By Andi Anderson
Excessive rainfall across Iowa during late summer and early fall has created favorable conditions for soybean seed diseases, raising concerns about seed quality and crop value. Wet weather not only promotes fungal growth but also delays harvest operations by preventing combines from entering fields. As a result, many farmers are observing increased levels of seed-related diseases in soybean crops.
Two of the most common diseases appearing this season are purple seed stain and Diaporthe seed decay. Both diseases can negatively affect seed quality and may create challenges for farmers planning to save seed for future planting.
Purple seed stain is caused by the same fungus responsible for Cercospora leaf blight. The disease is identified by light to dark purple discoloration on soybean seeds. While purple seed stain does not directly reduce yield, it can lower grain quality and market value because affected seed may receive dockage at delivery points. In some cases, the disease can also reduce seed germination potential.
Diaporthe seed decay is considered a more serious concern for seed quality. Infected seeds often appear cracked, shriveled, or covered with a white chalk-like mold. Small dark fungal structures may also develop on the seed surface. The disease is favored by warm and wet weather conditions, especially when prolonged rainfall occurs near harvest.
The fungus initially infects soybean pods before spreading to the developing seeds inside. Infection slows or stops when seed moisture drops below 19 percent. However, continued rainfall can raise seed moisture levels and allow the disease to keep developing, leading to further seed damage before harvest.
Diaporthe seed decay can significantly reduce test weight, germination rates, and seed vigor. Poor-quality seed may struggle to establish healthy seedlings the following spring, and infected seedlings can suffer from early disease problems.
For growers planning to use soybean seed for the 2027 planting season, seed testing is strongly recommended. Laboratory testing can determine germination rates and identify the percentage of seed infected with the Diaporthe fungus. These results help farmers and seed dealers make informed decisions about seed use and treatment options.
Fungicide seed treatments may improve establishment by protecting seeds and young seedlings from disease. Research has shown that appropriate seed treatments can increase germination rates by approximately 10 to 15 percent under certain conditions. Seed quality may also improve during winter storage because fungal growth declines under dry storage conditions. Experts recommend retesting lower-quality seed lots before planting.
However, seed lots with more than 20 percent Diaporthe infection are generally not recommended for planting. Severely infected or moldy seeds often fail to germinate, even after fungicide treatment.
For the current crop, management options are limited once disease has developed. Farmers are encouraged to harvest fields as soon as soil conditions allow and monitor disease levels while waiting for fields to dry. Information collected this season can help guide future management decisions.
Long-term disease management includes selecting resistant soybean varieties, planting disease-free seed, using effective fungicide seed treatments, and reducing disease-causing pathogens through crop rotation and tillage practices. In some situations, fungicide applications during the R3 to R5 growth stages may help reduce seed infection levels.
As Iowa farmers prepare for harvest, careful scouting and seed evaluation will be important steps in protecting seed quality and improving crop performance in future growing seasons.
Photo Credit: istock-sandramatic
Categories: Iowa, Crops, Soybeans