X. B. Yang

Effects of Herbicides on Fusarium solani f. sp. glycines and Development of Sudden Death Syndrome in Glyphosate-Tolerant Soybean

ABSTRACT Sudden death syndrome of soybean, caused by Fusarium solani f. sp. glycines, is a disease of increasing economic importance in the United States. Although the ecology of sudden death syndrome has been extensively studied in relation to crop management practices such as tillage, irrigation, and cultivar selection, there is no information on the effects of herbicides on this disease. Three herbicides (lactofen, glyphosate, and imazethapyr) commonly used in soybean were evaluated for their effects on the phenology of F. solani f. sp. glycines and the development of sudden death syndrome in four soybean cultivars varying in resistance to the disease and in tolerance to glyphosate. Conidial germination, mycelial growth, and sporulation in vitro were reduced by glyphosate and lactofen. In growth-chamber and greenhouse experiments, there was a significant increase in disease severity and frequency of isolation of F. solani f. sp. glycines from roots of all cultivars after application of imazethapyr or glyphosate compared with the control treatment (no herbicide applied). Conversely, disease severity and isolation frequency of F. solani f. sp. glycines decreased after application of lactofen. Across all herbicide treatments, severity of sudden death syndrome and isolation frequency were lower in disease-resistant than in susceptible cultivars. Results suggest that glyphosate-tolerant and -nontolerant cultivars respond similarly to infection by F. solani f. sp. glycines after herbicide application.

One fungus, one name: defining the genus Fusarium in a scientifically robust way that preserves longstanding use.

In this letter, we advocate recognizing the genus Fusarium as the sole name for a group that includes virtually all Fusarium species of importance in plant pathology, mycotoxicology, medicine, and basic research. This phylogenetically guided circumscription will free scientists from any obligation to use other genus names, including teleomorphs, for species nested within this clade, and preserve the application of the name Fusarium in the way it has been used for almost a century. Due to recent changes in the International Code of Nomenclature for algae, fungi, and plants, this is an urgent matter that requires community attention. The alternative is to break the longstanding concept of Fusarium into nine or more genera, and remove important taxa such as those in the F. solani species complex from the genus, a move we believe is unnecessary. Here we present taxonomic and nomenclatural proposals that will preserve established research connections and facilitate communication within and between research communities, and at the same time support strong scientific principles and good taxonomic practice.

Assessment of the potential year-round establishment of soybean rust throughout the world

Soybean rust (Phakopsora pachyrhizi Sydow) has been known to occur in eastern Asia and Aus-tralia for decades. In recent years, the disease entered Africa and South America and has spread rapidly in these continents. It has become a concern to the U.S. soybean industry. To assess the threat of soybean rust, we used a modeling approach to determine the potential geographical zones where the fungus might overwinter and serve as source areas for seasonal epidemics. Long-term meteorological averages were used to assess the temperature stresses by using CLIMEX, and the dry stress with an algorithm developed in this study. Integration of stresses was used to predict the likelihood of survival of the rust in a defined location. Our results suggest that the new soybean rust invasions in Africa and South America occurred in the areas where the fungus might persist year-round. The main regions where rust has not been reported but might overwinter are located in the western hemisphere, including northern South America, Central America, the Caribbean, Mexico, southern Texas, and Florida. Southeastern China and neighboring areas are suggested as the primary regions where initial spores for soybean rust epidemics in central China are produced. If the disease is to establish in the United States, it is likely to be restricted to parts of Florida and southern Texas during the winter in the frost-free areas or areas where the fungus could overcome short periods of below-freezing temperatures. Occurrence of rust epidemics within the U.S. soybean belt would depend on south-to-north dispersal of uredospores.

