included three isolates from Illinois, three from Indiana, and five from Nebraska. Species identification and lineage of F. graminearum were confirmed by sequencing the translation elongation factor gene (EF1-α) using EF-1H and EF-2T primers. A maximum likelihood analysis of the EF1-α, including voucher strains from nine lineages of F. graminearum (2), placed all 38 isolates into lineage 7, F. graminearum sensu stricto (representative GenBank accessions KJ415349 to KJ415352). To determine the trichothecene genotype of each isolate we used three multiplex PCR assays. The first two assays targeted a portion of trichothecene biosynthesis genes Tri3 and Tri12 (4), while the third assay targeted portions of the Tri3, Tri5, and Tri7 genes (3). The PCR for the first two assays was conducted as described by Ward et al. (4) using four sets of primers: 3CON, 3NA, 3D15A, and 3D3A; and 12CON, 12NF, 12-15F, and 12-3F for the Tri3 and Tri12 genes, respectively. The PCR for the third assay was conducted as described by Quarta et al. (3) using the following primers: Tri3F971, Tri3F1325, Tri3R1679, Tri7F340, Tri7R965, 3551H, and 4056H. The amplification products were analyzed by gel electrophoresis. All 38 isolates produced amplicons consistent with the 15-ADON genotype; ~610 and 670 bp for the Tri3 and Tri12 genes, respectively (4), and two amplicons of ~708 and 525 bp for the Tri3/Tri5 genes (3). Our results indicated that the dominant trichothecene genotype among isolates of F. graminearum from soybean is 15-ADON, and the introduction of 3-ADON isolates does not explain the recent host shift of F. graminearum to soybean in North America. To our knowledge, this is the first assessment of trichothecene genotypes in F. graminearum populations from soybean from the United States. References: (1) K. E. Broders et al. Plant Dis. 91:1155, 2007. (2) K. O'Donnell et al. Fungal Gen. Biol. 41:600, 2004. (3) A. Quarta et al. FEMS Microbiol. Lett. 259:7, 2006. (4) T. D. Ward et al. Fungal Gen. Biol. 45:473, 2008. (5) A. G. Zue et al. Can. J. Plant Pathol. 29:35, 2007.

Distribution and Frequency of Fusarium Species Associated with Soybean Roots in Iowa

A 3-year survey was conducted in Iowa to characterize the distribution and frequency of species of Fusarium associated with soybean roots. Ten plants were collected from each of 40 to 57 fields each year at V2 to V5 and R3 to R4 soybean growth stages. Fusarium colonies were isolated from symptomatic and symptomless roots and identified to species based on cultural and morphological characteristics. Species identification was confirmed by amplification and sequencing of the translation elongation factor (EF1-α) gene. Fifteen species were identified; Fusarium oxysporum was isolated most frequently, accounting for more than 30% of all isolates. F. acuminatum, F. graminearum, and F. solani were also among the most frequent and widespread species. Eleven other species were recovered from few fields, accounting for less than 10% of all isolates in a given year. No consistent trends were observed in geographic distribution of species. Variability in species frequency was found between soybean growth stages. Fusarium oxysporum was recovered at higher frequency during vegetative stages (40%) than reproductive stages (22%). Conversely, species such as F. acuminatum, F. graminearum, and F. solani were recovered more often from reproductive-stage plants. No significant differences in species composition were observed among fields differing in tillage practices and row spacing.

Zucchini yellow mosaic virus (ZYMV, Potyvirus): vertical transmission, seed infection and cryptic infections

The role played by seed transmission in the evolution and epidemiology of viral crop pathogens remains unclear. We determined the seed infection and vertical transmission rates of zucchini yellow mosaic virus (ZYMV), in addition to undertaking Illumina sequencing of nine vertically transmitted ZYMV populations. We previously determined the seed-to-seedling transmission rate of ZYMV in Cucurbita pepo ssp. texana (a wild gourd) to be 1.6%, and herein observed a similar rate (1.8%) in the subsequent generation. We also observed that the seed infection rate is substantially higher (21.9%) than the seed-to-seedling transmission rate, suggesting that a major population bottleneck occurs during seed germination and seedling growth. In contrast, that two thirds of the variants present in the horizontally transmitted inoculant population were also present in the vertically transmitted populations implies that the bottleneck at vertical transmission may not be particularly severe. Strikingly, all of the vertically infected plants were symptomless in contrast to those infected horizontally, suggesting that vertical infection may be cryptic. Although no known virulence determining mutations were observed in the vertically infected samples, the 5' untranslated region was highly variable, with at least 26 different major haplotypes in this region compared to the two major haplotypes observed in the horizontally transmitted population. That the regions necessary for vector transmission are retained in the vertically infected populations, combined with the cryptic nature of vertical infection, suggests that seed transmission may be a significant contributor to the spread of ZYMV.

Aggressiveness of Fusarium species and impact of root infection on growth and yield of soybeans

Fusarium spp. are commonly isolated from soybean roots but the pathogenic activity of most species is poorly documented. Aggressiveness and yield impact of nine species of Fusarium were determined on soybean in greenhouse (50 isolates) and field microplot (19 isolates) experiments. Root rot severity and shoot and root dry weights were compared at growth stages V3 or R1. Root systems were scanned and digital image analysis was conducted; yield was measured in microplots. Disease severity and root morphology impacts varied among and within species. Fusarium graminearum was highly aggressive (root rot severity >90%), followed by F. proliferatum and F. virguliforme. Significant variation in damping-off (20 to 75%) and root rot severity (<20 to >60%) was observed among F. oxysporum isolates. In artificially-infested microplots, root rot severity was low (<25%) and mean yield was not significantly reduced. However, there were significant linear relationships between yield and root symptoms for some isolates. Root morphological characteristics were more consistent indicators of yield loss than root rot severity. This study provides the first characterization of aggressiveness and yield impact of Fusarium root rot species on soybean at different plant stages and introduces root image analysis to assess the impact of root pathogens on soybean.

First Report of Fusarium commune Causing Damping-off, Seed Rot, and Seedling Root Rot on Soybean (Glycine max) in the United States

