Soybean Diseases

Foliar Diseases

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 (Figure 2). Crop rotation is the main method of disease prevention.