Soybean cultivar responses to Sclerotinia sclerotiorum in field and controlled environment studies

The responses of 12 soybean cultivars to Sclerotinia sclerotiorum were evaluated under field and controlled environment conditions. The 12 cultivars were planted in fields naturally or artificially infested with S. sclerotiorum and evaluated for disease incidence. In controlled environment studies, the cultivars were compared with respect to lesion size on detached leaves and stems inoculated with mycelial disks, incidence of stem rot following mycelial inoculation of foliage, lesion lengths on stems discolored by oxalic acid, and levels of soluble pigment(s) in stems. Significant differences (P ≤ 0.05) in disease incidence, lesion sizes, and levels of soluble pigment(s) were detected among the 12 cultivars in all evaluations, but ranking of cultivars varied among methods and experiments within methods. Corsoy 79 and S19-90 were consistently most resistant in all methods; whereas Kenwood, A2242, Bell, and Williams 82 were least resistant. Pearson correlation coefficients (R) for disease incidence between location years in field experiments (FE) ranged from 0.86 to 0.95. R values between FE and controlled environment experiments (CEE) ranged from 0.01 to 0.62 for detached leaf assays (DLA), -0.20 to 0.47 for lesion lengths on stems inoculated with mycelial disks (LLM), 0.38 to 0.45 for incidence of stem rot from mycelial inoculation of foliage (MIF), 0.08 to 0.66 for lesion lengths on stems discolored by oxalic acid (LLO), and -0.55 to -0.37 for levels of soluble stem pigment(s) (SSP). Absolute values of R between FE and CEE were 0.40 or greater in 40, 17, 33, 29, and 83% of all correlation analyses for DLA, LLM, MIF, LLO, and SSP, respectively. Spearmans coefficients of rank correlation (r s) between FE and CEE based on average performance were 0.55, -0.20, 0.40, 0.42, and -0.44 for DLA, LLM, MIF, LLO, and SSP, respectively. Disease development was slow when foliage was inoculated with a mycelial suspension. Based on experiment (E) by cultivar (C) interaction and rs values between replicate experiments within each method, determination of soluble stem pigment levels (0.86 ≤ rs ≤ 0.97; P < 0.001 for 100% of six analyses; P = 0.98 for E × C interaction) and measurement of lesion lengths on stems discolored by oxalic acid (0.27 ≤ rs ≤ 0.85; rs ≥ 0.58 and P ≤ 0.05 for 68% of 28 analyses; P = 0.07 for E × C interaction) were the most repeatable methods. The results from this study suggest that determination of levels of soluble pigments in stems, measurement of lesion lengths on stems discolored by oxalic acid, and the detached leaf assay may be better than mycelial inoculation of stems or foliage in evaluating soybean cultivars for field resistance to S. sclerotiorum.

Predicting severity of Asian soybean rust epidemics with empirical rainfall models

ABSTRACT Although Asian soybean rust occurs in a broad range of environmental conditions, the most explosive and severe epidemics have been reported in seasons with warm temperature and abundant moisture. Associations between weather and epidemics have been reported previously, but attempts to identify the major factors and model these relationships with field data have been limited to specific locations. Using data from 2002-03 to 2004-05 from 34 field experiments at 21 locations in Brazil that represented all major soybean production areas, we attempted to identify weather variables using a 1-month time window following disease detection to develop simple models to predict final disease severity. Four linear models were identified, and these models explained 85 to 93% of variation in disease severity. Temperature variables had lower correlation with disease severity compared with rainfall, and had minimal predictive value for final disease severity. A curvilinear relationship was observed between 1 month of accumulated rainfall and final disease severity, and a quadratic response model using this variable had the lowest prediction error. Linear response models using only rainfall or number of rainy days in the 1-month period tended to overestimate disease for severity <30%. The study highlights the importance of rainfall in influencing soybean rust epidemics in Brazil, as well as its potential use to provide quantitative risk assessments and seasonal forecasts for soybean rust, especially for regions where temperature is not a limiting factor for disease development.

Regional Assessment of Soybean Brown Stem Rot, Phytophthora sojae, and Heterodera glycines Using Area-Frame Sampling: Prevalence and Effects of Tillage.

