Leaf Diseases in Soybeans

Bacterial Diseases

• Bacterial pustule (Xanthomonas campestris pv. glycines) and bacterial blight (Pseudomonas syringae pv. glycinea) are two leaf diseases of soybeans caused by bacterial pathogens. Bacterial postules present as lesions on either the upper or lower leaf surface with raised postules mostly on the lower surface. Bacterial blight presents as leaf spots, which form yellow halos around the spot. The affected tissue then turns brown and falls out creating holes in the leaf. There are few treatments for affected plants. Copper fungicides may be used for bacterial blight if sprayed early enough. Preventative measures should be taken.

Viral Diseases

• Bud blight and soybean mosaic are two common diseases of soybean leaves caused by viral pathogens. Plants affected with bud blight do not present any discoloration, but the leaves will wilt and curl. Mosaic virus causes similar symptoms, with the addition of puckering of the leaves. Neither disease is serious enough to cause substantial losses. There is no chemical treatment recommended for control of either disease. Resistant varieties of soybean and virus-free seed should be selected.

Fungal Diseases

• Fungal pathogens are the most common cause of soybean leaf diseases. Symptoms vary from a white to gray coating on the leaves such as with powdery mildew, a yellowing or browning of the leaves such as with downey mildew, a red to orange speckled appearance such as with rust diseases and various leaf spots. Select disease resistant varieties and treated seed when possible. There are chemical treatments available for fungus diseases. Select only fungicides labeled for use on soybeans. Preventative sprays can be performed at 10 to 14 day intervals throughout the growing season. Treatment sprays should be done every 7 to 10 days.

Prevention

• The best way to control diseases on the leaves of soybeans is to take preventative measures. Start with treated seed and select disease-resistant varieties. Follow irrigation practices that keep the foliage dry as much as is possible such as by using drip irrigation. Plant soybeans in an area that receives good air circulation and full sun to help prevent the conditions that favor fungal diseases. Crop rotation should be practiced where possible and debris from the previous year's crop should be removed before replanting in areas where rotations are not possible.