Fungicide and Cultivar Effects on Sudden Death Syndrome and Yield of Soybean

Management of Soybean Cyst Nematode and Sudden Death Syndrome with Nematode-Protectant Seed Treatments Across Multiple Environments in Soybean

As soybean (Glycine max) production continues to expand in the United States and Canada, so do pathogens and pests that directly threaten soybean yield potential and economic returns for farmers. One such pathogen is the soybean cyst nematode (SCN; Heterodera glycines). SCN has traditionally been managed using SCN-resistant cultivars and rotation with nonhost crops, but the interaction of SCN with sudden death syndrome (SDS; caused by Fusarium virguliforme) in the field makes management more difficult. Nematode-protectant seed treatments have become options for SCN and SDS management. The objectives of this study were to evaluate nematode-protectant seed treatments for their effects on (i) early and full season SCN reproduction, (ii) foliar symptoms and root-rot caused by SDS, and (iii) soybean yield across environments accounting for the above factors. Using a standard protocol, field trials were implemented in 13 states and one Canadian province from 2019 to 2021 constituting 51 site-years. Six nematode-protectant seed treatment products were compared with a fungicide + insecticide base treatment and a nontreated check. Initial (at soybean planting) and final (at soybean harvest) SCN egg populations were enumerated, and SCN females were extracted from roots and counted at 30 to 35 days postplanting. Foliar disease index (FDX) and root rot caused by the SDS pathogen were evaluated, and yield data were collected for each plot. No seed treatment offered significant nematode control versus the nontreated check for in-season and full-season nematode response, no matter the initial SCN population or FDX level. Of all treatments, ILEVO (fluopyram) and Saltro (pydiflumetofen) provided more consistent increases in yield over the nontreated check in a broader range of SCN environments, even when FDX level was high.

Exploring the Potential Mechanism of Prothioconazole Resistance in Fusarium graminearum in China

The Fusarium head blight (FHB) caused by Fusarium graminearum is one of the most important diseases threatening wheat production in China. However, the triazole sterol 14α-demethylation inhibitor (DMI), prothioconazole, is known to exhibit high activity against F. graminearum. The current study indicated that three highly resistant laboratory mutants exhibited significantly (p < 0.05) altered growth and sporulation, although contrary to expectation, only one of the mutants exhibited reduced growth and sporulation, while the other two exhibited significant (p < 0.05) increases. Despite this, pathogenicity tests revealed that all of the mutants exhibited significantly (p < 0.05) reduced pathogenicity, indicating a substantial cost to fitness. Sequence analysis of the prothioconazole target protein, CYP51, of which F. graminearum has three homologues (FgCYP51A, FgCYP51B, and FgCYP51C), identified three mutations in the FgCYP51B sequence with a high likelihood of being associated with the observed resistance, as well as another three mutations in the FgCYP51B sequence, and two in the FgCYP51A sequence that are worthy of further investigation. Two of the prothioconazole-resistant mutants were also found to have several amino acid substitutions in their FgCYP51C sequences, and it was interesting to note that these two mutants exhibited significantly (p < 0.05) reduced pathogenicity compared to the other mutant. Expression analysis revealed that prothioconazole treatment (0.1 μg/mL) resulted in altered expression of all the FgCYP51 target genes, and that expression was also altered in the prothioconazole-resistant mutants compared to their wild-type parental isolates. Meanwhile, no evidence was found of any cross-resistance between prothioconazole and other commonly used fungicides, including carbendazim, pyraclostrobin, and fluazinam, as well as the triazole tebuconazole and the imidazole DMI prochloraz. Taken together, these results not only provide new insight into potential resistance mechanism in F. graminearum, and the biological characteristics associated with them, but also convincing evidence that prothioconazole can offer effective control of FHB.

Influence of Integrated Management Strategies on Soybean Sudden Death Syndrome (SDS) Root Infection, Foliar Symptoms, Yield and Net Returns

Three soybean field trials were conducted in Indiana to evaluate the integration of seed treatment, cultivar selection, and seeding rate on sudden death syndrome (SDS) root rot, pathogen load in the root, foliar symptoms, yield, and net return. Two soybean cultivars, one moderately resistant and one susceptible to SDS, were planted at three seeding rates (272,277 seeds/ha, 346,535 seeds/ha, and 420,792 seeds/ha). Fluopyram and pydiflumetofen seed treatments were applied to both cultivars, and the cultivars were then compared with a control. Low foliar SDS disease pressure was observed in our study. Seed treatment with either fluopyram or pydiflumetofen and the use of a moderately resistant cultivar decreased Fusarium virguliforme DNA concentration in the root relative to the control and the use of a susceptible cultivar. Fluopyram significantly reduced visual root rot severity by 8.8% and increased yield by 105 kg/ha relative to the control but was not different from pydiflumetofen. However, pydiflumetofen performed the same as the control with respect to root rot severity and yield. Findings from this study support the use of a seed treatment to protect roots from infection and the use of a moderately resistant cultivar planted at a seeding rate of 346,535 seeds/ha to protect yield and maximize net returns when a field has low foliar SDS pressure.

Field and Greenhouse Assessment of Seed Treatment Fungicides for Management of Sudden Death Syndrome and Yield Response of Soybean

Seed treatments for the management of sudden death syndrome (SDS) caused by Fusarium virguliforme are available in the United States and Canada; however, side-by-side comparisons of these seed treatments are lacking. Sixteen field experiments were established in Illinois, Indiana, Iowa, Michigan, and Wisconsin, United States, and Ontario, Canada, in 2019 and 2020 to evaluate seed treatment combinations. Treatments included a nontreated check (NTC), fungicide and insecticide base seed treatments (base), fluopyram, base + fluopyram, base + saponin extracts from Chenopodium quinoa, base + fluopyram + heat-killed Burkholderia rinojenses, base + pydiflumetofen, base + thiabendazole + heat-killed B. rinojenses, and base + thiabendazole + C. quinoa extracts + heat-killed B. rinojenses. Treatments were tested on SDS moderately resistant and susceptible soybean cultivars at each location. Overall, NTC and base had the most root rot, most foliar disease index (FDX), and lowest yield. Base + fluopyram and base + pydiflumetofen were most effective for managing SDS. Moderately resistant cultivars reduced FDX in both years but visual root rot was greater on the moderately resistant than the susceptible cultivars in 2020. Yield response to cultivar was also inconsistent between the 2 years. In 2020, the susceptible cultivar provided significantly more yield than the moderately resistant cultivar. Treatment effect for root rot and FDX was similar in field and greenhouse evaluations. These results reinforce the need to include root rot evaluations in addition to foliar disease evaluations in the breeding process for resistance to F. virguliforme and highlights the importance of an integrated SDS management plan because not a single management tactic alone provides adequate control of the disease.