ABSTRACT The prevalence of brown stem rot (caused by Phialophora gregata), Heterodera glycines, and Phytophthora sojae in the north central United States was investigated during the fall of 1995 and 1996. Soybean fields were randomly selected using an area-frame sampling design in collaboration with the National Agricultural Statistics Service. Soil and soybean stem samples, along with tillage information, were collected from 1,462 fields in Illinois, Iowa, Minnesota, Missouri, and Ohio. An additional 275 soil samples collected from Indiana were assessed for H. glycines. For each field, the incidence and prevalence of brown stem rot was assessed in 20 soybean stem pieces. The prevalence and recovery (expressed as the percentage of leaf disks colonized) of P. sojae and the prevalence and population densities of H. glycines were determined from the soil samples. The prevalence of brown stem rot ranged from 28% in Missouri to 73% in Illinois; 68 and 72% of the fields in Minnesota and Iowa, respectively, showed symptomatic samples. The incidence of brown stem rot was greater in conservation-till than in conventional-till fields in all states except Minnesota, which had few no-till fields. P. sojae was detected in two-thirds of the soybean fields in Ohio and Minnesota, whereas 63, 55, and 41% of the fields in Iowa, Missouri, and Illinois, respectively, were infested with the pathogen. The recovery rates of P. sojae were significantly greater (P </= 0.05) in conservation-till than in conventional-till fields in all states except Iowa. H. glycines was detected in 83% of the soybean fields in Illinois, 74% in Iowa, 71% in Missouri, 60% in Ohio, 54% in Minnesota, and 47% in Indiana. Both the prevalence and population densities of H. glycines were consistently greater in tilled than in no-till fields in all states for which tillage information was available.

Field Response of Glyphosate-Tolerant Soybean to Herbicides and Sudden Death Syndrome

Three-year field experiments were conducted to assess the development of sudden death syndrome (caused by Fusarium solani f. sp. glycines) in three soybean cultivars, tolerant (P9344 and A3071) and nontolerant (BSR101), to glyphosate following foliar application of four herbicides (acifluorfen, glyphosate, imazethapyr, and lactofen) commonly applied to soybeans in the north-central region of the United States. Cultivar A3071 is resistant to sudden death syndrome, whereas cultivars P9344 and BSR101 are susceptible to this disease. There was no statistically significant cultivar-herbicide interaction with respect to the severity of foliar symptoms of the disease and the frequency of isolation of F. solani f. sp. glycines from roots of soybean plants. Across all herbicide treatments, the level of sudden death syndrome was lower in the disease-resistant cultivar than in the susceptible ones. There was an increase in the disease levels under application of acifluorfen, glyphosate, and imazethapyr compared with nontreated or lactofen-treated plants. The results obtained indicate that the response of glyphosate-tolerant soybeans to sudden death syndrome is not different from the response of conventional soybeans to this disease following application of the selected herbicides, and the resistance of soybean to sudden death syndrome was not changed with application of glyphosate.

Development of a neural network for soybean rust epidemics

The objective of this study was to develop a neural network to predict soybean rust disease severity for a single soybean cultivar. Data available for development consisted of sequential weekly plantings of TK 5 soybean cultivar in 1980 and 1981, planted at the Asian Vegetable Research and Development Center in Taiwan. Plant and disease development were measured weekly throughout the season for each planting date experiment. Data from 73 epidemics were collected with a total of 577 scenarios (observations) over the two years. A total of seven inputs related to soybean and rust progress and weather were available for development. Validation was performed for two different assumptions about the independence of the epidemics. In the first validation, it was assumed that all epidemics were independent. The 73 epidemics and their associated observations of rust severity were selected so that 70% were used for training, 15% for testing, and 15% for validation. Several different three-layer feedforward neural networks were developed using different combinations of inputs and hidden nodes. The best NN had four inputs and four hidden nodes, and gave the highest coefficient of multiple determination (R2) of 0.793 for the validation scenarios. This configuration gave the lowest average error of 11.0% per observation. Cumulative probability of percent error between predicted and observed rust severity showed that 50% of the validation scenarios had errors within ±10% of measured values, and 80% had errors within ±20%. In the second validation, it was assumed that epidemics in 1980 were independent of epidemics in 1981. Epidemics for 1980 were used for training and testing, and epidemics in 1981 were used for validation. Several different three-layer feed forward networks were developed with different combinations of inputs and hidden nodes. Networks with three and four inputs gave very similar results. The three input network gave an R2 of 0.858, average error of 9.7%, and a maximum error of 43% for the validation scenarios. Cumulative probability of percent error between predicted and measured rust severity showed that 54% of the validation scenarios had errors within ±10%, and 84% of the scenarios had errors within ±20% of measured values.