During 2007 to 2009, symptomatic and asymptomatic soybean plants were collected from fields in 18 Iowa counties. Fusarium isolates were recovered from surface-sterilized root tissue on peptone PCNB agar (2). Single-spore isolates were transferred to synthetic low nutrient agar (SNA) overlain with pieces (1 × 2 cm) of sterile filter paper, and to potato dextrose agar (PDA), and placed in the dark for 10 to 14 days for morphological identification (4). Twenty-three isolates were identified as Fusarium commune K. Skovg., O'Donnell & Nirenberg, previously in the F. oxysporum species complex (4). Colonies on PDA had white, fluffy, aerial mycelium with magenta to violet pigmentation in the medium. On SNA, macroconidia, chlamydospores, and microconidia on monophialides and polyphialides were consistent with the species description (4). Identification of all 23 isolates was confirmed by DNA sequencing of the translation elongation factor (EF1-α) gene, using ef1 and ef2 primers, and the mitochondrial small subunit (mtSSU), using primers MS1 and MS2 (4) [GenBank accessions for two representative isolates: EF1-α (JX289892 and JX289893), and mtSSU (JX289894 and, JX289895)]. Pathogenicity of two representative isolates of F. commune was tested on soybean (cv. AG2403) in a greenhouse, in water baths set at 18°C, using autoclaved soil mixed with infested sand-cornmeal inoculum (3). The experiment entailed a completely randomized design (CRD) with five replications (single plant/150 ml cone) per treatment, and was conducted three times. Dry root and shoot weights, and root rot severity (visual estimate of percent root rot on the entire root system) were evaluated after 6 weeks. Mean seedling emergence in soil infested with F. commune was 47 and 40% for the two isolates; in contrast, non-inoculated control plants had 100% emergence. There were significant differences in root (P < 0.0001) and shoot (P < 0.0001) weights, and root rot severity (P < 0.0001), between inoculated and non-inoculated plants. Seedlings that emerged were severely stunted and had dark brown lesions. F. commune was reisolated from infected roots of inoculated plants, but not from non-inoculated plants. Pathogenicity of both isolates to soybean (cv. MN1805) was also tested using a petri dish assay, in which eight seeds were placed on a plate with a 4-day-old culture growing on 2% water agar (1). Plates were rated 7 days later for seed germination, seed rot, and lesion development, using an ordinal scale (1). The experiment entailed a CRD with three replicate plates/treatment, and was conducted three times. Germination of inoculated seeds ranged from 37.5 to 75.0%, and germinated seedlings had dark brown lesions on the taproots. There was a significant difference between isolates in the petri dish assay (P = 0.0030); one isolate was less aggressive, but both isolates resulted in significantly more disease than on the non-inoculated control plants, which had 100% germination and no symptoms (P < 0.0001). F. oxysporum is a known soybean pathogen (1), but isolates of F. commune may have been misidentified as F. oxysporum in previous studies. To our knowledge, this is the first report of F. commune as a pathogen on soybean in the U.S.A. References: (1) K. E. Broders et al. Plant Dis. 91:727, 2007. (2) J. F. Leslie and B. A. Summerell. The Fusarium Laboratory Manual. Blackwell Publishing, Oxford, UK, 2006. (3) G. P. Munkvold and J. K. O'Mara. Plant Dis. 86:143, 2002. (4) K. Skovgaard et al. Mycologia. 94:630, 2003.

First Report of Fusarium armeniacum Causing Seed Rot and Root Rot on Soybean (Glycine max) in the United States

In a survey for Fusarium root rot, soybean plants were sampled from eight counties across Iowa in 2008 to 2009. Fusarium isolates were recovered from surface-sterilized symptomatic and asymptomatic root tissue by culturing on peptone PCNB agar (2). Single-spore isolates were transferred to carnation leaf agar (CLA) and potato dextrose agar (PDA) for morphological identification; 11 isolates were identified as F. armeniacum (Forbes, Windels, and Burgess) Burgess and Summerell (previously F. acuminatum ssp. armeniacum) (2). Colonies on PDA produced white aerial mycelium, red to apricot pigment in agar, and bright orange sporodochia in the center of the culture. Some isolates produced a pionnotal form of slow-growing colonies with little aerial mycelium and abundant orange sporodochia. On CLA, macroconidia in orange sporodochia on carnation leaves and chlamydospores formed abundantly, but microconidia were absent (2). Species identity for the 11 isolates was confirmed by sequencing of the elongation factor gene (EF1-α) using ef1 and ef2 primers (4) (reference sequences deposited in GenBank JX101763 and JX101764). Pathogenicity of seven F. armeniacum isolates was tested using surface-sterilized soybean seed, cv. AG2403, in a petri dish assay with 3-day-old cultures on 2% water agar (1). Germination, seed rot, and lesion development were scored 7 dai using an ordinal scale (1). The experiment was a completely randomized design (CRD), had three replicate plates per isolate, and was conducted twice. All seven isolates were pathogenic on soybean, though variation in aggressiveness was observed among isolates (P < 0.0001) related to colony morphology on PDA. Seed germination was 0 to 40% when inoculated with four isolates showing white fluffy aerial mycelium on PDA. Seedlings were severely stunted with dark brown lesions covering a majority of the root system. When inoculated with three isolates showing the pionnotal form of slow-growing mycelium, germination was 70 to 100%, with few small brown lesions (~5 to 10 mm) on the roots. Noninoculated controls showed 100% germination and no symptoms. Pathogenicity was also tested in a growth chamber assay at 18°C using autoclaved soil mixed with an infested sand-cornmeal inoculum (3). Data for dry root and shoot weights and root rot severity (visually scored on a % scale) were collected at 6 weeks. The CRD experiment had five replications (single plant in a cone containing 150 ml infested soil), and was conducted twice. Root symptoms and similar variation in aggressiveness among isolates (based on colony morphology) was observed in inoculated plants. Isolates differed significantly for effects on root weight (P = 0.0125), shoot weight (P = 0.0035), and root rot severity (P = 0.0158). F. armeniacum was reisolated from infected root tissue, but not from noninoculated controls. Recovered isolates maintained their original colony morphology. F. armeniacum was previously reported in Minnesota on symptomless corn (2), but it has not been reported on soybean and its pathogenicity has not been established on any crop. To our knowledge, this is the first report of F. armeniacum as a pathogen on soybean in the United States. References: (1) K. E. Broders et al. Plant Dis. 91:727, 2007. (2) J. F. Leslie and B. A. Summerell. The Fusarium Laboratory Manual. Blackwell Publishing, Oxford, UK, 2006. (3) G. P. Munkvold and J. K. O'Mara. Plant Dis. 86:143, 2002. (4) K. O'Donnell et al. Proc. Natl. Acad. Sci. 95:2044, 1998.

First Report of Fusarium proliferatum Causing Root Rot on Soybean (Glycine max) in the United States

Fusarium spp. are widespread soilborne pathogens that cause important soybean diseases such as damping-off, root rot, Fusarium wilt, and sudden death syndrome. At least 12 species of Fusarium, including F. proliferatum, have been associated with soybean roots, but their relative aggressiveness as root rot pathogens is not known and pathogenicity has not been established for all reported species (2). In collaboration with 12 Iowa State University extension specialists, soybean roots were arbitrarily sampled from three fields in each of 98 Iowa counties from 2007 to 2009. Ten plants were collected from each field at V2-V3 and R3-R4 growth stages (2). Typical symptoms of Fusarium root rot (2) were observed. Symptomatic and asymptomatic root pieces were superficially sterilized in 0.5% NaOCl for 2 min, rinsed three times in sterile distilled water, and placed onto a Fusarium selective medium. Fusarium colonies were transferred to carnation leaf agar (CLA) and potato dextrose agar and later identified to species based on cultural and morphological characteristics. Of 1,230 Fusarium isolates identified, 50 were recognized as F. proliferatum based on morphological characteristics (3). F. proliferatum isolates produced abundant, aerial, white mycelium and a violet-to-dark purple pigmentation characteristic of Fusarium section Liseola. On CLA, microconidia were abundant, single celled, oval, and in chains on monophialides and polyphialides (3). Species identity was confirmed for two isolates by sequencing of the elongation factor (EF1-α) gene using the ef1 and ef2 primers (1). Identities of the resulting sequences (~680 bp) were confirmed by BLAST analysis and the FUSARIUM-ID database. Analysis resulted in a 99% match for five accessions of F. proliferatum (e.g., FD01389 and FD01858). To complete Koch's postulates, four F. proliferatum isolates were tested for pathogenicity on soybean in a greenhouse. Soybean seeds of cv. AG2306 were planted in cones (150 ml) in autoclaved soil infested with each isolate; Fusarium inoculum was applied by mixing an infested cornmeal/sand mix with soil prior to planting (4). Noninoculated control plants were grown in autoclaved soil amended with a sterile cornmeal/sand mix. Soil temperature was maintained at 18 ± 1°C by placing cones in water baths. The experiment was a completely randomized design with five replicates (single plant in a cone) per isolate and was repeated three times. Root rot severity (visually scored on a percentage scale), shoot dry weight, and root dry weight were assessed at the V3 soybean growth stage. All F. proliferatum isolates tested were pathogenic. Plants inoculated with these isolates were significantly different from the control plants in root rot severity (P = 0.001) and shoot (P = 0.023) and root (P = 0.013) dry weight. Infected plants showed dark brown lesions in the root system as well as decay of the entire taproot. F. proliferatum was reisolated from symptomatic root tissue of infected plants but not from similar tissues of control plants. To our knowledge, this is the first report of F. proliferatum causing root rot on soybean in the United States. References: (1) D. M. Geiser et al. Eur. J. Plant Pathol. 110:473, 2004. (2) G. L. Hartman et al. Compendium of Soybean Diseases. 4th ed. The American Phytopathologic Society, St. Paul, MN, 1999. (3) J. F. Leslie and B. A. Summerell. The Fusarium Laboratory Manual. Blackwell Publishing, Oxford, UK, 2006. (4) G. P. Munkvold and J. K. O'Mara. Plant Dis. 86:143, 2002.