Deep learning-based phenotyping for genome wide association studies of sudden death syndrome in soybean

Using a reliable and accurate method to phenotype disease incidence and severity is essential to unravel the complex genetic architecture of disease resistance in plants, and to develop disease resistant cultivars. Genome-wide association studies (GWAS) involve phenotyping large numbers of accessions, and have been used for a myriad of traits. In field studies, genetic accessions are phenotyped across multiple environments and replications, which takes a significant amount of labor and resources. Deep Learning (DL) techniques can be effective for analyzing image-based tasks; thus DL methods are becoming more routine for phenotyping traits to save time and effort. This research aims to conduct GWAS on sudden death syndrome (SDS) of soybean [Glycine max L. (Merr.)] using disease severity from both visual field ratings and DL-based (using images) severity ratings collected from 473 accessions. Images were processed through a DL framework that identified soybean leaflets with SDS symptoms, and then quantified the disease severity on those leaflets into a few classes with mean Average Precision of 0.34 on unseen test data. Both visual field ratings and image-based ratings identified significant single nucleotide polymorphism (SNP) markers associated with disease resistance. These significant SNP markers are either in the proximity of previously reported candidate genes for SDS or near potentially novel candidate genes. Four previously reported SDS QTL were identified that contained a significant SNPs, from this study, from both a visual field rating and an image-based rating. The results of this study provide an exciting avenue of using DL to capture complex phenotypic traits from images to get comparable or more insightful results compared to subjective visual field phenotyping of traits for disease symptoms.

Time of Soybean Sudden Death Syndrome Foliar Symptom Onset Influences Final Disease Intensity, Yield, and Yield Components

Sudden death syndrome (SDS), caused by Fusarium virguliforme, causes substantial yield losses in soybean. However, relationships between soybean yield and components of disease progress, including time of disease onset, are poorly understood. Individual soybean plants (2018) and quadrats (2016 to 2018) were monitored in commercial fields and experimental plots in Iowa to quantify the impact of SDS foliar symptom onset on final SDS intensity, soybean yield components, and yield. The date when SDS foliar symptoms were first detected (onset time) and progress of SDS incidence and severity were recorded weekly. Individual soybean plants and quadrats were harvested at the end of each season. Beta-regression showed that date of SDS onset had a consistent and stable effect on final disease intensity both at individual plant and quadrat levels. The slope of the relationship between date of SDS onset and final SDS severity was common across all field sites and years. Weighted linear regression revealed that SDS onset explained 60 to 83% of the variation in number of pods, number of seeds, and total seed weight in individual plants, and 94 to 97% of the variation in seed yield in quadrats. Soybean yield damage functions (slopes) indicated that for each day SDS onset was delayed, soybean yield increased by 30.5 to 31.3 kg/ha. This new quantitative information improves understanding of the impact of SDS on final disease intensity and soybean yield. Further experiments are needed to determine how this relationship is affected by site-specific factors.

Genome-wide transcriptional response of the causal soybean sudden death syndrome pathogen Fusarium virguliforme to a succinate dehydrogenase inhibitor fluopyram

BACKGROUND

Succinate dehydrogenase inhibitors (SDHIs) have been widely used to manage plant diseases caused by phytopathogenic fungi. Although attention to and use of SDHI fungicides has recently increased, molecular responses of fungal pathogens to SDHIs have often not been investigated. A SDHI fungicide, fluopyram, has been used as a soybean seed treatment and has displayed effective control of Fusarium virguliforme, one of the causal agents of soybean sudden death syndrome. To examine genome-wide gene expression of F. virguliforme to fluopyram, RNA-seq analysis was conducted on two field strains of F. virguliforme with differing SDHI fungicide sensitivity in the absence and presence of fluopyram.

RESULTS

The analysis indicated that several xenobiotic detoxification-related genes, such as those of deoxygenase, transferases and transporters, were highly induced by fluopyram. Among the genes, four ATP-binding cassette (ABC) transporters were characterized by the yeast expression system. The results revealed that expression of three ABCG transporters was associated with reduced sensitivity to multiple fungicides including fluopyram. In addition, heterologous expression of a major facilitator superfamily (MFS) transporter that was highly expressed in the fluopyram-insensitive F. virguliforme strain in the yeast system conferred decreased sensitivity to fluopyram.

CONCLUSION

This study demonstrated that xenobiotic detoxification-related genes were highly upregulated in response to fluopyram, and expression of ABC or MFS transporter genes was associated with reduced sensitivity to the SDHI fungicide. This is the first transcriptomic analysis of the fungal species response to fluopyram and the finding will help elucidate the molecular mechanisms of SDHI resistance. © 2021 Society of Chemical Industry.

Current recommendations and novel strategies for sustainable management of soybean sudden death syndrome

The increase in food production requires reduction of the damage caused by plant pathogens, minimizing the environmental impact of management practices. Soil-borne pathogens are among the most relevant pathogens that affect soybean crop yield. Soybean sudden death syndrome (SDS), caused by several distinct species of Fusarium, produces significant yield losses in the leading soybean-producing countries in North and South America. Current management strategies for SDS are scarce since there are no highly resistant cultivars and only a few fungicide seed treatments are available. Because of this, innovative approaches for SDS management need to be developed. Here, we summarize recently explored strategies based on plant nutrition, biological control, priming of plant defenses, host-induced gene silencing, and the development of new SDS-resistance cultivars using precision breeding techniques. Finally, sustainable management of SDS should also consider cultural control practices with minimal environmental impact. © 2021 Society of Chemical Industry.

Influence of Fusarium virguliforme Temporal Colonization of Corn, Tillage, and Residue Management on Soybean Sudden Death Syndrome and Soybean Yield

The asymptomatic host range of Fusarium virguliforme includes corn, a common crop rotated with soybean that we hypothesize may alter F. virguliforme population dynamics and disease management. A field-based approach explored the temporal dynamics of F. virguliforme colonization of corn and soybean roots under different tillage and residue managements. Experiments were conducted in Iowa, Indiana, Michigan, and Wisconsin, United States and Ontario, Canada from 2016 to 2018. Corn and soybean roots were sampled at consecutive timepoints between 1 and 16 weeks after planting. DNA was extracted from all roots and analyzed by real-time quantitative PCR for F. virguliforme quantification. Trials were rotated between corn and soybean, containing a two-by-two factorial of tillage (no-tilled or tilled) and corn residue (with or without) in several experimental designs. In 2016, low amounts (approximately 100 fg per 10 mg of root tissue) of F. virguliforme were detected in the inoculated Iowa, Indiana, and Michigan locations and noninoculated Wisconsin corn fields. However, in 2017, greater levels of F. virguliforme DNA were detected in Iowa, Indiana, and Michigan across sampling timepoints. Tillage practices showed inconsistent effects on F. virguliforme root colonization and sudden death syndrome (SDS) foliar symptoms among trials and locations. However, residue management did not alter root colonization of corn or soybean by F. virguliforme. Plots with corn residue had greater SDS foliar disease index in Iowa in 2016. However, this trend was not observed across the site-years, indicating that corn residue may occasionally increase SDS foliar symptoms depending on the disease level and soil and weather factors.

A machine learning interpretation of the contribution of foliar fungicides to soybean yield in the north-central United States

Foliar fungicide usage in soybeans in the north-central United States increased steadily over the past two decades. An agronomically-interpretable machine learning framework was used to understand the importance of foliar fungicides relative to other factors associated with realized soybean yields, as reported by growers surveyed from 2014 to 2016. A database of 2738 spatially referenced fields (of which 30% had been sprayed with foliar fungicides) was fit to a random forest model explaining soybean yield. Latitude (a proxy for unmeasured agronomic factors) and sowing date were the two most important factors associated with yield. Foliar fungicides ranked 7th out of 20 factors in terms of relative importance. Pairwise interactions between latitude, sowing date and foliar fungicide use indicated more yield benefit to using foliar fungicides in late-planted fields and in lower latitudes. There was a greater yield response to foliar fungicides in higher-yield environments, but less than a 100 kg/ha yield penalty for not using foliar fungicides in such environments. Except in a few production environments, yield gains due to foliar fungicides sufficiently offset the associated costs of the intervention when soybean prices are near-to-above average but do not negate the importance of disease scouting and fungicide resistance management.

