Soy Bean Seed Rot

Causal Agent

• A number of diseases lead to seed rots in soybeans. These include phomopsis seed decay, anthracnose, downy mildew, purple seed stain and frogeye leaf spot. Phomopsis seed decay is the most damaging and the most common among these, cites the North Carolina State University Extension. Three different fungi cause phomopsis seed rot including Diaporthe phaseolorum var. caulivora, Diaporthe phaseolorum var. sojae and Phomopsis longicolla.

Symptoms

• Phomopsis seed rot causes cracking and shriveling of seeds and growth of white mold on the seed surface. Seed with a greater level of infection do not germinate. Seedlings emerging from the less-infected seeds have colored lesions on stems above or below the soil line. Similar symptoms are caused by the fungi causing other seed rot diseases of soybeans. Other seed rot fungi are less common, however. The Phomopsis spp. fungi are present in almost all soybean fields at maturity and usually infect seeds when the crop is harvested late.

Favorable Conditions

• Rainy, wet or humid weather at the time of harvest is the primary reason for seed infection in soybeans. Soybean crops that have not been rotated with other crops such as corn or wheat have higher incidence of seed rot. The fungal fruiting bodies or pycnidia are left behind in fields from prior crops and infect new plants in fields that are not properly cleared of debris prior to planting.

Management

• Reduce chances of seed rot by harvesting when the seeds are at 13 to 16 percent moisture level. Rotate crops with corn or wheat. Treat pods with recommended fungicides during the midflowering or late pod stages in order to reduce chances of phomopsis infection. Bury all crop residues after harvest. Treat seeds with carboxin, captan or metalaxyl prior to planting to control any possible fungi in disease.