Meta-analysis of yield response of hybrid field corn to foliar fungicides in the U.S. Corn Belt

The use of foliar fungicides on field corn has increased greatly over the past 5 years in the United States in an attempt to increase yields, despite limited evidence that use of the fungicides is consistently profitable. To assess the value of using fungicides in grain corn production, random-effects meta-analyses were performed on results from foliar fungicide experiments conducted during 2002 to 2009 in 14 states across the United States to determine the mean yield response to the fungicides azoxystrobin, pyraclostrobin, propiconazole + trifloxystrobin, and propiconazole + azoxystrobin. For all fungicides, the yield difference between treated and nontreated plots was highly variable among studies. All four fungicides resulted in a significant mean yield increase relative to the nontreated plots (P < 0.05). Mean yield difference was highest for propiconazole + trifloxystrobin (390 kg/ha), followed by propiconazole + azoxystrobin (331 kg/ha) and pyraclostrobin (256 kg/ha), and lowest for azoxystrobin (230 kg/ha). Baseline yield (mean yield in the nontreated plots) had a significant effect on yield for propiconazole + azoxystrobin (P < 0.05), whereas baseline foliar disease severity (mean severity in the nontreated plots) significantly affected the yield response to pyraclostrobin, propiconazole + trifloxystrobin, and propiconazole + azoxystrobin but not to azoxystrobin. Mean yield difference was generally higher in the lowest yield and higher disease severity categories than in the highest yield and lower disease categories. The probability of failing to recover the fungicide application cost (p(loss)) also was estimated for a range of grain corn prices and application costs. At the 10-year average corn grain price of $0.12/kg ($2.97/bushel) and application costs of $40 to 95/ha, p(loss) for disease severity <5% was 0.55 to 0.98 for pyraclostrobin, 0.62 to 0.93 for propiconazole + trifloxystrobin, 0.58 to 0.89 for propiconazole + azoxystrobin, and 0.91 to 0.99 for azoxystrobin. When disease severity was >5%, the corresponding probabilities were 0.36 to 95, 0.25 to 0.69, 0.25 to 0.64, and 0.37 to 0.98 for the four fungicides. In conclusion, the high p(loss) values found in most scenarios suggest that the use of these foliar fungicides is unlikely to be profitable when foliar disease severity is low and yield expectation is high.

Effects of Virus Infection on Susceptibility of Soybean Plants to Phomopsis longicolla

Infection of soybean by Bean pod mottle virus (BPMV) or Soybean mosaic virus (SMV) has been reported to increase susceptibility to seed infection by Phomopsis spp., but the mechanism is unclear. Effects of virus infection on susceptibility to Phomopsis longicolla were studied in greenhouse experiments. Three soybean cultivars were inoculated with BPMV at growth stage V2 to V3, and with P. longicolla at R3, R5, or R7. Inoculation with BPMV did not increase the incidence of stem infection by P. longicolla, but it increased susceptibility to seed infection of cultivars Spansoy 201 at R5, and Pioneer brand 92M02 at R3, R5, and R7. A delay in maturity was observed only in 92M02. Thus, BPMV predisposed soybean plants to seed infection by P. longicolla, but this predisposition was not due solely to prolonging maturation. In separate experiments, two soybean cultivars were inoculated with SMV (V2 to V3) and P. longicolla (R3 and R5). Inoculation with SMV did not increase the incidence of stem or seed infection by P. longicolla. The SMV-Phomopsis spp. relationship may be cultivar and strain dependent. Results suggest that the risk of soybean seed infection by P. longicolla may be higher when BPMV vector populations are high and BPMV infection is widespread.

Seed Pathology Progress in Academia and Industry

Seed pathology involves the study and management of diseases affecting seed production and utilization, as well as disease management practices applied to seeds. In this paper, three aspects of seed pathology are discussed: research innovations in detection of seedborne pathogens and elucidation of their epidemiology; advances in development and use of seed treatments; and progress toward standardization of phytosanitary regulations and seed health testing methods. The application of nucleic-acid based detection methods in seed health testing has been facilitated by integrating conventional or real-time PCR with other technologies (e.g., BIO-PCR, IMS-PCR, MCH-PCR). PCR-based methods and pathogen marker technologies are being applied to epidemiological research on seedborne pathogens, e.g., seed transmission mechanisms, the influence of external biotic and abiotic factors on seed transmission, and tracking progress of seed-transmitted pathogens. Seed treatment use is discussed in terms of the revolutionary expansion in seed-applied insecticide use, impacts of new fungicide active ingredients, and the effects of some seed treatments on crop physiology. International seed trade has been affected significantly by changing phytosanitary regulations, not always based on science. Efforts are underway to revise phytosanitary regulations to reflect pest risk analysis outcomes and to develop standards for seed health testing methods that facilitate safe and efficient international trade in seeds.

Systemic Infection by Fusarium verticillioides in Maize Plants Grown Under Three Temperature Regimes

Fusarium verticillioides causes seedling decay, stalk rot, ear rot, and mycotoxin contamination (primarily fumonisins) in maize. Systemic infection of maize plants by F. verticillioides can lead to kernel infection, but the frequency of this phenomenon has varied widely among experiments. Variation in the incidence of systemic infection has been attributed to environmental factors. In order to better understand the influence of environment, we investigated the effect of temperature on systemic development of F. verticillioides during vegetative and reproductive stages of plant development. Maize seeds were inoculated with a green fluorescent protein-expressing strain of F. verticillioides, and grown in growth chambers under three different temperature regimes. In the vegetative-stage and reproductive-stage experiments, plants were evaluated at tasseling (VT stage), and at physiological maturity (R6 stage), respectively. Independently of the temperature treatment, F. verticillioides was reisolated from nearly 100% of belowground plant tissues. Frequency of reisolation of the inoculated strain declined acropetally in aboveground internodes at all temperature regimes. At VT, the high-temperature treatment had the highest systemic development of F. verticillioides in aboveground tissues. At R6, incidence of systemic infection was greater at both the high- and low-temperature regimes than at the average-temperature regime. F. verticillioides was isolated from higher internodes in plants at R6, compared to stage VT. The seed-inoculated strain was recovered from kernels of mature plants, although incidence of kernel infection did not differ significantly among treatments. During the vegetative growth stages, temperature had a significant effect on systemic development of F. verticillioides in stalks. At R6, the fungus reached higher internodes in the high-temperature treatment, but temperature did not have an effect on the incidence of kernels (either symptomatic or asymptomatic) or ear peduncles infected with the inoculated strain. These results support the role of high temperatures in promoting systemic infection of maize by F. verticillioides, but plant-to-seed transmission may be limited by other environmental factors that interact with temperature during the reproductive stages.