Effect of In-Furrow Application of Fluopyram on Leaf Spot Diseases of Peanut

In peanut (Arachis hypogaea) production, in-furrow applications of the premix combination of the succinate-dehydrogenase-inhibitor (SDHI) fungicide and nematicide fluopyram and the insecticide imidacloprid are used primarily for management of nematode pests and for preventing feeding damage on foliage caused by tobacco thrips (Frankliniella fusca). Fluopyram is also active against many fungal pathogens. However, the effect of in-furrow applications of fluopyram on early leaf spot (Passalora arachidicola) or late leaf spot (Nothopassalora personata) has not been characterized. The purpose of this study was to determine the effects of in-furrow applications of fluopyram + imidacloprid or fluopyram alone on leaf spot epidemics. Field experiments were conducted in Tifton, GA in 2015, 2016, and 2018 to 2020. In all experiments, in-furrow applications of fluopyram + imidacloprid provided extended suppression of early leaf spot and late leaf spot epidemics compared with the nontreated control. In 2020, there was no difference between the effects of fluopyram + imidacloprid and fluopyram alone on leaf spot epidemics. Results indicated that fluopyram could complement early-season leaf spot management programs. Use of in-furrow applications of fluopyram should be considered as an SDHI fungicide application for resistance management purposes.

Biological Control of Chili Damping-Off Disease, Caused by Pythium myriotylum

Pythium myriotylum is a notorious soil-borne oomycete that causes post-emergence damping-off in chili pepper. Of various disease management strategies, utilization of plant growth promoting rhizobacteria (PGPR) in disease suppression and plant growth promotion is an interesting strategy. The present study was performed to isolate and characterize PGPR indigenous to the chili rhizosphere in Pakistan, and to test the potential to suppress the damping-off and plant growth promotion in chili. Out of a total of 28 antagonists, eight bacterial isolates (4a2, JHL-8, JHL-12, 1C2, RH-24, 1D, 5C, and RH-87) significantly suppressed the colony growth of P. myriotylum in a dual culture experiment. All the tested bacterial isolates were characterized for biochemical attributes, and 16S rRNA sequence based phylogenetic analysis identified these isolates as Flavobacterium spp., Bacillus megaterium, Pseudomonas putida, Bacillus cereus, and Pseudomonas libanensis. All the tested bacterial isolates showed positive test results for ammonia production, starch hydrolase (except 4a2), and hydrogen cyanide production (except 4a2 and 1D). All the tested antagonists produced indole-3-acetic acid (13.4–39.0 μg mL^–1), solubilized inorganic phosphate (75–103 μg mL^–1), and produced siderophores (17.1–23.7%) in vitro. All the tested bacterial isolates showed varying levels of susceptibility and resistance response against different antibiotics and all these bacterial isolates were found to be non-pathogenic to chili seeds and notably enhanced percentage seed germination, plumule, redical length, and vigor index over un-inoculated control. Additionally, under pathogen pressure, bacterization increased the defense related enzymes such as Peroxidase (PO), polyphenol oxidase (PPO), and phenylalanine ammonia-lyase (PAL) activates. Moreover, the treatment of chili seeds with these bacterial isolates significantly suppressed the damping-off caused by P. myriotylum and improved PGP traits compared to the control. In addition, a positive correlation was noticed between shoot, root length, and dry shoot and root weight, and there was a negative correlation between dry shoot, root weight, and seedling percentage mortality. These results showed that native PGPR possesses multiple traits beneficial to the chili plants and can be used to develop eco-friendly and effective seed treatment formulation as an alternative to synthetic chemical fungicides.

Economic Impact of Fluopyram-Amended Seed Treatments to Reduce Soybean Yield Loss Associated with Sudden Death Syndrome

Soybean (Glycine max) sudden death syndrome (SDS), caused by Fusarium virguliforme, is a key limitation in reaching soybean yield potential, stemming from incomplete disease management through cultural practices and partial host resistance. A fungicidal seed treatment was released in 2014 with the active ingredient fluopyram and was the first chemical management strategy to reduce soybean yield loss stemming from SDS. Although farm level studies have found fluopyram profitable, we were curious to discover whether fluopyram would be beneficial nationally if targeted to soybean fields at risk for SDS yield loss. To estimate economic benefits of fluopyram adoption in SDS at-risk acres, in the light of U.S. public research and outreach from a privately developed product, we applied an economic surplus approach, calculating ex ante net benefits from 2018 to 2032. Through this framework of logistic adoption of fluopyram for alleviation of SDS-associated yield losses, we projected a net benefit of $5.8 billion over 15 years, considering the costs of public seed treatment research and future extension communication. Although the sensitivity analysis indicates that overall net benefits from fluopyram adoption on SDS at-risk acres are highly dependent upon the market price of soybean, the incidence of SDS, the adoption path, and ceiling of this seed treatment, the net benefits still exceeded $407 million in the worst-case scenario.

Effects of ILeVO Seed Treatment on Heterodera glycines Reproduction and Soybean Yield in Small-Plot and Strip-Trial Experiments in Iowa

ILeVO (fluopyram) is a fungicide seed treatment for soybean sudden death syndrome (SDS) that also has nematicidal activity. ILeVO is sold with a base of insecticide Poncho (clothianidin), nematode-protectant VOTiVO (Bacillus firmus), and Acceleron fungicides (metalaxyl, fluxapyroxad, and pyraclostrobin). Yield and reproduction of the soybean cyst nematode (SCN) (Heterodera glycines) on soybean plants grown from seed treated with ILeVO plus the base were compared with those treated with only the base in 27 small-plot experiments and 12 strip-trial experiments across Iowa from 2015 to 2017. To increase the likelihood that yield results were related to effects on SCN, data were used only from 26 small-plot experiments and 12 strip trials in which symptoms of SDS were low or nonexistent. An SCN reproductive factor (RF) was calculated for each experimental unit by dividing the SCN population density at harvest by the population density at planting. ILeVO significantly reduced SCN RF by 50% in one strip-trial experiment and by 36 to 60% in four small-plot experiments but yields were not increased by ILeVO in any of those five experiments. Soybean yields were 2.8 to 3.7 bushels/acre (bu/ac) (188.3 to 248.8 kg/ha) greater with ILeVO in three small-plot experiments but SCN RF was not reduced in those experiments. Also, yield was 1.9 bu/ac (127.8 kg/ha) greater with ILeVO in one strip-trial experiment in 2016 but SCN samples were not collected at harvest from the study to assess the possible effects of ILeVO on SCN reproduction. When strip-trial data from 2015 and 2016 were combined, there was a small but significant 0.8 bu/ac (52.2 kg/ha) yield increase with ILeVO. Overall, the effects of ILeVO on SCN reproduction and soybean yield were variable in these field studies.

Characterization of native plant growth promoting rhizobacteria and their anti-oomycete potential against Phytophthora capsici affecting chilli pepper (Capsicum annum L.)

