How to Manage Pests

UC Pest Management Guidelines


Powdery Mildew on Greenhouse and Coastal Tomatoes

Pathogens: Oidium neolycopersici

(Reviewed 12/13, updated 12/13)

In this Guideline:


Symptoms of powdery mildew are limited to leaves. Symptoms initially appear as light green to yellow blotches or spots that range from 0.125 to 0.5 inch (3–12 mm) in diameter on the upper surface of the leaf. A white, powdery growth of the fungal mycelia and spores is obvious on the top of leaves. As spots coalesce, the leaf tissue dies. The entire leaf eventually turns brown and shrivels but remains attached to the stem.

Comments on the Disease

In California, powdery mildew caused by O. neolycopersici is limited to greenhouses and fields close to the coast. Conidia are easily windborne and are carried long distances. The conidia land on leaves where they germinate and enter the leaf stomata. The fungus grows at moderate to cool temperatures. Little moisture is required for the fungus to establish itself on a plant. There is experimental evidence that the pathogen has a wide host range and probably survives on other hosts or volunteer tomato plants from season to season.


This powdery mildew is generally not severe in coastal fields and control measures are usually not warranted. Greenhouse-grown tomatoes, however, can suffer to the point of severe economical damage. Registered fungicides, such as sulfur, may be required to control the disease in the greenhouse. Begin applications when the disease first appears.

Organically Acceptable Methods

Sulfur sprays are acceptable for use on organically certified produce.

Common name Amount per acre** R.E.I.‡ P.H.I.‡
(example trade name)   (hours) (days)

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Bee precaution pesticide ratings
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A. SULFUR# Label rates 24 0
  MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M2)
  COMMENTS: Do not use if an oil was applied recently or will be in the near future.
** See label for dilution rate.
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 until harvest can take place. In some cases the R.E.I. exceeds the P.H.I. The longer of these two intervals is the minimum time that must elapse before harvest may take place.
# Acceptable for use on organically grown produce.
1 Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of actions (for more information, see 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.



[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Tomato
UC ANR Publication 3470


R. M. Davis, Plant Pathology, UC Davis
G. Miyao, UC Cooperative Extension, Solano and Yolo counties
K. V. Subbarao, USDA Agricultural Research Station, Salinas, CA
J. J. Stapleton, UC IPM Program and Kearney Agricultural Center, Parlier
B. J. Aegerter, UC Cooperative Extension, San Joaquin County (powdery mildew on field-grown tomatoes)
Acknowledgments for contributions to Diseases:
B. W. Falk, Plant Pathology, UC Davis

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