Response of the Soybean Seed-Borne Pathogens Diaporthe spp. and Cercospora spp. to Foliar Fungicides in Arkansas

Few studies have determined the effect of foliar fungicides on the incidence of soybean seed infection by the pathogens Diaporthe spp. and Cercospora spp. in Arkansas. Three fungicides (propiconazole, pyraclostrobin, and thiophane-methyl) were compared to an untreated control (water) at two locations in Arkansas in 2018 and 2019. The fungicides were applied at the R3, R5, or at the R3 and R5 growth stages to an early maturity group 4 (MG4) cultivar (CZ4044LL in 2018 and CZ4105LL in 2019) and a late MG4 cultivar (CZ4748LL). Yellow pods were sampled from the lower half of the plants at physiological maturity (PM) and placed in a moist chamber for three days, the seeds removed, surface disinfested, placed on Petri dishes filled with solidified potato dextrose agar and the incidence of seed infected with Diaporthe spp. and Cercospora spp. determined. At harvest, yield data were collected, and seeds were assessed for the incidence of discolored seed (chalky, brown, purple, and total) and the incidence of seed infection. There was no significant effect of fungicide on Diaporthe spp. seed infection. Cercospora sp. seed infection under propiconazole and thiophanate-methyl treatments was significantly greater than the control at PM, but significantly lower than the control at harvest. The incidence of Cercospora spp. seed infection increased significantly from PM to harvest. All fungicides significantly reduced the incidence of purple seed and CZ4748LL had significantly more Cercospora spp. seed infection and seed discoloration than CZ4044LL/CZ4105LL. Yields were significantly greater with pyraclostrobin than the control.