Phytophthora capsici is a notorious fungus which infects many crop plants at their early and late growth stages. In the present study, twelve P. capsici isolates were morphologically characterized, and based on pathogenicity assays; two highly virulent isolates causing post-emergence damping-off on locally cultivated chilli pepper were screened. Two P. capsici isolates, HydPak1 (MF322868) and HydPk2 (MF322869) were identified based on internal transcribed spacer (ITS) sequence homology. Plant growth promoting rhizobacteria (PGPR) play a significant role in disease suppression and plant growth promotion in various crops. Out of fifteen bacterial strains recovered from chilli rhizosphere, eight were found potential antagonists to P. capsici in vitro. Bacterial strains with strong antifungal potential were subjected to biochemical and molecular analysis. All tested bacterial strains, were positive for hydrogen cyanide (HCN), catalase production and indole-3-acetic acid (IAA) production (ranging from 6.10 to 56.23 µg ml^-1), while siderophore production varied between 12.5 and 33.5%. The 16S rRNA sequence analysis of tested bacterial strains showed 98-100% identity with Pseudomonas putida, P. libanensis, P. aeruginosa, Bacillus subtilis, B. megaterium, and B. cereus sequences available in the National Center for Biotechnology Information (NCBI) GenBank nucleotide database. All sequences of identified bacteria were submitted to GenBank for accessions numbers (MH796347-50, MH796355-56, MH801129 and MH801071). Greenhouse studies concluded that all tested bacterial strains significantly suppressed the P. capsici infections (52.3-63%) and enhanced the plant growth characters in chilli pepper. Efficacy of many of these tested rhizobacteria is being first time reported against P. capsici from Pakistan. Plant growth promoting rhizobacteria (PGPR) exhibiting multiple traits may be used in the development of new, eco-friendly, and effective bioformulations as an alternative to synthetic fungicides.

Trichoderma Isolates Inhibit Fusarium virguliforme Growth, Reduce Root Rot, and Induce Defense-Related Genes on Soybean Seedlings

Sudden death syndrome (SDS) caused by Fusarium virguliforme is among the most important diseases affecting soybean in the United States. The use of biological control agents (BCAs) such as Trichoderma spp. can be a valuable resource to suppress F. virguliforme populations. Therefore, this research focused on screening possible BCAs against F. virguliforme and evaluating mycoparasitism and the induction of systemic resistance as mechanisms underlying the antagonistic activity of selected BCAs against F. virguliforme. In total, 47 potential BCAs, including 41 Trichoderma isolates and 6 Mortierella isolates, were screened in a dual-plate assay. The most effective isolates belonged to the Trichoderma harzianum species and were able to inhibit F. virguliforme radial growth by up to 92%. Selected Trichoderma isolates were tested in the greenhouse and in a microplot study. They reduced root rot caused by F. virguliforme when the plants were coinoculated with the pathogen and the BCA. The tested BCA's ability to reduce F. virguliforme growth may be related to several mechanisms of action, including mycoparasitism and induction of defense-related genes in plants, as revealed by monitoring the expression of defense-related genes in soybean. Our results highlight the potential of native Trichoderma isolates to inhibit F. virguliforme growth and reduce SDS severity, providing the basis for future implementation of biological control in soybean production. More efforts are needed to implement the use of these approaches in production fields, and to deepen the current knowledge on the biology of these highly antagonistic isolates.

Relationship Between Sudden Death Syndrome caused by Fusarium virguliforme and Soybean Yield: A Meta-Analysis

In total, 52 uniform field experiments were conducted in Illinois, Indiana, Iowa, Michigan, South Dakota, and Wisconsin in the United States and Ontario, Canada from 2013 to 2017 comparing crop protection products against sudden death syndrome (SDS) of soybean. Data were analyzed using meta-analytic models to summarize the relationship between foliar disease index (FDX) and yield. For each study, correlation and regression analyses were performed separately to determine three effect sizes: Fisher's transformation of correlation coefficients (Z r ), intercept (β0), and slope (β1). Random- and mixed-effect meta-analyses were used to summarize the effect sizes. Study- and location-specific moderator variables FDX (low < 10% and high ≥ 10%), date of planting (early = prior to 7 May, conventional = 7 to 21 May, and late = after 21 May) cultivar (susceptible and partially resistant to SDS), study location, and growing season were used as fixed effects. The overall mean effect sizes of transformed correlation coefficient [Formula: see text] r was -0.41 and different from zero (P < 0.001), indicating that yield was negatively correlated with FDX. The [Formula: see text] r was affected by disease level (P < 0.01) and cultivar (P = 0.02), with a greater effect at higher disease levels and with susceptible cultivars. The mean [Formula: see text] 0 was 4,121 kg/ha and mean [Formula: see text] 1 was -21 kg/ha/% FDX and were different from zero (P < 0.01). Results from these data indicate that, for every unit of FDX increase, yield was decreased by 0.5%. Study locations and year affected the [Formula: see text] 0 , whereas none of the moderator variables significantly affected [Formula: see text] 1.

Fluopyram Suppresses Population Densities of Heterodera glycines in Field and Greenhouse Studies in Michigan

The soybean cyst nematode (SCN), Heterodera glycines Ichinohe, causes significant damage to soybean production annually. Fluopyram is a fungicide commonly used in soybean seed treatments intended to control soilborne fungal pathogens; however, recent studies have also suggested inhibitory effects on SCN. We examined the effects of a fluopyram seed treatment, ILeVO, on SCN reproduction, sudden death syndrome (SDS) development, and yield in a 3-year field study. Overall, fluopyram had a significant effect on yield (P = 0.046) and end-of-season SCN eggs and second-stage juveniles (Pf, P = 0.033) but no significant effect on SCN reproduction (Rf) or SDS disease index (P > 0.05). Post hoc tests indicated that fluopyram increased yield and suppressed SCN quantities. However, Rf was consistently greater than 1 whether or not the seed was treated with fluopyram, indicating that SCN populations were still increasing in the presence of fluopyram. A follow-up greenhouse study indicated that fluopyram reduced SCN relative to nontreated controls, as observed in the field, but only reduced SCN DNA within roots of a susceptible cultivar. These results indicate that fluopyram can suppress SCN quantities relative to nontreated seed but may not successfully reduce nematode populations without the use of additional management strategies.

The Efficacy of Ethaboxam as a Soybean Seed Treatment Toward Phytophthora, Phytopythium, and Pythium in Ohio

Phytophthora, Phytopythium, and Pythium species that cause early-season seed decay and pre-emergence and post-emergence damping off of soybean are most commonly managed with seed treatments. The phenylamide fungicides metalaxyl and mefenoxam, and ethaboxam are effective toward some but not all species. The primary objective of this study was to evaluate the efficacy of ethaboxam in fungicide mixtures and compare those with other fungicides as seed treatments to protect soybean against Pythium, Phytopythium, and Phytophthora species in both high-disease field environments and laboratory seed plate assays. The second objective was to evaluate these seed treatment mixtures on cultivars that have varying levels and combinations of resistance to these soilborne pathogens. Five of eight environments received adequate precipitation in the 14 days after planting for high levels of seedling disease development and treatment evaluations. Three environments had significantly greater stands, and three had significantly greater yield when ethaboxam was used in the seed treatment mixture compared with treatments containing metalaxyl or mefenoxam alone. Three fungicide formulations significantly reduced disease severity compared with nontreated in the seed plate assay for 17 species. However, the combination of ethaboxam plus metalaxyl in a mixture was more effective than either fungicide alone against some Pythium and Phytopythium species. Overall, our results indicate that the addition of ethaboxam to a fungicide seed treatment is effective in reducing seed rot caused by these pathogens commonly isolated from soybean in Ohio but that these effects can be masked when cultivars with resistance are planted.