Mycotoxins in ethanol co-products: modeling economic impacts on the livestock industry and management strategies

The rapidly expanding U.S. ethanol industry is generating a growing supply of co-products, mostly in the form of dried distillers' grain and solubles (DDGS) or wet distillers' grains (WDG). In the United States, 90% of the co-products of maize-based ethanol are fed to livestock. An unintended consequence is that animals are likely to be fed higher levels of mycotoxins, which are concentrated up to three times in DDGS compared to grain. The model developed in this study estimates current losses to the swine industry from weight gain reduction due to fumonisins in added DDGS at $9 million ($2-18 million) annually. If there is complete market penetration of DDGS in swine feed with 20% DDGS inclusion in swine feed and fumonisins are not controlled, losses may increase to $147 million ($29-293 million) annually. These values represent only those losses attributable to one mycotoxin on one adverse outcome on one species. The total loss due to mycotoxins in DDGS could be significantly higher due to additive or multiplicative effects of multiple mycotoxins on animal health. If mycotoxin surveillance is implemented by ethanol producers, losses are shifted among multiple stakeholders. Solutions to this problem include methods to reduce mycotoxin contamination in both pre- and postharvest maize.

Seed Transmission of Fusarium verticillioides in Maize Plants Grown Under Three Different Temperature Regimes

Fusarium verticillioides can be seed transmitted and cause systemic infection of maize; however, the frequency of these phenomena has varied widely among and within individual studies. In order to better understand this variability, we evaluated the effect of temperature on the first step in the systemic infection process, the transmission of F. verticillioides from seed to seedling. Seed of a commercial maize hybrid were inoculated with a strain of F. verticillioides that had been transformed with a gene for green fluorescent protein (GFP). The seed were planted in a greenhouse potting mix and incubated in growth chambers. Plants were incubated at one of three temperature regimes designed to simulate average and extreme temperatures occurring in Iowa during the weeks following planting. Root, mesocotyl, and stem tissues were sampled at growth stages V2 and V6, surface disinfested, and cultured on a semiselective medium. At V2, >90% of root and mesocotyl tissues was infected by the GFP-expressing strain at all three temperature regimes. Also at V2, infection was detected in 68 to 75% of stems. At V6, infection of root and mesocotyl tissues persisted and was detected in 97 to 100% of plants at all three temperature regimes. Plants also had symptomless systemic infection of belowground and aboveground internodes at V6. Infection of the three basal aboveground internodes was 24, 6, and 3% for the low-temperature regime; 35, 9, and 0% for the average-temperature regime; and 46, 24, and 9% for the high-temperature regime. Seed transmission and systemic infection occurred at all temperatures and did not differ significantly among treatments. These results indicate that, if maize seed is infected with F. verticillioides, seed transmission is common and symptomless systemic infection can be initiated under a broad range of temperature conditions.

Effects of Tillage Practices on Dissemination and Spatial Patterns of Heterodera glycines and Soybean Yield

Two field experiments were conducted in central Iowa to assess the effects of tillage on Heterodera glycines dissemination and reproduction and soybean (Glycine max) yield. Plots in both experiments were artificially infested with equivalent numbers of H. glycines cysts. In one experiment, plots were left noninfested or received aggregated or uniform infestation, and a susceptible soybean cultivar was grown for 3 years. By the end of the first growing season and through the second, H. glycines population densities were consistently greater (P ≤ 0.05) in uniformly infested plots than in plots with aggregated infestations. No differences in soybean yield among the treatments were detected. In a second experiment, a 1-m² area of each plot was infested with H. glycines cysts, susceptible soybeans were grown for four seasons, and crop residue was managed with either ridge-, conventional-, reduced-, or no-tillage. After 1 year, nematode population densities were significantly (P ≤ 0.05) greater in conventional- and reduced-tillage treatments than in no- and ridge-tillage treatments. After 2 years, H. glycines had been disseminated 6.9 m from the infestation site in conventional- and reduced-tillage treatments but only 0.5 and 1.4 m for no-tillage and ridge-tillage treatments, respectively. After 3 years, H. glycines population densities were 10 times greater in conventional- and reduced-tillage treatments than in the no-tillage treatment; conventional-tillage was the only treatment with yield significantly lower (P ≤ 0.05) than the noninfested control. Aggregation of H. glycines eggs was greater (P ≤ 0.05) in no- and ridge-tillage treatments than in conventional- and reduced-tillage treatments. Results indicate tillage can quickly disseminate H. glycines in newly infested fields, facilitating more rapid nematode reproduction and subsequent yield loss.

Reduced Incidence of Bacterial Rot on Transgenic Insect-Resistant Maize in the Philippines

In the Philippines and parts of Southeast Asia, Asian corn borer (Ostrinia furnacalis) is a serious pest of maize, and injury from this insect often is associated with the occurrence of bacterial stalk and ear rot (caused by Erwinia chrysanthemi pv. zeae). The effect of transgenic insect protection on the incidence of bacterial stalk and ear rot was studied in the Philippines with seven field trials in Mindanao and two trials in Laguna. Three transgenic hybrids (expressing Bt protein Cry1Ab) and their conventional near-isogenic counterparts were included in Mindanao, and one transgenic/conventional hybrid pair was used in Laguna (Los Banos). In the Mindanao trials, bacterial stalk rot was rated on a 1 to 9 scale approximately 2 weeks before harvest, while in Laguna, bacterial rot mortality and bacterial ear rot incidence were assessed 10 days before and at harvest, respectively. In all trials, the number of Asian corn borer tunnels was assessed by splitting stalks at harvest. Results of the trials showed significant differences between the transgenic hybrids and their conventional counterparts in terms of bacterial stalk and ear rot incidence, number of Asian corn borer tunnels, and yield. Transgenic hybrids invariably showed significantly lower bacterial stalk rot mortality and ear rot incidence, no Asian corn borer infestation, and higher yield compared with their conventional counterparts. Average yield advantage of transgenic hybrids ranged from 1.2 to 5.1 t/ha. Results confirm the important role of Asian corn borer in the initiation and spread of bacterial stalk and ear rot in maize; hence, the use of transgenic insect-resistant hybrids will have an added value in areas where this disease is prevalent.

