Soybean seed diseases are showing up across Iowa, which isn’t surprising considering the wet weather experienced in parts of the state over the past month (Fig 1). The rain is a double whammy for farmers this time of year as it favors continued fungal development and keeps combines from getting in the field. Some of the primary seed diseases showing up are purple seed stain and Diaporthe seed decay.
Purple seed stain is caused by the same pathogen responsible for Cercospora leaf blight. Soybean seeds with this disease show light to dark purple discoloration and can have reduced germination (Fig 2). Purple seed stain doesn’t reduce yield, but it can lower the value of the crop through dockage.
Diaporthe seed decay causes cracked, shriveled seed with white chalk-colored mold on the seed surface (Fig 3). Small, dark fungal structures (pycnidia) can sometimes be seen on infected seeds. The large amounts of rain that occurred throughout the last month favor the Diaporthe fungus. This fungus first infects pods before infecting the seeds within. Seed infection stops once moisture is below 19 percent. However, wet and warm weather can allow for continued seed infection and colonization if seed moisture increases to more than 19 percent.
Diaporthe seed decay can reduce test weight and seed quality. Seeds with this disease can have lower than normal germination rates and vigor in the spring and subsequent seedlings may have blight.
For those growing soybean seed for 2027, the Iowa State University Seed Lab is able to test the germination rate of seeds to verify if and what percentage of seeds are infected with the Diaporthe fungus. Depending on the germination rate and incidence of infection, seed dealers may want to have seed treated with a fungicide to increase the chances of germination and prevent seedling diseases. Appropriate seed treatments can increase germination rates by 10-15 percent. Consult the Crop Protection Network's Fungicide Efficacy for Management of Soybean Seedling Diseases for the seed treatments with efficacy against Diaporthe.
Winter storage of seed can also increase germination rates, due to the fact that under dry storage conditions, the mycelium of the fungus will die, improving the seed quality. A suggested practice is to dry-store low germ seeds over winter and retest the seed in February. It is not recommended to use seed lots with more than 20 percent Diaporthe infection because severely infected or moldy seed will fail to germinate even after being treated.
For soybeans in the field in 2026, there’s not much to be done beyond getting into the fields as soon as the soil is dry enough, but that hardly needs to be said. Scouting for these diseases while waiting for fields to dry can help inform if seed testing should occur or to help plan for 2027. Seed and seedling disease management in subsequent seasons includes the use of resistant varieties, planting pathogen-free seed, fungicide seed treatments, and reducing disease causing inoculum through rotation and tillage. Seed infection may be reduced by an R3 to R5 fungicide application.
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- Professor
- Extension Plant Pathologist