Symptoms and Signs
White mold appears as water-soaked lesions covered by a white, cottony mycelial mat on lower 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 associated with girdling cankers.
Comments on the Disease
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. Petals of flowering potatoes are particularly susceptible and when the petals fall and are associated with lower stems, infection results. Spores can also 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—PDF). 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, if appropriate, 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. Varieties with excessive vine growth can be more prone to the disease. For best results in the Tule Lake region avoid overwatering and prolonged leaf wetness. Time the first fungicide application when plants are in full bloom and follow with a second application 14 days later.
|Common name||Amount per acre||REI‡||PHI‡|
|(Example trade name)||(hours)||(days)|
|Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least likely to cause resistance are at the top of the table. When choosing a pesticide, consider information relating to the pesticide's properties and application timing, honey bees, and environmental impact. Always read the label of the product being used.|
|(Endura)||5.5–10 fl oz||12||10|
|MODE OF ACTION GROUP NAME (NUMBER1): Succinate dehydrogenase inhibitors (7)|
|(Vertisan)||14–24 fl oz||12||7|
|MODE-OF-ACTION GROUP NAME (NUMBER1): succinate dehydrogenase inhibitors (7)|
|COMMENTS: For in-furrow use and after planting. Medium to high risk for resistance development.|
|(Luna Tranquility)||11.2 fl oz (ground or chemigation)||12||7|
|MODE-OF-ACTION GROUP NAME (NUMBER1): Succinate dehydrogenase inhibitors (7)|
|(Omega 500F)||5.5–8 fl oz||12||14|
|MODE-OF-ACTION GROUP NAME (NUMBER1): 29|
|‡||Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest.|
|1||Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of actions. Fungicides with a different group number are suitable to alternate in a resistance management program. In California, make no more than one application of fungicides with mode of action group numbers 1, 4, 9, 11, or 17 before rotating to a fungicide with a different mode of action group number; for fungicides with other group numbers, make no more than two consecutive applications before rotating to fungicide with a different mode of action group number.|