Linkage Mapping for Foliar Necrosis of Soybean Sudden Death Syndrome

Sudden death syndrome (SDS) foliar symptoms consist of foliar chlorosis, foliar necrosis, leaf marginal curling, and premature defoliation, but resistance screening has been evaluated mostly based on the overall SDS foliar severity rather than on a specific foliar symptom. This study generated an F2 population derived from crossing the susceptible variety Sloan and the resistant germplasm line PI 243518, which exhibits resistance to both foliar chlorosis and necrosis. A total of 400 F2 lines were evaluated for foliar chlorosis, foliar necrosis, and overall SDS foliar symptoms, separately. Genotyping-by-sequencing was applied to obtain single nucleotide polymorphisms (SNPs) in the F2 population, and linkage mapping using 135 F2 lines with 969 high-quality SNPs identified a locus on chromosome 13 for foliar necrosis and SDS foliar symptoms. The locus partially overlaps with loci previously reported for SDS on chromosome 13, which is the third time the region from 15.98 to 21.00 Mbp has been reproduced independently and therefore qualifies this locus for a new nomenclature proposed as Rfv13-02. In summary, this study generated a new biparental population that enables not only the discovery of a locus for foliar necrosis and SDS foliar symptoms on chromosome 13 but also the potential for advanced exploration of SDS foliar resistance derived from the germplasm line PI 243518.

Metalloid and Metal Oxide Nanoparticles Suppress Sudden Death Syndrome of Soybean

Soybeans (Glycine max) (V3 stage) were sprayed once with nanoparticles (NPs) of AgO, B, CeO, CuO, MnO, MoO3, SiO, TiO, or ZnO and exposed to Fusarium virguliforme, the cause of sudden death syndrome. Up to 80% root rot was observed in greenhouse experiments. However, NP CuO, B, MoO3, or ZnO reduced the root rot severity by 17-25%. Infected roots and shoots had significant changes in B, Mg, P, S, Si, and Zn, but NP treatment restored levels to that of the healthy control. For example, the increased root Mg and Mn contents induced by disease were reversed by NP B and Mn amendments. In vitro assays found that the NPs did not inhibit the pathogen. This, along with the restoration of altered nutrient levels in the plant tissue, suggests that modulated plant nutrition increased disease defense. Treatment of seedlings with nanoscale micronutrients may be a new tool in promoting soybean health.

Biocontrol of Rhizoctonia solani (Kühn) and Fusarium solani (Marti) causing damping-off disease in tomato with Azotobacter chroococcum and Pseudomonas fluorescens

BACKGROUND AND OBJECTIVE

The Damping-off disease is one of the most reasons for low productively of tomato in the world, especially in Iraq. In the current study, two types of bacteria (Azotobacter chroococcum and Pseudomonas fluorescens) were used to evaluate their efficacy in inhibiting the growth of pathogenic fungi Rhizoctonia solani and Fusarium solani and protecting the seeds of tomato and increasing their germination percentage.

MATERIALS AND METHODS

Dual culture technique and Food poisoning technique were used to study the effect of bacteria on the growth of fungi understudy, and study the effect of bacterial filtrates on germination of tomato seeds.

RESULTS

A. chroococcum showed the strongest antagonistic activity followed by P. fluorescens with the percentage of inhibition ranging between 72.9-77.1 and 69.5-70.3% for R. solani and F. solani respectively after 7 days of incubation. The effect of A. chroococcum and P. fluorescens filtrates were increased and also increased the inhibition of growth of fungi understudy, A. chroococcum filtrate also showed the strongest inhibitory effect followed by P. fluorescens with the percentage of inhibition ranging between 86.0-87.0 and 83.0-83.5% for R. solani and F. solani respectively at 20% concentration of filtrate. The percentage of seeds germination reached 90% in the treatment of A. chroococcum filtrate and 80% in the treatment of P. fluorescens filtrate.

CONCLUSION

It can be concluded that the filtrates of A. chroococcum and P. fluorescens have antifungal properties against R. solani and F. solani and provided a high protection and increasing tomato seeds germination percentage.

Predicting Soybean Yield and Sudden Death Syndrome Development Using At-Planting Risk Factors

In the United States, sudden death syndrome (SDS) of soybean is caused by the fungal pathogen Fusarium virguliforme and is responsible for important yield losses each year. Understanding the risk of SDS development and subsequent yield loss could provide growers with valuable information for management of this challenging disease. Current management strategies for F. virguliforme use partially resistant cultivars, fungicide seed treatments, and extended crop rotations with diverse crops. The aim of this study was to develop models to predict SDS severity and soybean yield loss using at-planting risk factors to integrate with current SDS management strategies. In 2014 and 2015, field studies were conducted in adjacent fields in Decatur, MI, which were intensively monitored for F. virguliforme and nematode quantities at-planting, plant health throughout the growing season, end-of-season SDS severity, and yield using an unbiased grid sampling scheme. In both years, F. virguliforme and soybean cyst nematode (SCN) quantities were unevenly distributed throughout the field. The distribution of F. virguliforme at-planting had a significant correlation with end-of-season SDS severity in 2015, and a significant correlation to yield in 2014 (P < 0.05). SCN distributions at-planting were significantly correlated with end-of-season SDS severity and yield in 2015 (P < 0.05). Prediction models developed through multiple linear regression showed that F. virguliforme abundance (P < 0.001), SCN egg quantity (P < 0.001), and year (P < 0.01) explained the most variation in end-of-season SDS (R2 = 0.32), whereas end-of-season SDS (P < 0.001) and end-of-season root dry weight (P < 0.001) explained the most variation in soybean yield (R2 = 0.53). Further, multivariate analyses support a synergistic relationship between F. virguliforme and SCN, enhancing the severity of foliar SDS. These models indicate that it is possible to predict patches of SDS severity using at-planting risk factors. Verifying these models and incorporating additional data types may help improve SDS management and forecast soybean markets in response to SDS threats.

Effect of Seed Treatment and Foliar Crop Protection Products on Sudden Death Syndrome and Yield of Soybean

Sudden death syndrome (SDS), caused by Fusarium virguliforme, is an important soilborne disease of soybean. Risk of SDS increases when cool and wet conditions occur soon after planting. Recently, multiple seed treatment and foliar products have been registered and advertised for management of SDS but not all have been tested side by side in the same field experiment at multiple field locations. In 2015 and 2016, seed treatment fungicides fluopyram and thiabendazole; seed treatment biochemical pesticides citric acid and saponins extract of Chenopodium quinoa; foliar fungicides fluoxastrobin + flutriafol; and an herbicide, lactofen, were evaluated in Illinois, Indiana, Iowa, Michigan, South Dakota, Wisconsin, and Ontario for SDS management. Treatments were tested on SDS-resistant and -susceptible cultivars at each location. Overall, fluopyram provided the highest level of control of root rot and foliar symptoms of SDS among all the treatments. Foliar application of lactofen reduced foliar symptoms in some cases but produced the lowest yield. In 2015, fluopyram reduced the foliar disease index (FDX) by over 50% in both resistant and susceptible cultivars and provided 8.9% yield benefit in susceptible cultivars and 3.5% yield benefit in resistant cultivars compared with the base seed treatment (control). In 2016, fluopyram reduced FDX in both cultivars by over 40% compared with the base seed treatment. For yield in 2016, treatment effect was not significant in the susceptible cultivar while, in the resistant cultivar, fluopyram provided 3.5% greater yield than the base seed treatment. In this study, planting resistant cultivars and using fluopyram seed treatment were the most effective tools for SDS management. However, plant resistance provided an overall better yield-advantage than using fluopyram seed treatment alone. Effective seed treatments can be an economically viable consideration to complement resistant cultivars for managing SDS.

