Soybean Diseases

Soybean Foliar Diseases

Bacterial Blight (Pseudomonas syringae)

Bacterial blight thrives in cool, wet conditions. Bacteria overwinter in residue and seed, and can be spread by rain and wind. It can occur at any development stage, and young leaves are most susceptible. Leaf and pod symptoms include small, angular, reddish-brown lesions with water-soaked margins, surrounded by a yellow halo. Lesions eventually grow together producing irregularly-shaped dead areas.

Management options include crop rotation, incorporation of infected residue, and avoiding field activities when foliage is wet. Foliar fungicides do not provide protection against bacterial diseases. Bacterial blight has a low impact on soybean yield and quality in Manitoba, but it is of greater concern when soybeans are grown for seed.

Septoria Brown Spot (Septoria glycines)

Septoria brown spot thrives in warm, wet conditions. The fungus survives on infected leaf and stem residue. It can occur at any development stage. Symptoms include purple lesions on young soybean leaves, and small irregularly-shaped, dark brown lesions on older leaves. Lesions are visible on upper and lower leaf surfaces. Similar lesions can occur on stems, petioles and pods. Infected leaves can eventually drop. This disease starts in the lower canopy, progressing upward if favourable conditions persist.

Management options include crop rotation, incorporation of infected residue, and foliar fungicide. Brown spot has a moderate impact on soybean yield and quality in Manitoba. Yield loss depends on how far the disease has progressed up the canopy during grain fill.

Downy Mildew (Peronospora manshurica)

Downy mildew is a fungal, foliar disease of soybeans that generally occurs at low levels and seldom causes significant yield loss. It can appear in Manitoba anytime from late June until early September. Incidence of this disease has increased over the past few years in Manitoba. Overall, there is a lack of data indicating what level of downy mildew infection will cause yield loss.

Scout for downy mildew by examining upper and lower leaf surfaces. It is characterized by yellow lesions on the upper leaf surface. Opposite to these lesions are grey tufts of mycelial growth on the undersides of leaves (Figure 1). This disease first occurs on new growth at the top of the canopy and progresses downward. Older lesions appear brown with yellow-green lesions. The main methods of downy mildew management include crop rotation and burial of infested crop residue.

Frogeye Leaf Spot (Cercospora sojina)

Frogeye leaf spot is a relatively new soybean pest to Manitoba. It was first confirmed in 2016 in the western, central and eastern regions by Dr. Brian Cassone of Brandon University. The presence of this fungal, foliar disease is due to more frequent soybean production. Symptoms are most likely to appear during July or August. Warm, humid conditions promote spore production, infection and disease development. Soybeans remain at low risk of yield loss from frogeye leaf spot in Manitoba.

Scout for frogeye leaf spot from July to September. Its symptoms include small irregular to circular lesions with dark reddish-brown margins and a white-grey centre. Crop rotation is the main method of disease prevention.


Additional Resources

Extension Disease Handbook – Bryan Cassone

Bacterial Blight, Septoria Brown Spot, Downy Mildew and Frogeye Leaf Spot of Soybean – Crop Protection Network
Soybean Disease Diagnostic Series – North Dakota State University