How to Manage Pests
UC Pest Management Guidelines
White mold appears as water-soaked lesions covered by a white, cottony mycelial mat on leaves and stems. In severely affected plants, the stem is girdled and plants die. Hard, black, irregularly shaped sclerotia (about 0.25–0.5 inch in diameter) develop inside dying potato stems.
The fungus overwinters as sclerotia in the soil. When exposed to moisture for prolonged periods, sclerotia germinate and grow into mushroomlike bodies that eject airborne spores, which may infect nearby plants. The spores germinate and infect leaves or stems when free moisture is present for at least 48 hours. Cool temperatures (60° to 70°F) and high relative humidity (95–100%) favor rapid disease development. White mold is most serious in the Klamath-Tule Lake Basin area on late maturing, large vine-type cultivars. Sclerotinia sclerotiorum has a wide host range, attacking many broadleaf crops and weeds.
Watch for disease symptoms during routine field monitoring, and keep records of your results (example form— ). After vine closure, apply water less often so plant surfaces do not remain wet continuously for periods of 48 hours or longer. In between crops, flood soils for 3 to 6 weeks to kill sclerotia. Avoid excess nitrogen, which promotes heavy canopy growth and conditions favorable for the development of white mold. Grow early maturing varieties. For best results in the Tule Lake region, time the first fungicide application to coincide with the senescence of potato flowers, and follow with a second application 14 days later.
UC IPM Pest Management Guidelines: Potato