Influence of Temperature and Relative Humidity on Sporulation of Cercospora zeae-maydis and Expansion of Gray Leaf Spot Lesions on Maize Leaves

Controlled environment studies were conducted to determine the effects of temperature on the expansion of lesions of gray leaf spot, and the effects of temperature and relative humidity on the sporulation of Cercospora zeae-maydis on maize (Zea mays). For the lesion expansion experiment, potted maize plants were spray inoculated at growth stage V6, bagged, and incubated at 25 to 28°C and 100% relative humidity for 36 to 40 h. Symptomatic plants were transferred to growth chambers and exposed to constant temperatures of 25, 30, and 35°C. Lesion area (length by width) was measured at 4-day intervals for 17 days. For sporulation studies, lesions were excised from naturally infected maize leaves, measured, and incubated at constant temperature (20, 25, 30, or 35°C) and relative humidity (70, 80, 90, or 100%) for 72 h. Sporulation was estimated as the number of conidia per square centimeter of diseased leaf tissue. A quadratic function was used to model the relationship between log-transformed conidia per square centimeter at 100% relative humidity and temperature. Temperature had a significant effect on lesion expansion (P ≤ 0.05). At 25 and 30°C, the rate of lesion expansion was significantly higher than at 35°C (P ≤ 0.05). The largest lesions and the highest mean rate of lesion expansion were observed at 30°C; however, the mean lesion expansion rate at this temperature was not significantly different from that at 25°C. The interaction effect of temperature and relative humidity on the log of conidia per square centimeter of diseased tissue was significant (P ≤ 0.05). At 100% relative humidity, the effect of temperature on sporulation was significant (P ≤ 0.05), with maximum spore production occurring at 25 and 30°C. The quadratic model explained between 49 and 80% of the variation in the log of conidia per square centimeter at 100% with variation in temperature. These results suggest that the rapid increase in gray leaf spot severity generally observed during mid- and late summer may be due to favorable conditions for lesion expansion during this period. When relative humidity is >95%, expanding lesions may serve as a source of inoculum for secondary infections.

Regression and artificial neural network modeling for the prediction of gray leaf spot of maize

ABSTRACT Regression and artificial neural network (ANN) modeling approaches were combined to develop models to predict the severity of gray leaf spot of maize, caused by Cercospora zeae-maydis. In all, 329 cases consisting of environmental, cultural, and location-specific variables were collected for field plots in Iowa between 1998 and 2002. Disease severity on the ear leaf at the dough to dent plant growth stage was used as the response variable. Correlation and regression analyses were performed to select potentially useful predictor variables. Predictors from the best 9 of 80 regression models were used to develop ANN models. A random sample of 60% of the cases was used to train the networks, and 20% each for testing and validation. Model performance was evaluated based on coefficient of determination (R(2)) and mean square error (MSE) for the validation data set. The best models had R(2) ranging from 0.70 to 0.75 and MSE ranging from 174.7 to 202.8. The most useful predictor variables were hours of daily temperatures between 22 and 30 degrees C (85.50 to 230.50 h) and hours of nightly relative humidity >/=90% (122 to 330 h) for the period between growth stages V4 and V12, mean nightly temperature (65.26 to 76.56 degrees C) for the period between growth stages V12 and R2, longitude (90.08 to 95.14 degrees W), maize residue on the soil surface (0 to 100%), planting date (in day of the year; 112 to 182), and gray leaf spot resistance rating (2 to 7; based on a 1-to-9 scale, where 1 = most susceptible to 9 = most resistant).

A model-based approach to preplanting risk assessment for gray leaf spot of maize

ABSTRACT Risk assessment models for gray leaf spot of maize, caused by Cercospora zeae-maydis, were developed using preplanting site and maize genotype data as predictors. Disease severity at the dough/dent plant growth stage was categorized into classes and used as the response variable. Logistic regression and classification and regression tree (CART) modeling approaches were used to predict severity classes as a function of planting date (PD), amount of maize soil surface residue (SR), cropping sequence, genotype maturity and gray leaf spot resistance (GLSR) ratings, and longitude (LON). Models were development using 332 cases collected between 1998 and 2001. Thirty cases collected in 2002 were used to validate the models. Preplanting data showed a strong relationship with late-season gray leaf spot severity classes. The most important predictors were SR, PD, GLSR, and LON. Logistic regression models correctly classified 60 to 70% of the validation cases, whereas the CART models correctly classified 57 to 77% of these cases. Cases misclassified by the CART models were mostly due to overestimation, whereas the logistic regression models tended to misclassify cases by underestimation. Both the CART and logistic regression models have potential as management decision-making tools. Early quantitative assessment of gray leaf spot risk would allow for more sound management decisions being made when warranted.

Cultural and genetic approaches to managing mycotoxins in maize

Infection of maize kernels by toxigenic fungi remains a challenging problem despite decades of research progress. Cultural practices, including crop rotation, tillage, planting date, and management of irrigation and fertilization, have limited effects on infection and subsequent mycotoxin accumulation. Current infrastructure and grain storage practices in developed countries can prevent postharvest development of mycotoxins, but this aspect remains a threat in developing countries, especially in tropical areas. Because most mycotoxin problems develop in the field, strategies are needed to prevent infection of growing plants by toxigenic fungi. Developing genetic resistance to Aspergillus flavus, Gibberella zeae, and Fusarium spp. (particularly F. verticillioides) in maize is a high priority. Sources of resistance to each of these pathogens have been identified and have been incorporated into public and private breeding programs. However, few, if any, commercial cultivars have adequate levels of resistance. Efforts to control infection or mycotoxin development through conventional breeding and genetic engineering are reviewed. The role of transgenic insect control in the prevention of mycotoxins in maize is discussed.

FUM1--a gene required for fumonisin biosynthesis but not for maize ear rot and ear infection by Gibberella moniliformis in field tests

We have analyzed the role of fumonisins in infection of maize (Zea mays) by Gibberella moniliformis (anamorph Fusarium verticillioides) in field tests in Illinois and Iowa, United States. Fumonisin-nonproducing mutants were obtained by disrupting FUM1 (previously FUM5), the gene encoding a polyketide synthase required for fumonisin biosynthesis. Maize ear rot, ear infection, and fumonisin contamination were assessed by silk-channel injection in 1999 and 2000 and also by spray application onto maize silks, injection into maize stalks, and application with maize seeds at planting in 1999. Ear rot was evaluated by visual assessment of whole ears and by calculating percentage of symptomatic kernels by weight. Fumonisin levels in kernels were determined by high-performance liquid chromatography. The presence of applied strains in kernels was determined by analysis of recovered isolates for genetic markers and fumonisin production. Two independent fumonisin-nonproducing (fum1-3 and fum1-4) mutants were similar to their respective fumonisin-producing (FUM1-1) progenitor strains in ability to cause ear rot following silk-channel injection and also were similar in ability to infect maize ears following application by all four methods tested. This evidence confirms that fumonisins are not required for G. moniliformis to cause maize ear rot and ear infection.

A Comparison of Maize Stalk Rot Occurrence in Bt and Non-Bt Hybrids

Stalk rots, caused by a complex of fungal species, are among the most widespread and destructive diseases of maize. Larvae of the European corn borer (ECB) (Ostrinia nubilalis) promote stalk rot development by creating entry points for fungi, serving as vectors of pathogens, and causing physiological stress that may predispose plants to stalk decay. Field experiments were conducted in 1998, 1999, and 2000 to determine whether the use of transgenic Bt hybrids expressing insecticidal proteins would influence stalk rot symptoms (pith disintegration, pith discoloration, and lodging). Five hybrids representing different Bt types (or "Bt events") (176, BT11, MON810, DBT418, and CBH351) were paired with their near-isogenic, non-Bt counterparts and subjected to treatments of manual and natural infestation with ECB larvae. Manual infestation resulted in significantly more ECB tunneling than natural infestation in 1998 and 1999 and significantly more lodging in 1998. There were significant linear correlations between ECB injury and stalk rot symptoms in non-Bt hybrids in 1998 and 1999, but not in 2000. A standard foliar insecticide treatment for ECB did not significantly affect stalk rot symptoms. In 1998, Bt hybrids had significantly less ECB tunneling, stalk discoloration, pith disintegration, and lodging compared with non-Bt hybrids, but these effects depended upon the Bt event and the infestation treatment. Similar but less pronounced effects of Bt events were observed in 1999. The 2000 results were more variable; the amount of pith disintegration was significantly lower but discoloration was significantly higher in the BT11 hybrid compared with its non-Bt counterpart, and the amount of lodging was significantly higher in the event 176 hybrid compared with its non-Bt counterpart. The ratio of stalk strength to grain weight did not consistently differ between Bt and non-Bt hybrids. These results indicate that, although specific Bt events in some years may cause reductions in stalk rot, the overall effect of Bt transformation on stalk rot occurrence is highly variable.

