NPIPM:Powdery mildew on soybean
Authors: Buyung Hadi and Dean Malvick
Powdery mildew on soybean was first reported from North Carolina in 1947. Since then, the disease has been reported from other soybean producing regions across the US.
Causal Organism
Powdery mildew on soybean is caused by the fungus Microsphaera diffusa. Initially identified as Erysiphe polygoni, M. diffusa is now accepted as the cause of soybean powdery mildew.
Symptoms and Signs
Common signs of powdery mildew are white powdery patches of fungal mycelium and conidia forming on the upper surface of soybean leaves, cotyledons, stems and pods. Initially, the patches are small and later grow in size and coalesce to form larger whitish area. Symptoms of powdery mildew infection are less common than the signs. Symptoms include green and yellow islands on the leaves, interveinal necrosis, necrotic specks and crinkling of the leaves. Usually, the presence of white powdery patches on plant surface is enough to diagnose powdery mildew of soybean.
Disease Impacts
When found on soybean, powdery mildew often occurs too late in the season to reduce much yield. Powdery mildew can reduce photosynthesis and respiration on infected plants. An yield loss of 13% due to powdery mildew infection was estimated in one field study in Iowa. Powdery mildew is not a common disease on soybean.
Life Cycle and Epidemiology
Cleistothecia, the fruiting bodies of M. diffusa, are believed to survive on plant debris and the produced ascospores from these cleistothecia act as primary inoculum the following season. Powdery mildew of soybean requires cool temperatures (66-74°F or 19-23°C) and low relative humidity to develop. Powdery mildew can be more severe in late-planted fields.
Management Approaches
Cultural methods
Crop rotation is not effective to manage powdery mildew since M. diffusa's conidia can easily be introduced to a field by blowing wind. Since powdery mildew is frequently more severe in late-planted fields, planting date management may have an effect on powdery mildew incidence.
Host Plant Resistance
Resistance towards M. diffusa in soybean germplasm is conditioned by Rmd genes. Generally, most determinate commercial varieties are resistant while many indeterminate varieties are susceptible towards powdery mildew.
Chemical Control
Fungicides can control powdery mildew effectively. However, due to cost consideration of fungicide application, decision to apply fungicide may be more economically prudent on fields grown for seed production.
