White mold, also called Sclerotinia stem rot, is a destructive disease that can hinder the potato production process. The white mold pathogen is prevalent across the Pacific Northwest, Upper Midwest and Northeast regions, specifically located in Washington, Idaho, Michigan and Maine.
White mold thrives in high-moisture areas and moderate temperatures below 85 degrees Fahrenheit – two factors that contribute to the growth of fungus. The infectious disease arises from Sclerotinia sclerotia, a pathogenic fungus that can affect more than 400 plant species.
Sclerotia germinate under shady canopies with free moisture and produce apothecia, which release ascospores that infect plant debris. If located in a high moisture, low air environment, apothecia can germinate and form hyphae. This formation can contaminate remaining plant parts as they fall to the ground.
If the fungus begins to grow on healthy stems or leaves, lesions will develop. These white, cottony lesions first appear at the intersections of stems and branches, or if the stems and branches come in contact with the soil. These lesions can grow and spread quickly, especially in humid climates, girdling stems and causing foliage to wilt.
Researchers and professors at the University of Idaho Extension say white mold pathogens can survive for several years due to the inoculum levels secreted in the soil. However, if Sclerotia does not have a sufficient host, the fungus will be incapable of continuing its life cycle and will no longer germinate.
Therefore, it is important to manage for white mold until environmental conditions are no longer conducive to development. Cultural practices to limit white mold include eradication of weed hosts and crop rotation with non-susceptible hosts, such as corn, for multiple years if a field has a history of the disease.
Another approach is to consider chemical control by using a preventive fungicide like Omega. Omega is a group 29 fungicide for white mold control. Applying Omega at the initial full bloom of the potato crop will remain effective in reducing the number of infected stems and maximize yields.