Fungal Species Composition in Maize Stalks in Relation to European Corn Borer Injury and Transgenic Insect Protection

The maize stalk rot complex is comprised of several fungal pathogens, including Gibberella zeae, Colletotrichum graminicola, Stenocarpella maydis, and several members of the genus Fusarium. The European corn borer (ECB) (Ostrinia nubilalis) can contribute to stalk rot development by creating entry wounds and by serving as a vector of some stalk rot pathogens, particularly Fusarium verticillioides. Transgenic insect protection of maize hybrids with insecticidal proteins derived from Bacillus thuringiensis greatly reduces ECB injury and may therefore alter the species composition and diversity of the stalk rot complex. Field experiments were conducted in 1998, 1999, and 2000 to compare the species composition and diversity of fungi infecting stalks of Bt and non-Bt maize hybrids. Hybrids representing five Bt types (or "events") and their near-isogenic non-Bt counterparts were subjected to manual and natural infestations with ECB larvae. Stalk tissue samples were cultured to determine fungal species composition. At least one species was isolated from nearly every stalk and from both diseased and symptomless tissues. G. zeae was the most common species in 1998 and 1999, but C. graminicola was most common in 2000. The mean proportions of stalks infected with F. verticillioides, F. proliferatum, and F. subglutinans were significantly lower in Bt hybrids than in non-Bt hybrids in 2 of the 3 years. Conversely, the mean proportion of stalks infected with G. zeae was higher in some Bt hybrids than their non-Bt counterparts in two of the three years. F. verticillioides was more likely to be isolated from ECB-injured tissue, whereas G. zeae and C. graminicola were more likely to be isolated from tissue not associated with ECB injury. The overall species diversity of the stalk rot complex was lower in some Bt hybrids compared with their non-Bt counterparts in 1998 and 1999. ECB activity appeared to alter fungal species composition in stalks, reflecting the association between ECB injury and specific stalk rot pathogens, particularly F. verticillioides. The species composition of fungi infecting stalks of Bt hybrids differed from that of non-Bt hybrids, but the implications of this result are not yet clear.

Relationships of Environmental and Cultural Factors with Severity of Gray Leaf Spot in Maize

Gray leaf spot of maize caused by Cercospora zeae-maydis is a major foliar disease in the United States and other parts of the world. Efficient management of gray leaf spot is hindered by a lack of quantitative information regarding environmental and cultural influences on disease severity. We collected environmental, cultural, and disease severity data in southern Iowa at 13 locations in 1998 and 11 locations in 1999. The independent variables that we considered included temperature, relative humidity, leaf wetness, percent maize residue cover, distance to nearest maize residue, planting date, and previous crop. A time-duration value (TDV) variable was created to represent cumulative hours of favorable temperature (22 ≤ T ≤ 30°C) and relative humidity (≥95%). Disease severity was assessed at 2-week intervals on three to eight maize genotypes differing in gray leaf spot resistance and maturity at each location. Environmental, cultural, and disease data were summarized for four different periods during the growing season and analyzed by stepwise multiple linear regression in order to determine which variables significantly contributed to gray leaf spot severity at the dough or dent growth stages of maize. In 1998, genotype resistance, planting date, distance to nearest maize residue, wetness duration, and TDV had significant effects on disease severity. R² values were similar among the four periods. The best-fitting model for the 1998 data had an R² of 0.65. With 1998 and 1999 data combined, results were similar except that percent maize residue cover was significant rather than distance to nearest maize residue. The best-fitting model had an R² of 0.55. The 2-year model utilizing only the weather variables from emergence to 2 weeks before silking had an R² value of 0.43. Strong linear relationships existed between gray leaf spot severity and genotype resistance, maize surface residue, planting date, and TDV. These results can serve as a foundation for the development of a prediction model for gray leaf spot severity on maize.

Laboratory and Growth Chamber Evaluation of Fungicidal Seed Treatments for Maize Seedling Blight Caused by Fusarium Species

The performance of seed treatment products for maize usually is evaluated in field experiments, where it is difficult to assess their effects on specific important pathogens such as fungi in the genus Fusarium. To evaluate three fungicidal seed treatments (captan, difenoconazole, and fludioxonil) against six Fusarium species that infect maize seed or seedlings, we conducted experiments in the laboratory and in growth chambers. In the laboratory experiments, treated and nontreated seeds of two maize hybrids were incubated on the surface of an agar medium colonized by each of 12 Fusarium isolates. The fungi did not reduce seed germination, but most Fusarium isolates caused decay of the seed and radicle, and arrested the development of the radicle. All three fungicides significantly reduced the colonization and decay of the seeds and radicles by Fusarium isolates and resulted in greater radicle lengths, but there were significant interactions between the effects of fungicide treatments and Fusarium isolates. Overall, difenoconazole was the most effective fungicide for the prevention of seed colonization and decay. Fludioxonil was overall the most effective fungicide in terms of increased radicle length, particularly when seed was exposed to isolates of F. graminearum, which were among the most aggressive isolates in the experiments. In the growth chamber experiments, seeds were planted in a Fusarium-infested potting medium, which resulted in lower emergence, shoot length, root length, and dry weight of seedlings compared to the noninfested control. Some isolates also caused root rot symptoms. All three fungicides significantly improved shoot and root length and root health, difenoconazole and fludioxonil significantly improved emergence, and only difeno-conazole significantly improved dry weight compared to the nontreated control. There were significant rank correlations between the results of the laboratory and growth chamber experiments in terms of relative aggressiveness of the isolates and relative efficacy of the fungicides. The laboratory experiments were more sensitive in terms of detecting differences in fungicide performance. These results indicate that all three fungicides were effective against Fusarium, but difenoconazole and fludioxonil generally were more effective than captan; the fungicides also differed in efficacy against different Fusarium species.

Relationships Between Tillage and Spatial Patterns of Heterodera glycines

ABSTRACT The dynamics of Heterodera glycines spatial patterns were studied under different tillage systems in two naturally infested soybean fields in Iowa from 1994 to 1997. At each location, there were four different tillage treatments (conventional tillage, reduced tillage, ridge tillage, and no tillage). Soil samples were taken from 98 contiguous quadrats (5.2 m(2)) per plot in the fall of 1994, before any tillage was performed, and in the spring of the following 3 years shortly after planting. Cysts were extracted from soil samples by elutriation and counted, and eggs were extracted from cysts and enumerated. Spatial patterns of H. glycines populations were characterized by geostatistical analysis and variance-to-mean (VM) ratios. Semivariance values were calculated for cyst and egg densities and semivariograms were constructed. In general, there was greater spatial dependence among cyst populations than egg populations. In one field with a strongly aggregated initial H. glycines population, tillage practices resulted in changes in spatial patterns of H. glycines populations, characterized by spherical-model semivariogram parameters (sill, nugget effect, and range of spatial dependence). These parameters indicated increasing aggregation over time in no tillage and ridge tillage treatments, but decreasing aggregation in reduced and conventional tillage treatments. There was an increase of 350% in sill values (maximum semivariance) for cyst populations after 3 years of no tillage, but in the conventional tillage treatment, sill values remained unchanged or decreased over time as tillage was implemented. Semivariograms for cyst and egg population densities revealed strong anisotropy (directional spatial dependence) along soybean rows, coincident with the direction of tillage practices. VM ratios for cyst counts increased each year in the no tillage and ridge tillage treatments, but decreased for 2 years in reduced tillage and conventional tillage treatments. Final VM ratios for cyst and egg counts were highest in the no tillage treatment. In a second field, with low initial aggregation of H. glycines populations, there was little measurable change in semivariogram parameters after 3 years of no tillage, but in the conventional tillage treatment, populations became less aggregated, as the range, sill, and the proportion of the sill explained by spatial dependence decreased for cyst population densities. Our results indicated that in soybean fields with initially aggregated populations of H. glycines, no tillage and ridge tillage systems promoted aggregation of the nematode population, whereas conventional and reduced tillage systems resulted in a less aggregated spatial pattern.

