Quantifying the effects of fungicides, cultivars and row spacing on foliar diseases and soybean yield in the southern United States

Soybean field trials were established in Alabama, Kentucky, and Tennessee in 2020 and 2021 to determine the effect of different strategies on management of foliar diseases. Cultivars Asgrow 53X0 (susceptible to target spot, caused by Corynespora cassiicola) and Asgrow 52X9 (moderately resistant to target spot) were planted using two different row spacings (38 and 76 cm). Foliar fungicides (pyraclostrobin or pydiflumetofen + difenoconazole) were applied at beginning pod development, and nontreated plots served as a control. Results from Kentucky trials showed lower target spot severity for Asgrow 52X9 compared to Asgrow 53X0. Both fungicide treatments significantly reduced target spot severity in the susceptible cultivar when compared to the nontreated control. Additionally, the severity of frogeye leaf spot (caused by Cercospora sojina) was evaluated, and significant effects of cultivars and fungicides were detected only in the 2020 crop season. In Tennessee, the main effect of cultivar was significant for target spot, and plots treated with pydiflumetofen + difenoconazole had significantly lower frogeye leaf spot severity compared to other treatments during the 2021 season. In Alabama, soybean rust (caused by Phakopsora pachyrhizi) was observed during both seasons, and fungicides significantly reduced disease severity compared to the nontreated control. The effect of row spacing was not significant for disease severity at any location, but the combination of cultivars and fungicides were found to be effective in protecting soybean plants against foliar diseases.