Soybean Seed Quality Issues During A Wet Fall

By Daren S Mueller

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.