Probabilities for profitable fungicide use against gray leaf spot in hybrid maize

ABSTRACT Gray leaf spot, caused by the fungus Cercospora zeae-maydis, causes considerable yield losses in hybrid maize grown in the north-central United States and elsewhere. Nonchemical management tactics have not adequately prevented these losses. The probability of profitably using fungicide application as a management tool for gray leaf spot was evaluated in 10 field experiments under conditions of natural inoculum in Iowa. Gray leaf spot severity in untreated control plots ranged from 2.6 to 72.8% for the ear leaf and from 3.0 to 7.7 (1 to 9 scale) for whole-plot ratings. In each experiment, fungicide applications with propiconazole or mancozeb significantly reduced gray leaf spot severity. Fungicide treatment significantly (P </= 0.05) increased yield by as much as 1.65 t/ha with a single propiconazole application. There were significant (P < 0.05) correlations between gray leaf spot severity and yield. We used a Bayesian inference method to calculate for each experiment the probability of achieving a positive net return with one or two propiconazole applications, based on the mean yields and standard deviations for treated and untreated plots, the price of grain, and the costs of the fungicide applications. For one application, the probability ranged from approximately 0.06 to more than 0.99, and exceeded 0.50 in six of nine scenarios (specific experiment/hybrid). The highest probabilities occurred in the 1995 experiments with the most susceptible hybrid. Probabilities were almost always higher for a single application of propiconazole than for two applications. These results indicate that a single application of propiconazole frequently can be profitable for gray leaf spot management in Iowa, but the probability of a profitable application is strongly influenced by hybrid susceptibility. The calculation of probabilities for positive net returns was more informative than mean separation in terms of assessing the economic success of the fungicide applications.

Virulence of Aphanomyces euteiches Isolates from Iowa and Wisconsin and Benefits of Resistance to A. euteiches in Alfalfa Cultivars

Aphanomyces euteiches has become recognized as an important root rot pathogen of alfalfa in the north-central United States, and resistant cultivars are now commonly planted. Recent evidence indicates the existence of A. euteiches strains, designated as race 2, that are virulent on resistant cultivars, but there is little information on the prevalence of such strains or their impact on the performance of A. euteiches-resistant cultivars. The purpose of this study was to assess the virulence of A. euteiches isolates obtained from Iowa and Wisconsin soils and to determine the frequency of isolates virulent on race 1-resistant alfalfa populations. In addition, the yield performance of susceptible and resistant alfalfa populations was compared in four Iowa locations and one Wisconsin location. Fourteen isolates of A. euteiches from different Iowa locations were used to challenge two race 1-resistant cultivars (Paramount and Quantum), a susceptible cultivar (Agate or Vernal), and two resistant breeding populations (WAPH-1 and WAPH-2). Fifty-nine isolates of A. euteiches from one location in Wisconsin were used to challenge one susceptible cultivar (Saranac) and WAPH-1 and WAPH-2. Every isolate was virulent to one or more alfalfa cultivars or populations. Emergence of seedlings in growth chamber experiments did not differ significantly among isolates or alfalfa populations. Alfalfa population and A. euteiches isolate had significant effects on disease severity index (DSI, 1-5 scale), but there were significant interactions (P < 0.05) between these two effects. All 14 Iowa isolates of A. euteiches were virulent (DSI ≥ 3.0) on Agate (mean DSI = 4.4, range 3.8 to 4.9), WAPH-1 (mean DSI = 3.9, range 3.0 to 4.4), and the two commercial resistant cultivars (mean DSI = 3.9 and 4.1, range 3.2 to 4.4). On WAPH-2, only three isolates were virulent (mean DSI = 2.5, range 1.8 to 3.2). Of 59 Wisconsin isolates, all were virulent on Saranac (mean DSI = 4.6, range 3.9 to 5.0), 21 were virulent on WAPH-1 (mean DSI = 2.9, range 1.8 to 4.8), and only four were virulent on WAPH-2 (mean DSI = 2.3, range 1.8 to 3.4). In field studies, we compared yield performance of alfalfa cultivars that were resistant or susceptible to A. euteiches or Phytophthora medicaginis at four Iowa locations for one to three harvest years, and one Wisconsin location for two harvest years. Mean yields of cultivars with resistance to one or both pathogens were significantly higher than those of susceptible cultivars in only one of the four Iowa locations. In Wisconsin, WAPH-4, a Race 2-resistant alfalfa population, expressed a significant yield advantage when compared with both WAPH-1, a Race 1-resistant alfalfa population, and Columbia 2000, a cultivar susceptible to both race 1 and 2 of A. euteiches. These results indicate that race 2 of A. euteiches is prevalent in Iowa and Wisconsin soils and may be limiting the yield benefits of currently available race 1-resistant alfalfa cultivars. Incorporation of race 2 resistance is likely to improve the performance of alfalfa cultivars in A. euteiches-infested soils.

Effects of temperature, incubation period and substrate on production of fusaproliferin by Fusarium subglutinans ITEM 2404

The kinetics of the production of fusaproliferin by Fusarium subglutinans ITEM 2404 in maize and rice cultures was investigated at various incubation temperatures. The growth rate of F. subglutinans was highest at 20 degrees C and 25 degrees C in maize cultures and at 15 degrees C in rice cultures. Although the growth rate was higher in rice than in maize, the maximal production of fusaproliferin was obtained in maize cultures, with a maximum yield (4309 microg g(-1)) at 20 degrees C for 6 weeks. In rice cultures the optimal incubation regimen was at 15 degrees C for 6 weeks, with a fusaproliferin level of 1557 microg g(-1). The production of fusaproliferin at 25 degrees C and 30 degrees C in both substrates was very low, with maximal yield at 25 degrees C of 979 microg g(-1) after 2 weeks and 143 microg g(-1) after 3 weeks in maize and rice cultures, respectively.