Soybean Sudden Death Syndrome Causal Agent Fusarium brasiliense Present in Michigan

Sudden death syndrome (SDS), caused by members of Fusarium solani species complex (FSSC) clade 2, is a major and economically important disease in soybean worldwide. The primary causal agent of SDS isolated to date in North America has been F. virguliforme. In 2014 and 2016, SDS symptoms were found in two soybean fields located on the same farm in Michigan. Seventy Fusarium strains were isolated from roots of the SDS-symptomatic soybeans in two fields. Phylogenetic analysis of partial sequences of elongation factor-1α, the nuclear ribosomal DNA intergenic spacer region, and the RNA polymerase II beta subunit revealed that the primary FSSC species isolated was F. brasiliense (58 and 36% in each field) and the remaining Fusarium strains were identified as F. cuneirostrum, F. phaseoli, an undescribed Fusarium sp. from FSSC clade 2, and strains in FSSC clade 5 and FSSC clade 11. Molecular identification was supported with morphological analysis and a pathogenicity assay. The soybean seedling pathogenicity assay indicated that F. brasiliense was capable of causing typical foliar SDS symptoms. Both root rot and foliar disease severity were variable by strain, just as they are in F. virguliforme. Both FSSC 5 and FSSC 11 strains were also capable of causing root rot, but SDS foliar symptoms were not detected. To our knowledge, this is the first report of F. brasiliense causing SDS in soybean in the United States and the first report of F. cuneirostrum, F. phaseoli, an as-yet-unnamed Fusarium sp., and strains in FSSC clade 5 and FSSC clade 11 associated with or causing root rot of soybean in Michigan.

The Effects of ILeVO and VOTiVO on Root Penetration and Behavior of the Soybean Cyst Nematode, Heterodera glycines

The objective of this study was to determine the effects of ILeVO (fluopyram) and VOTiVO (Bacillus firmus I-1582) seed treatments on Heterodera glycines second-stage juvenile (J2) root penetration and behavior. In a growth chamber experiment, roots of soybeans grown from treated or untreated seeds were inoculated with H. glycines J2s at soil depths of 2.5, 5, or 7.5 cm. ILeVO significantly reduced H. glycines root penetration compared with the untreated control, but only when J2s were inoculated at a soil depth of 2.5 cm, which was near the seed. Changes in nematode behavior were assessed by collecting 60-s videos of J2s after 2 h of exposure to exudates from treated seeds or radicles from treated seeds or from soil leachates in which treated seeds were planted. X- and y-coordinates of each of the 13 reference points were recorded every hour for 24 h. A custom program analyzed and transformed the coordinates into nematode motion parameters (speed and total change in curvature). ILeVO, but not VOTiVO, seed exudates significantly reduced J2 speed relative to the untreated control. Soil leachates from ILeVO or VOTiVO treatments had no consistent effect on H. glycines speed or total change in curvature compared with the untreated control. In another experiment, treated or untreated seeds were incubated in wells of 6-well tissue culture plates containing 11.5% Pluronic gel. Seeds were removed after 2 h, and approximately 50 J2s then were pipetted into each well. The plates were scanned every 60 min for 24 h, and the number of J2s in each well that moved a minimum distance of ≥300 µm was determined using another custom software program. ILeVO, but not VOTiVO, significantly reduced the movement of J2 populations relative to control wells in which no seeds were added. And wells that had seeds, treated or not, yielded significantly less J2 movement compared with the no-seed control. The results of these experiments indicate that ILeVO reduces activity on H. glycines J2s but may not affect nematodes beyond a limited area surrounding the treated seed.

Fluopyram Sensitivity and Functional Characterization of SdhB in the Fusarium solani Species Complex Causing Soybean Sudden Death Syndrome

The succinate dehydrogenase inhibitor (SDHI) fungicide, fluopyram, is used as a soybean seed treatment to manage Fusarium virguliforme, the casual agent of sudden death syndrome (SDS). More recently, other species within clade 2 of the Fusarium solani species, F. tucumaniae in South America and F. brasiliense in America and Africa, have been recognized as additional agents capable of causing SDS. To determine if fluopyram could be used for management of SDS caused by these species, in vitro sensitivity tests of the three Fusarium species to fluopyram were conducted. The mean EC50 values of F. brasiliense and F. virguliforme strains to fluopyram were 1.96 and 2.21 μg ml-1, respectively, but interestingly F. tucumaniae strains were highly sensitive (mean EC50 = 0.25 μg ml-1) to fluopyram compared to strains of the other two species. A sequence analysis of Sdh genes of Fusarium strains revealed that the F. tucumaniae strains contain an arginine at codon 277 in the SdhB gene instead of a glycine as in other Fusarium species. Replacement of glycine to arginine in SdhB-277 in a F. virguliforme wild-type strain Mont-1 through genetic transformation resulted in increased sensitivity to two SDHI fungicides, fluopyram and boscalid. Similar to a F. tucumaniae strain, the Mont-1 (SdhBG277R) mutant caused less SDS and root rot disease than Mont-1 on soybean seedlings with the fluopyram seed treatment. Our study suggests the amino acid difference in the SdhB in F. tucumaniae results in fluopyram being efficacious if used as a seed treatment for management of F. tucumaniae, which is the most abundant SDS causing species in South America. The establishment of baseline sensitivity of Fusarium species to fluopyram will contribute to effective strategies for managing Fusarium diseases in soybean and other pathosystems such as dry bean.

Primary metabolism changes triggered in soybean leaves by Fusarium tucumaniae infection

Sudden death syndrome (SDS) of soybean can be caused by at least four distinct Fusarium species, with F. tucumaniae being the main causal agent in Argentina. The fungus is a soil-borne pathogen that is largely confined to the roots, but damage also reaches aerial part of the plant and interveinal chlorosis and necrosis, followed by premature defoliation can be observed. In this study, two genetically diverse soybean cultivars, one susceptible (NA 4613) and one partially resistant (DM 4670) to SDS infection, were inoculated with F. tucumaniae or kept uninoculated. Leaf samples at 7, 10, 14 and 25 days post-inoculation (dpi) were chosen for analysis. With the aim of detecting early markers that could potentially discriminate the cultivar response to SDS, gas chromatography-mass spectrometry (GC-MS) analyses and biochemical studies were performed. Metabolic analyses show higher levels of several amino acids in the inoculated than in the uninoculated susceptible cultivar starting at 10 dpi. Biochemical studies indicate that pigment contents and Rubisco level were reduced while class III peroxidase activity was increased in the inoculated susceptible plant at 10 dpi. Taken together, our results indicate that the pathogen induced an accumulation of amino acids, a decrease of the photosynthetic activity, and an increase of plant-specific peroxidase activity in the susceptible cultivar before differences of visible foliar symptoms between genotypes could be observed, thus suggesting that metabolic and biochemical approaches may contribute to a rapid characterization of the cultivar response to SDS.

