Effect of seed treatments and soybean cultivars on root rot caused by Fusarium virguliforme

Fusarium virguliforme, the causal agent of sudden death syndrome (SDS), is one of the most important soybean pathogens in the United States and Canada. Cultivar selection, seed treatments and some cultural practices are currently the main management practices for this disease. Fusarium virguliforme is a soilborne pathogen that infects soybean roots early in development (root rot phase), but also leads to the characteristic foliar symptoms (foliar phase) later in the season. Recently, the product fluopyram (ILeVO®, Bayer CropScience) was registered for management of SDS. The research described in this manuscript explains how fluopyram, in combination with cultivar selection, affects the two phases of SDS under field conditions. Two different greenhouse techniques were also evaluated for their ability to quickly and accurately screen seed treatments for efficacy against SDS.\nAll field experiments evaluated two seed treatments: (1) base seed treatment (control), which was prothioconazole + penflufen + metalaxyl (0.019 mg a.i/seed) + metalaxyl (0.02 mg a.i/seed) + clothianidin + Bacillus firmus I-1582 (0.13 mg a.i/seed) and (2) base treatment + fluopyram (0.15 mg a.i/seed). For the Roland, IA and Muscatine, IA locations in 2015, a third seed treatment was included; (3) base treatment + fluopyram (0.075 mg a.i./seed). All three seed treatments were included in all of the 2016 locations. Three soybean cultivars, categorized as susceptible, moderately resistant or resistant, were planted at each location. Fluopyram significantly reduced root rot at 7 of the 10 locations and foliar symptoms at 5 of the 9 locations where foliar symptoms occurred. Yield was significantly increased by fluopyram at 5 of the 10 locations. Our findings suggest that yields could be reduced by the root rot phase and should be managed in addition to the foliar phase of SDS.\nTwo greenhouse techniques: (1) soil-incorporated technique and (2) layer technique used the aforementioned seed treatments in the greenhouse. Neither technique consistently differentiated between treatments for foliar symptoms. However, both experiments showed approximately 20% reductions in root rot severity between the base seed treatment and both rates of fluopyram. Further work is needed to identify a controlled experiment in the greenhouse for evaluating seed treatments for both phases of SDS.\nOur findings suggest that at locations with known SDS history in years with favorable weather for the F. virguliforme pathogen, both phases of SDS can cause significant yield loss. The use of fluopyram as a seed treatment, along with resistant cultivars can protect soybean yields from both phases of SDS. However, more work is needed to identify an integrated management plan for both the root rot and foliar phases of SDS

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