Outbreak of Smut Caused by Tilletia maclaganii on Cultivated Switchgrass in Iowa

Switchgrass (Panicum virgatum L.), a prairie grass native to Iowa, is cultivated for forage and biomass production. During the late 1990s, biomass and seed yields of switchgrass grown in southern Iowa began to decline, and the reduction has been attributed to unidentified diseases. In 1999, many plants in previously low-yielding fields were stunted and flowered prematurely. Glumes had an uncharacteristic purple pigmentation, and seeds had been replaced by fungal spores. A smut fungus identified as Tilletia maclaganii (Berk.) G.P. Clinton (1) was associated consistently with fields that yielded poorly. Teliospores were red-orange when immature and turned dark brown as they matured. Teliospores were globose to slightly irregular, ≈18 to 25 µm in diameter, finely verrucose, with a thick exospore. True sterile cells also were present. T. maclaganii infects switchgrass and has been reported previously in Iowa (2), although it is found only occasionally on the state's native switchgrass. The prevalence and incidence of disease was surveyed in late August 1999. A weighted random sampling procedure was used to select switchgrass production fields from 60 fields involved in the Chariton Valley Biomass Project. Fields were located in Appanoose, Lucas, Monroe, and Wayne counties in southern Iowa. The sampling procedure was designed so the probability of each field being chosen was proportional to its area. This resulted in samples being taken from 17 fields representing ≈50% of the total area of the 60 fields. All sampled fields were planted with the predominant cultivar, Cave-in-Rock. In each field, five 1-m² samples (≈60 to 250 tillers) were taken from arbitrary points. The incidence of smut (percentage of tillers with smut) was calculated for each sample. Smut was found in 15 of 17 fields. We estimated that 50 to 82% of the area in switchgrass production in these counties was infested with T. maclaganii. The mean incidence of smut was estimated at 10.1% of all tillers in the area. Incidence in individual fields ranged from 0 to 70%. Fields with incidence >50% yielded less than half of the expected biomass. Some infested seed-production fields were a total loss in 1999. This disease presents a serious threat to the cultivation of switchgrass for biomass production in southern Iowa. The disease cycle for T. maclaganii is poorly documented, but because switchgrass is a perennial species, it is likely that affected fields will have recurring epidemics. Susceptibility of other cultivars is unknown but needs to be investigated. References: (1) G. W. Fischer. 1953. Manual of the North American Smut Fungi. Ronald Press, NY. (2) J. C. Gilman and W. A. Archer. The fungi of Iowa parasitic on plants. Iowa State College J. Sci. 3:299, 1929.

Genetic analysis of eutypa strains from california supports the presence of two pathogenic species

ABSTRACT Eutypa dieback is a perennial canker disease that adversely affects grape (Vitis vinifera) production throughout the world. The causal agent has been known as either Eutypa armeniacae or E. lata, and it has been unclear whether the two taxa are separate species. We analyzed 115 isolates of Eutypa and conspecific strains, including 106 from California, using amplified fragment length polymorphism (AFLP) and sequence analysis of the ribosomal DNA (rDNA) internal transcribed spacer (ITS) sequence. Strains from cultivated plant species exhibited an average genetic distance of 0.34, as calculated by the DICE coefficient (NTSYS-pc software). An unweighted pair-group method with arithmetic averages dendrogram revealed a genetically distinct (distance of 0.73) group of Eutypa strains from valley oak (Quercus lobata) and madrone (Arbutus menziesii) and a strain from grape. Analysis of rDNA ITS sequences strongly supported the genetically distinct cluster detected in the AFLP data. Combined data indicated the presence of two species of Eutypa (E. armeniacae and E. lata) in our sample population. However, both Eutypa species were capable of infecting native and cultivated hosts, suggesting the potential for native tree species to serve as inoculum sources for grape infection in California. Further investigations of E. armeniacae and E. lata would contribute to the development of a successful disease management strategy.

Comparison of Fumonisin Concentrations in Kernels of Transgenic Bt Maize Hybrids and Nontransgenic Hybrids

Maize hybrids genetically engineered with genes from the bacterium Bacillus thuringiensis (Bt maize) express CryIA(b) and other Cry proteins that are toxic to certain insects, particularly the European corn borer (Ostrinia nubilalis). Maize kernel feeding by O. nubilalis often leads to infection by fungi in the genus Fusarium, including the fumonisin-producing species F. verticillioides and F. proliferatum. In field experiments in 1995, 1996, and 1997, transgenic maize hybrids and near-isogenic, nontransgenic hybrids were manually infested with neonatal European corn borer larvae. Manual infestation increased Fusarium ear rot severity and fumonisin concentrations in kernels of nontransgenic hybrids. Transgenic hybrids with kernel expression of CryIA(b) consistently experienced less insect feeding on kernels and less Fusarium ear rot than their nontransgenic counterparts. In manually infested treatments, these hybrids also exhibited lower concentrations of fumonisins in kernels compared with their nontransgenic counterparts. In manually infested treatments in 1995, mean fumonisin B₁ concentrations were 8.8 μg/g in the nontransgenic hybrid and 6.7 or 3.0 μg/g in transgenic hybrids. In 1996, mean fumonisin B₁ concentrations in manually infested treatments were 4.9 μg/g (range 2.3 to 8.8) for nontransgenic and 1.2 μg/g (range 1.0 to 1.3) for transgenic hybrids with kernel expression. Mean total fumonisin concentrations (fumonisin B1 + B2 + B3) were 7.0 μg/g (range 3.0 to 12.2) for nontransgenic and 1.7 μg/g (range 1.5 to 1.9) for transgenic hybrids with kernel expression. In 1997, mean fumonisin B₁ concentrations in manually infested treatments were 11.8 μg/g (range 7.6 to 17.3) for nontransgenic and 1.3 μg/g (range 0.8 to 2.2) for transgenic hybrids with kernel expression of CryIA(b) or Cry9C. Mean total fumonisin concentrations were 16.5 μg/g (range 10.7 to 24.0) for nontransgenic and 2.1 μg/g (range 1.5 to 3.1) for transgenic hybrids with kernel expression. Transgenic hybrids that do not express CryIA(b) or Cry9C in kernels did not consistently have fumonisin concentrations different from the nontransgenic hybrids. Higher fumonisin concentrations in nontransgenic hybrids were associated with high European corn borer populations during the early reproductive stages of the maize plants. These results indicate that under some conditions, genetic engineering of maize for insect resistance may enhance its safety for animal and human consumption.

Survival of Fusarium moniliforme, F. proliferatum, and F. subglutinans in Maize Stalk Residue

ABSTRACT The roles of residue size and burial depth were assessed in the survival of Fusarium moniliforme, F. proliferatum, and F. subglutinans in maize stalk residue. Stalk pieces (small or large sizes) were soaked in a spore suspension of F. moniliforme, F. proliferatum, or F. subglutinans and placed in a field on the soil surface or buried at 15- or 30-cm depths. Residue pieces were recovered periodically, cultured on a selective medium, and microscopically examined for the presence of the inoculated Fusarium species. After 630 days, the inoculated Fusarium species were recovered from 0 to 50% of the inoculated stalk pieces in a long-term, continuous maize field, from 0 to 28% of the inoculated stalk pieces placed in a maize/soybean/oat rotation field, and from 0 to 25% of the noninoculated stalk pieces at both locations. Residue size and residue depth had significant effects on survival, but there were significant interactions among strain, depth, residue size, and time. Up to 343 days after placement in the field, survival of the three Fusarium species was not consistently different between buried residues and surface residues, but after 630 days, survival was greater from surface residues. Overall, fungus survival decreased more slowly in the surface residues than in the buried residues. Linear coefficients of determination ranged from 0.35 to 0.82 for the surface residues and from 0.81 to 0.98 for the buried residues. Decline in survival over time followed a more linear pattern in buried residues than in surface residues. Vegetative compatibility tests confirmed that F. moniliforme, F. proliferatum, and F. subglutinans strains can survive at least 630 days in surface or buried maize residue. These results demonstrate that maize residue can act as a long-term source of inoculum for infection of maize plants by these three Fusarium species.