Temporal Dynamics of Fusarium virguliforme Colonization of Soybean Roots.

Soybean sudden death syndrome (SDS) caused by Fusarium virguliforme is one of the most yield limiting soybean diseases in the United States. SDS disease symptoms include root rot and foliar symptoms induced by fungal toxins. Soybean cultivar resistance is one of the most effective SDS disease management options, but no cultivar displays complete resistance. Soybean SDS foliar symptoms are the primary phenotype used to screen and breed for SDS resistance. Root rot or root colonization measures are seldom utilized, partly due to the lack of convenient and accurate methods for quantification of F. virguliforme. In this study, greenhouse and field experiments were conducted to determine the temporal dynamics of F. virguliforme colonization of soybean roots using quantitative real-time PCR (qPCR). The infection coefficient (IC), or ratio of F. virguliforme DNA to soybean DNA, was determined in soybean cultivars with different SDS foliar resistance ratings. In greenhouse experiments, F. virguliforme was detected in all cultivars 7 days after planting (DAP), with a peak in IC at 14 DAP. All soybean cultivars developed SDS foliar symptoms, but F. virguliforme soybean root colonization levels did not significantly correlate with SDS foliar symptom severity. In field experiments, SDS foliar symptoms developed among soybean cultivars in alignment with provided foliar resistance ratings; however, the F. virguliforme IC were not significantly different between SDS foliar symptomatic and asymptomatic cultivars. F. virguliforme was detected in all cultivars at the first sample collection point 25 DAP (V3 vegetative growth stage), and the IC increased throughout the season, peaking at the last sample collection point 153 DAP (postharvest). Collectively, appearance and disease severity ratings of SDS foliar symptoms were not associated with F. virguliforme quantity in roots, suggesting a need to include F. virguliforme root colonization in breeding efforts to screen soybean germplasm for F. virguliforme root infection resistance. The findings also demonstrates root colonization of the pathogen on nonsymptomatic soybean cultivars leading to persistence of the pathogen in the field, and possible hidden yield loss.