Soybean sudden death syndrome (SDS), caused by Fusarium virguliforme, is one of the most devastating diseases of soybean responsible for yield losses in both North America and South America. In the United States, F. virguliforme is the predominant SDS causal pathogen, while four Fusarium species including F. virguliforme can cause SDS in South America. All four SDS-causing Fusarium species are located in clade2 of the Fusarium solani species complex (FSSC) along with three bean root rot (BRR) Fusarium pathogens. Within this group of fungi, it is difficult to identify to species level based on morphological traits. We developed a specific and sensitive diagnostic real-time quantitative PCR assay (qPCR) for detection and quantification of F. virguliforme from plant or environmental samples. Furthermore, this assay was applied in characterization of temporal dynamics of F. virguliforme infection and colonization of soybean roots. The quantity of F. virguliforme increased over time and reached a plateau at the end of the season. The severity or appearance of SDS foliar symptoms was not associated with quantity of F. virguliforme infection, and cultivars with varied SDS resistance levels did not differ in their quantity of F. virguliforme in roots. The fungicide fluopyram has been demonstrated to be effective in reducing SDS foliar symptoms in field trials as a seed treatment; however, in vitro evaluation of fungicide baseline sensitivity of F. virguliforme had not been determined. In this study, 185 F. virguliforme isolates collected from multiple locations in the United States were selected for estimation of fungicide sensitivity to fluopyram. Overall, the US F. virguliforme population appears to be sensitive to fluopyram. The effective concentration to inhibit 50% growth for 95.1% of isolates was determined to be between 0.81 to 5 μg/ml, while only nine isolates were determined to be less sensitive.Since the first report of SDS in Arkansas in 1971, SDS had been reported in surrounding states with an apparent pattern of dispersal. To date, although SDS has been reported in most soybean producing areas in the United States, limited research had been conducted to study the population biology of F. virguliforme. We utilized 539 isolates from North and South America in a population genetics study to test the hypothesis that Arkansas was the center of introduction within the United States, and investigate possible intercontinental movement. The Arkansas population demonstrated the highest genotypic diversity and most diverse population structure. Coalescence based migration analysis also supported a directional migration model from Arkansas to Indiana and Michigan. Within the United States, there was a weak positive correlation (P = 0.08) between genetic dissimilarity and geographical distance, suggesting a mixed dispersal pattern of F. virguliforme in the United States. Although South America has been proposed as the center of origin in previous studies, this hypothesis was only supported in the migrate analysis, while the genotypic diversity and population structure compositions detected in the United States cannot be explained by this hypothesis. Therefore, Arkansas as the center of origin in the United States hypothesis is supported by the population genetic analyses, but the South America as the center of origin hypothesis does not have strong support in our analysis.
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