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.