Introduction

White mold, caused by the soil-borne fungus Sclerotinia sclerotiorum, is an important cause of yield loss on soybean (Glycine max L.) production in the United States. This disease has been reported affecting soybean in the top 28 United States producing states and Southern Canada; however, S. sclerotiorum is more severe in Northern states of the United States. Although it has been reported affecting soybean in North Carolina, it usually does not represent a major threat to soybean production in the state.

Pathogen

S. sclerotiorum is a necrotrophic fungus in phylum Ascomycota. In culture, the fungus produces white to grey white colonies, with abundant irregularly shaped, black, sclerotia (overwintering structures) that vary in size. On soybean plants, a white fungal mat with black, irregularly shaped sclerotia may be found on and inside of the plant stems.

Host Range

S. sclerotiorum has a broad host range and it has been reported affecting many economically important crops including, legumes, sunflower, canola, tobacco, and most vegetables. Other known hosts are flowering bedding plants and stone fruits.

Life Cycle and Favorable Conditions for Disease

Sclerotia serve as the overwintering structures in the absence of a plant host and initial source of inoculum. In the spring, sclerotia germinate to form fruiting bodies (apothecia) or directly infect lower stems of soybeans. The spores (ascospores) are released from the fruiting bodies and land on lower nodes of soybean plants. Conducive environmental conditions for sclerotia germination and spore release are saturated soils, prolonged periods of leaf wetness, and continuous cool temperatures ranging from 12 to 24°C. The fungus invades the plant stems and colonizes the vascular tissue of the plants. Fungal mats may be observed on the surface of plants. As the fungus depletes available nutrients, sclerotia are formed. At the end of the growing season, sclerotia are released into the soil or survive in crop debris left in the field.

Symptoms and Signs

Symptoms may vary depending on plant growth stage at which infections take place. Foliar symptoms are often observed in July and August, although they may occur earlier when plants become water stressed. Above-ground typical symptoms include, water-soaked stem lesions with a distinct margin, wilting, bleaching, and stem shredding. Pods may also be affected. Leaves wilt and turn brown, but remain attached to the stem. Distinct signs, which include white cottony mycelium and black, irregularly-shaped sclerotia, appear in the later stages of the disease development and can be found on the surface of diseased tissue.

Management

Planting densities that allow for air-flow in the plant canopy may reduce conducive microclimates that favor disease. Row spacing and orientation, fertilization, and cultivar selection may also be adjusted for canopy management to promote a disease-suppressive microclimate. Crop rotations with non-host crops may be beneficial when disease pressure is high. Chemical controls are often not economical in North Carolina, but timing is important for efficacy. Protectant fungicides need to be applied before the canopy closes in order for the active ingredient to reach the lower canopy of plants. Fungicide efficacy ratings for white mold can be found on the NC State Extension Soybean portal.

Useful Resources

The NC State University Plant Disease and Insect Clinic provides diagnostics and control recommendations.

The NC State Extension Plant Pathology portal provides information on crop disease management.

The North Carolina Agricultural Chemicals Manual provides pesticide information for common diseases of North Carolina. The manual recommendations do not replace those described on the pesticide label, and the label must be followed.

Acknowledgements

This factsheet was prepared by the NC State University Field Crops and Tobacco Pathology Lab in 2020.

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