Soybean leaves (left) and plants exhibiting symptoms of Alternaria leaf spot.
Soybean leaves (left) and plants exhibiting symptoms of Alternaria leaf spot.

Alternaria leaf spot has been found in soybeans in Red Willow County in southwest Nebraska.

Symptoms and Pathogen

Alternaria leaf spot of soybean, caused by a fungal species called Alternaria spp., is usually a secondary invader following mechanical injury, insect damage, or another disease. Alternaria leaf spot occasionally appears in seedlings but generally attacks leaves and pods of plants approaching maturity. Because the disease usually occurs late in the growing season, yield losses typically are minimal.

Diseased lesions are round or restricted by a major vein or merge with another lesion. Some have brown concentric rings with a well-defined border. The lesions expand and may combine to yield larger dead areas on the leaves. Infected leaves eventually dry out and fall.

Alternaria leaf spot of soybeanDisease Cycle and Epidemiology

Alternaria leaf spot is a seed-borne pathogen. Seed invasions occur through breaks in the pod walls. The disease is favored by warm, moist weather late in the growing season.

Management

Generally this disease does not require specific management, but if it occurs in the early soybean growth stages with favorable environmental conditions, consider a fungicide application. A strobilurin fungicide, such as Quadris or Headline, can be used. Delaying treatment until a large outbreak is identified may be too late for successful control, thus early scouting and diagnosis is important. Planting high quality, certified, disease-free seed that has been treated with a strobilurin fungicide is critical to managing this disease.