Cropping System Diversification Reduces Severity and Incidence of Soybean Sudden Death Syndrome Caused by Fusarium virguliforme

Current management of sudden death syndrome (SDS) of soybean, caused by Fusarium virguliforme, focuses on planting resistant varieties and improving soil drainage; however, these measures are not completely effective. A 6-year study evaluated the effects of cropping system diversification on SDS and soybean yield. SDS, root health, yield, and F. virguliforme density in soil were assessed in a naturally infested field trial comparing a 2-year cropping system consisting of a corn-soybean rotation and synthetic fertilizer applications with 3- and 4-year cropping systems consisting of corn-soybean-oat + red clover and corn-soybean-oat +alfalfa-alfalfa rotations, respectively, with both manure and low synthetic fertilizer rates. In 5 of 6 years, SDS incidence and severity were lower and yield higher in the 3- and 4-year systems than in the 2-year system. SDS severity and incidence were up to 17-fold lower in the diversified systems than in the 2-year system. Incidence and severity of SDS explained 45 to 87% of the variation in yield. Plants in the 2-year system generally showed more severe root rot and lower plant weights than plants in the diversified systems. F. virguliforme density in soil was up to fivefold greater in the 2-year system compared with the 4-year system. The processes responsible for the suppression of SDS and yield protection in the diversified cropping systems still need to be determined.

Benefits and Profitability of Fluopyram-Amended Seed Treatments for Suppressing Sudden Death Syndrome and Protecting Soybean Yield: A Meta-Analysis

A meta-analytic approach was used to summarize data on the effects of fluopyram-amended seed treatment on sudden death syndrome (SDS) and yield of soybean (Glycine max L.) in over 200 field trials conducted in 12 U.S. states and Ontario, Canada from 2013 to 2015. In those trials, two treatments-the commercial base (CB), and CB plus fluopyram (CBF)-were tested, and all disease and yield data were combined to conduct a random-effects and mixed-effects meta-analysis (test of moderators) to estimate percent control and yield response relative to CB. Overall, a 35% reduction in foliar disease and 295 kg/ha (7.6%) increase in yield were estimated for CBF relative to CB. Sowing date and geographic region affected both estimates. The variation in yield response was explained partially by disease severity (19%), geographic region (8%), and sowing date (10%) but not by the resistance level of the cultivar. The probability of not offsetting the cost of fluopyram was estimated on a range of grain prices and treatment cost combinations. There was a high probability (>80%) of yield gains when disease level was high in any cost-price combinations tested but very low when the foliar symptoms of the disease were absent.

Integration of sudden death syndrome resistance loci in the soybean genome

KEY MESSAGE

Complexity and inconsistencies in resistance mapping publications of soybean sudden death syndrome (SDS) result in interpretation difficulty. This review integrates SDS mapping literature and proposes a new nomenclature system for reproducible SDS resistance loci. Soybean resistance to sudden death syndrome (SDS) is composed of foliar resistance to phytotoxins and root resistance to pathogen invasion. There are more than 80 quantitative trait loci (QTL) and dozens of single nucleotide polymorphisms (SNPs) associated with soybean resistance to SDS. The validity of these QTL and SNPs is questionable because of the complexity in phenotyping methodologies, the disease synergism between SDS and soybean cyst nematode (SCN), the variability from the interactions between soybean genotypes and environments, and the inconsistencies in the QTL nomenclature. This review organizes SDS mapping results and proposes the Rfv (resistance to Fusarium virguliforme) nomenclature based on supporting criteria described in the text. Among ten reproducible loci receiving our Rfv nomenclature, Rfv18-01 is mostly supported by field studies and it co-localizes to the SCN resistance locus rhg1. The possibility that Rfv18-01 is a pleiotropic resistance locus and the concern about Rfv18-01 being confounded with Rhg1 is discussed. On the other hand, Rfv06-01, Rfv06-02, Rfv09-01, Rfv13-01, and Rfv16-01 were identified both by screening soybean leaves against phytotoxic culture filtrates and by evaluating SDS severity in fields. Future phenotyping using leaf- and root-specific resistance screening methodologies may improve the precision of SDS resistance, and advanced genetic studies may further clarify the interactions among soybean genotypes, F. virguliforme, SCN, and environments. The review provides a summary of the SDS resistance literature and proposes a framework for communicating SDS resistance loci for future research considering molecular interactions and genetic breeding for soybean SDS resistance.

Statistical Power in Plant Pathology Research

In null hypothesis testing, failure to reject a null hypothesis may have two potential interpretations. One interpretation is that the treatments being evaluated do not have a significant effect, and a correct conclusion was reached in the analysis. Alternatively, a treatment effect may have existed but the conclusion of the study was that there was none. This is termed a Type II error, which is most likely to occur when studies lack sufficient statistical power to detect a treatment effect. In basic terms, the power of a study is the ability to identify a true effect through a statistical test. The power of a statistical test is 1 - (the probability of Type II errors), and depends on the size of treatment effect (termed the effect size), variance, sample size, and significance criterion (the probability of a Type I error, α). Low statistical power is prevalent in scientific literature in general, including plant pathology. However, power is rarely reported, creating uncertainty in the interpretation of nonsignificant results and potentially underestimating small, yet biologically significant relationships. The appropriate level of power for a study depends on the impact of Type I versus Type II errors and no single level of power is acceptable for all purposes. Nonetheless, by convention 0.8 is often considered an acceptable threshold and studies with power less than 0.5 generally should not be conducted if the results are to be conclusive. The emphasis on power analysis should be in the planning stages of an experiment. Commonly employed strategies to increase power include increasing sample sizes, selecting a less stringent threshold probability for Type I errors, increasing the hypothesized or detectable effect size, including as few treatment groups as possible, reducing measurement variability, and including relevant covariates in analyses. Power analysis will lead to more efficient use of resources and more precisely structured hypotheses, and may even indicate some studies should not be undertaken. However, the conclusions of adequately powered studies are less prone to erroneous conclusions and inflated estimates of treatment effectiveness, especially when effect sizes are small.

Assessing the Effects of ILeVO and VOTiVO Seed Treatments on Reproduction, Hatching, Motility, and Root Penetration of the Soybean Cyst Nematode, Heterodera glycines

Successful management of the soybean cyst nematode Heterodera glycines is limited by increased virulence of nematode populations on resistant soybean cultivars and persistence of the nematode in the soil in the absence of hosts. Seed treatments are now available for H. glycines management. However, it is unclear how these treatments affect specific life stages of the nematode. The objectives of this study were to assess the effects of ILeVO (with active ingredient fluopyram) and VOTiVO (with active ingredient Bacillus firmus I-1582) seed treatments on H. glycines reproduction and important processes in the nematode life cycle, such as second-stage juvenile (J2) hatching, motility, and root penetration. The effects of seed treated with formulated (ILeVO and VOTiVO) and nonformulated active ingredient (fluopyram and B. firmus I-1582) on H. glycines reproduction were conducted in a greenhouse. Nematode reproduction on plants grown from seed treated with ILeVO or technical fluopyram (active ingredient only) was reduced by 35 to 97% relative to the nontreated control, suggesting that the fluopyram active ingredient was affecting H. glycines directly and was not an inert ingredient in the seed treatment formulation. Hatching, motility, and root penetration experiments also were conducted using only the formulated seed treatments. Exudates collected from ILeVO-treated seed reduced J2 hatching and motility by more than 95% in laboratory assays. Exudates from radicles grown from ILeVO-treated seed reduced hatching in vitro by 48% in one run but had no significant effect in the second run compared with the nontreated control exudates. There also were no consistent effects of radicle exudates, regardless of treatment, on hatching and motility of the J2. ILeVO reduced root penetration of H. glycines J2 at different inoculation densities in a growth chamber experiment. VOTiVO did not affect any of the processes or life stages of the nematode studied. The results of this study indicate that the use of nematode-protectant seed treatments may be useful in controlling H. glycines; however, additional investigations into the precise effects of ILeVO and VOTiVO on H. glycines life processes and in different parts of the soil profile are necessary.

