With the warm temperatures following the recent rains in Nebraska, we are starting to see more Purple Seed Stain and Blight symptoms in some soybean fields.
Purple Seed Stain and Blight is caused by the fungus Cercospora kikuchii. Its inoculum source is infected seed and previous soybean crop debris. Sporulation of Cercospora kikuchii is favored by periods of high humidity and temperatures of 73-80°F.
Symptoms of Purple Seed Stain are usually observed in the seed as pink to purple spots that range in size from specks to large blotches. Blight symptoms often become prevalent during the mid to late reproductive stages (R3-R6) and are typically on young leaves on the upper portion of the canopy.
These symptoms include dark reddish-purple bronzing of the leaf. Lesions typically occur on both the upper and lower leaf surfaces and can range from specks to blotches up to ½ inch in diameter. Severely affected upper leaves can drop with the petioles remaining on the plant while lower leaves are still attached.
There are commercially available resistant varieties for Cercospora Blight, but no known sources of resistance for Purple Seed Stain. There is also no clear relationship between the severity of Cercospora Blight and the severity of Purple Seed Stain.
Extended crop rotations and residue incorporation will reduce the inoculum by breaking down infested residue. Foliar fungicides are registered for Cercospora Blight and can be applied during R3-R5 pod stages which can reduce blight incidence and severity, but this may not affect soybean seed stain symptoms.
Seed lots with a high percent of infected seed should be treated with a seed treatment fungicide when used for seed. In most cases there will not be enough seed symptoms for any docking in price, but in severe cases this could happen.