Purple seed stain is caused by a fungus that infects the seed coat of soybeans, resulting in delayed germination. The greatest loss in regards to yield is grain dockage or denial of seed certification due to the discoloration of the seeds.
The pathogen responsible for this disease survives year to year in plant debris or seed that has been infected. It tends to become active during warm, humid spells, usually infecting the plants from flowering to maturity. After initial infection, symptoms may be not be visibly present until the R4 stage. Infection happens when the wind or rainwater carries spores onto new growth of the soybean plant.
Symptoms begin after flowering as circular, red-brown or purple spots evident on both sides of the leaves. As these spots grow, they become larger lesions. This ultimately gives the leaf a leathery texture. The lesions eventually grow together, creating dead spots within the leaves. In severe cases, the infected leaves are shed, leaving the petiole still attached to the stem. Lesions may also develop on the stems and pods. If lesions develop on the pods, there's a good chance that the seeds will become infected as well. The seed coat gets covered purple to pink blotches, giving it a cosmetically damaging appearance. The coloration usually radiates out from the seed helium ranging from specks to complete coverage. Sometimes seeds carry the fungus but show no symptoms of it.
Damage caused by this fungus is minimal in terms of seed production, but it may cause the value of seeds to go down due to discoloration. In the case of severe leaf blight, some leaves on the top of the plant may die and fall off. This loss of green leaf area will not significantly impact yield.
Tillage is effective in minimizing survival of the fungus, but it's important to take into account the impact it may have on the overall soil composition and operating costs.
Fungicide seed treatment has been effective in controlling the fungus as well as foliar treatments during early pod stages (R3-R5). Please contact your local plant pathologist to learn what is most effective in your area and how to use it best. Always read and follow all label instructions.
There are varieties that are known to be resistant to this fungus. Producers must be sure to buy and plant seed that is clean. Crop rotation away from soybeans is also known to be a helpful method for reducing potential for infection, as it causes viable spore populations to decrease.