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UC Pest Management Guidelines

Water-soaked lesion and cottony mycelium of white mold.


White Mold

Pathogen: Sclerotinia sclerotiorum

(Reviewed 8/07, updated 5/08)

In this Guideline:


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 form108 KB, 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, 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.

Common name Amount/Acre R.E.I.+ P.H.I.+
(trade name)   (hours) (days)

Calculate impact of pesticide on air quality
Bee precaution pesticide ratings
When choosing a pesticide, consider information relating to the impact on environmental quality Not all registered pesticides are listed. Always read label of product being used.
  (Endura) 5.5–10 fl oz 12 30
+ Restricted entry interval (R.E.I.) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (P.H.I.) 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 (for more information, see http://www.frac.info/). Fungicides with a different group number are suitable to alternate in a resistance management program. For fungicides with mode of action Group numbers 1, 4, 9, 11, or 17, make no more than one application 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.




[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Potato
UC ANR Publication 3463
R. M. Davis, Plant Pathology, UC Davis
J. Nuñez, UC Cooperative Extension, Kern Co.
B. J. Aegerter, UC Cooperative Extension, San Joaquin Co.
Acknowledgment for contributions to the disease section:
C. Smart, Plant Pathology, UC Davis

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