by Daren Mueller
Soybean seed diseases are showing up across Iowa, which isn’t surprising considering the wet weather conditions over the past month in parts of the state. 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.
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. Small and dark fungal structures (pycnidia) can sometimes be seen on infected seeds.
The large amount of rain that occurred throughout the past month favored the Diaporthe fungus. This fungus first infects pods before infecting the seeds within. Seed infection stops once moisture is below 19%. However, wet and warm weather conditions can allow for continued seed infection and colonization if seed moisture increases to more than 19%.
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.
Germination testing
For those growing soybean seed for 2027, the Iowa State University Seed Lab can 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% to 15%. Consult the Crop Protection Network's publication titled "Fungicide Efficacy for Management of Soybean Seedling Diseases" for the seed treatments with efficacy against Diaporthe.
Winter storage of seed also can increase germination rates because 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% Diaporthe infection because severely infected or moldy seed will fail to germinate even after being treated.
What to do at harvest
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 whether seed testing should occur or to help plan for 2027.
Seed and seedling disease management in subsequent seasons includes actions such as:
- using resistant varieties
- planting pathogen-free seed
- applying fungicide seed treatments
- reducing disease-causing inoculum through rotation and tillage
Seed infection may be reduced by an R3 (beginning pod) to R5 (beginning seed) fungicide application.
Mueller is an Iowa State University Extension and Outreach plant pathologist.