Estimating the Potential for Soybean Seed Infection by Diaporthe spp. and Cercospora spp. at Physiological Maturity

Diaporthe spp. and Cercospora spp. are the most important soybean seed pathogens. To determine the effect of physiological maturity (PM, soybean seed and pod yellow) and pod moisture on infection, four soybean accessions were planted at the Vegetable Research Station, Kibler, AR, in 2014 and eight accessions were planted in 2015. Pods were collected at PM and at harvest from the lower half of the plant. At PM, seed from half of the pods were immediately removed, surface disinfested, and placed on acidified potato dextrose agar. The other pods were incubated in a moist chamber for three days at room temperature (23°C) and then the seeds were assayed. Pods collected at harvest seeds did not receive the moisture treatment before the seeds were assayed. Diaporthe spp. and Cercospora spp. growing from the seed were identified morphologically. At PM, 0 to 12% of the seeds were infected with Diaporthe spp. before the moisture treatment but increased significantly to 18 to 62% after the treatment with all accessions in both years. Seeds were already infected by Cercospora spp. at PM (2 to 37% in 2014 and 0 to 11% in 2015). Pod moisture after PM had little or no significant effect on Cercospora spp. seed infection. Above average precipitation after PM was associated with increased seed infection by Diaporthe spp. at harvest, while above average precipitation before PM was associated with Cercospora spp. seed infection. Antagonism between these fungi at PM and harvest affected accession response to both pathogens.