Effect of Soybean Cyst Nematode Resistance Source and Seed Treatment on Population Densities of Heterodera glycines, Sudden Death Syndrome, and Yield of Soybean

A three-year study was conducted in Illinois, Indiana, Iowa, Michigan, and Ontario, Canada, from 2013 through 2015 to determine the effect of soybean (Glycine max) cultivars' source of soybean cyst nematode (SCN; Heterodera glycines) resistance on SCN population densities, sudden death syndrome (SDS; caused by Fusarium virguliforme), and yield of soybean. Five cultivars were evaluated with and without fluopyram seed treatment at each location. Cultivars with no SCN resistance had greater SDS severity, greater postharvest SCN egg counts (Pf), and lower yield than cultivars with plant introduction (PI) 548402 (Peking) and PI 88788-type of SCN resistance (P < 0.05). Cultivars with Peking-type resistance had lower Pf than those with PI 888788-type and no SCN resistance. In two locations with HG type 1.2-, cultivars with Peking-type resistance had greater foliar disease index (FDX) than cultivars with PI 88788-type. Fluopyram seed treatment reduced SDS and improved yield compared with a base seed treatment but did not affect SCN reproduction and Pf (P > 0.05). FDX and Pf were positively correlated in all three years (P < 0.01). Our results indicate that SDS severity may be influenced by SCN population density and HG type, which are important to consider when selecting cultivars for SCN management.

'MN1606SP' by 'Spencer' filial soybean population reveals novel quantitative trait loci and interactions among loci conditioning SDS resistance

KEY MESSAGE

Four novel QTL and interactions among QTL were identified in this research, using as a parent line the most SDS-resistant genotype within soybean cultivars of the US early maturity groups. Soybean sudden death syndrome (SDS) reduces soybean yield in most of the growing areas of the world. The causal agent of SDS, soilborne fungus Fusarium virguliforme (Fv), releases phytotoxins taken up by the plant to produce chlorosis and necrosis in the leaves. Planting resistant cultivars is the most successful management practice to control the disease. The objective of this study was to identify quantitative trait loci (QTL) associated with the resistance response of MN1606SP to SDS. A mapping population of F 2:3 lines created by crossing the highly resistant cultivar 'MN1606SP' and the susceptible cultivar 'Spencer' was phenotyped in the greenhouse at three different planting times, each with three replications. Plants were artificially inoculated using SDS infested sorghum homogeneously mixed with the soil. Data were collected on three disease criteria, foliar disease incidence (DI), foliar leaf scorch disease severity (DS), and root rot severity. Disease index (DX) was calculated as DI × DS. Ten QTL were identified for the different disease assessment criteria, three for DI, four for DX, and three for root rot severity. Three QTL identified for root rot severity and one QTL for disease incidence are considered novel, since no previous reports related to these QTL are available. Among QTL, two interactions were detected between four different QTL. The interactions suggest that resistance to SDS is not only dependent on additive gene effects. The novel QTL and the interactions observed in this study will be useful to soybean breeders for improvement of SDS resistance in soybean germplasm.

FvSTR1, a striatin orthologue in Fusarium virguliforme, is required for asexual development and virulence

The soil-borne fungus Fusarium virguliforme causes sudden death syndrome (SDS), one of the most devastating diseases of soybean in North and South America. Despite the importance of SDS, a clear understanding of the fungal pathogenicity factors that affect the development of this disease is still lacking. We have identified FvSTR1, a F. virguliforme gene, which encodes a protein similar to a family of striatin proteins previously reported to regulate signalling pathways, cell differentiation, conidiation, sexual development, and virulence in filamentous fungi. Striatins are multi-domain proteins that serve as scaffolding units in the striatin-interacting phosphatase and kinase (STRIPAK) complex in fungi and animals. To address the function of a striatin homologue in F. virguliforme, FvSTR1 was disrupted and functionally characterized using a gene knock out strategy. The resulting Fvstr1 mutants were largely impaired in conidiation and pigmentation, and displayed defective conidia and conidiophore morphology compared to the wild-type and ectopic transformants. Greenhouse virulence assays revealed that the disruption of FvSTR1 resulted in complete loss of virulence in F. virguliforme. Microtome studies using fluorescence microscopy showed that the Fvstr1 mutants were defective in their ability to colonize the vascular system. The Fvstr1 mutants also showed a reduced transcript level of genes involved in asexual reproduction and in the production of secondary metabolites. These results suggest that FvSTR1 has a critical role in asexual development and virulence in F. virguliforme.

Baseline Sensitivity of Fusarium virguliforme to Fluopyram Fungicide

Fluopyram, a succinate dehydrogenase inhibitor (SDHI) fungicide, was recently registered for use as a soybean seed treatment for management of sudden death syndrome (SDS) caused by Fusarium virguliforme. Although registered and now used commercially, in vitro baseline fungicide sensitivity of F. virguliforme to fluopyram has not yet been established. In this study, the baseline sensitivity of F. virguliforme to fluopyram was determined using in vitro growth of mycelium and germination of conidia assays with two collections of F. virguliforme isolates. A total of 130 and 75 F. virguliforme isolates were tested using the mycelial growth and conidia germination assays, respectively, including a core set of isolates that were tested with both assays. In the mycelial growth inhibition assay, 113 out of 130 isolates (86.9%) were inhibited 50% by effective concentrations (EC₅₀) less than 5 µg/ml with a mean EC₅₀ of 3.35 µg/ml. For the conidia germination assay, 73 out of 75 isolates (97%) were determined to have an estimated EC₅₀ of less than 5 µg/ml with a mean EC₅₀ value of 2.28 µg/ml. In a subset of 20 common isolates that were phenotyped with both assays, conidia germination of F. virguliforme was determined to be more sensitive to fluopyram (mean EC₅₀ = 2.28 µg/ml) than mycelial growth (mean EC₅₀ = 3.35 µg/ml). Hormetic effects were observed in the mycelial growth inhibition assay as 22% of the isolates demonstrated more growth on medium amended with the lowest fluopyram concentration (1 µg/ml), as compared with the nonfluopyram amended control. It was not possible to determine EC₅₀ values for nine out of 185 isolates (4.8%), as those isolates were not inhibited by 50% even at the highest fluopyram concentrations of 100 µg/ml for mycelial growth and 20 µg/ml for conidia germination inhibition assays. On the whole, the F. virguliforme population appears to be sensitive to fluopyram, and this study enables future monitoring of fungicide sensitivity.