Long-Term Prediction of Soybean Rust Entry into the Continental United States

This special report demonstrates the feasibility of long-term prediction of intercontinental dispersal of Phakopsora pachyrhizi spores, the causal agent of the devastating Asian soybean rust (SBR) that invaded the continental United States in 2004. The climate-dispersion integrated model system used for the prediction is the combination of the particle transport and dispersion model (HYSPLIT_4) with the regional climate prediction model (MM5). The integrated model system predicts the trajectory and concentration of P. pachyrhizi spores based on three-dimensional wind advection and turbulent transport while incorporating simple viability criteria for aerial spores. The weather input of the model system is from a seasonal global climate prediction. The spore source strength and distribution were estimated from detected SBR disease severity and spread. The model system was applied to the known P. pachyrhizi spore dispersal between and within continents while focusing on the disease entry into the United States. Prediction validation using confirmed disease activity demonstrated that the model predicted the 2004 U.S. entry months in advance and reasonably forecast disease spread from the south coast states in the 2005 growing season. The model also simulated the dispersal from Africa to South America and from southern South America to Columbia across the equator. These validations indicate that the integrated model system, when furnished with detailed source distribution, can be a useful tool for P. pachyrhizi and possibly other airborne pathogen prediction.

Soil Variables Associated with Sudden Death Syndrome in Soybean Fields in Iowa

Sudden death syndrome, caused by Fusarium solani f. sp. glycines, has increased in prevalence in soybean production regions in the North-Central United States. Little is known about soil factors and environmental conditions that influence disease severity in this pathosystem. We studied associations between biological, chemical, and physical soil variables and severity of foliar symptoms of sudden death syndrome in nine commercial soybean fields in Iowa during 1995 and 1996. Disease was patchy in all fields, and soil samples were collected in each field along a transect that ran from a symptomless area through a diseased area. There were 25 sampling stops along each transect, separated by distances of 1.5 to 2.5 m. At each stop, soil samples were collected and soil strength, soil moisture, and foliar disease severity (at plant growth stage R6) were measured. Soil samples were assayed for population densities of F. solani f. sp. glycines, cysts of the soybean cyst nematode (Heterodera glycines), and for chemical variables (soluble salts, pH, organic matter, cation exchange capacity, and concentrations of P, K, Ca, Mg, Mn, and Fe). Cross-correlation analyses were carried out to test for associations between soil variables and disease severity in individual fields, while discriminant analysis was used to assess the effects of soil variables across all fields. Disease severity showed consistent associations with F. solani f. sp. glycines populations (strong effect) and H. glycines cyst counts (minor effect). Available K was identified as a possible disease-enhancing factor, but the magnitude of its effect was dependent on the overall K-concentrations in the fields. For example, as the median K-concentration increased, the correlation between K and disease decreased. None of the other soil variables showed consistent associations with disease. The results suggest that localized presence or absence of F. solani f. sp. glycines is the chief reason for the patchiness of sudden death syndrome in affected fields. Thus, manipulation of soil nutrient status or fertility level appears to have limited potential for reducing disease in the high-yield soybean production environment of Iowa. Instead, producers should focus on preventing the establishment or reducing populations of F. solani f. sp. glycines and H. glycines in their fields.