How to Manage Pests

UC Pest Management Guidelines


Downy Mildew

Pathogen: Pseudoperonospora cubensis

(Reviewed 12/09, updated 6/12, pesticides updated 5/16)

In this Guideline:

Symptoms and Signs

Downy mildew first appears as small, pale green to yellow, angular spots delimited by leaf veins that give the foliage a mottled appearance. Eventually the spots coalesce and the leaf will turn brown. During moist weather, the lower surface of the leaf may be covered with a white to purple growth. Older leaves become infected first.

Comments on the Disease

Downy mildew occurs in the Sacramento, upper San Joaquin, Imperial valleys, and in coastal areas. It attacks all cucurbits, although cucumber is the most commonly infected. Spores of the fungus are carried by air currents or by rain-splash or sprinklers. Rain, dew, cool weather, and sprinkler irrigation favor this disease. The disease is more common in the late season.


Use resistant cucumber varieties. There are low levels of resistance in some varieties of melons and watermelons. Avoid overhead irrigation. Start monitoring for downy mildew during the vegetative growth stage and continue through fruit development. Apply a treatment when disease symptoms first occur and repeat if symptoms worsen. Planting early may help to avoid conditions conducive to the disease later in the season.

Common name Amount per acre REI‡ PHI‡
(Example trade name)   (hours) (days)

Calculate impact of pesticide on air quality
Bee precaution pesticide ratings
When choosing a pesticide, consider its usefulness in an IPM program by reviewing the pesticide's properties, efficacy, application timing, and information relating to resistance management, honey bees, and environmental impact. Not all registered pesticides are listed. Always read the label of the product being used.
  (Ranman 400SC) 2.1–2.75 fl oz 12 0
  MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone inside inhibitor (21)
  COMMENTS: Do not apply more than six applications of Ranman per growing season in cucurbits.
  (Presidio) 3–4 fl oz 12 2
  MODE-OF-ACTION GROUP NAME (NUMBER1): delocalization of spectrin-like proteins (43)
  COMMENTS: A labeled rate of another product with a different mode of action effective on the target pathogen must be mixed with Presidio fungicide
  (Previcur Flex) 1.2 pt or 0.6 to 1.2 pt in tank mix 12 2
  COMMENTS: Do not apply more than 6 pt/season in cucurbits
  (Tanos) 8 oz 12 3
  MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11) and Cyanoacetamide (27)
  COMMENTS: Do not make more than one application without alternating with a fungicide that has a mode of action other than Qol (Group 11) .
  (Reason 500SC) 5.5 fl oz 12 14
  MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
  COMMENTS: Alternate with a fungicide that has a different mode of action (i.e., group number).
  (Dithane DF) 2–3 lb 24 5
  (Dithane F-45) 1.6–2.4 qt 24 5
  (Dithane M-45) 2–3 lb 24 5
  (Penncozeb 75 DF) Label rates 24 5
  MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-site contact (M3)
  COMMENTS: Labeled for cucumbers, melons, watermelons, and summer squash only.
  (Bravo Ultrex, etc.) 1.4–1.8 lb 12 0
  MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-site contact (M5)
  COMMENTS: Do not apply more than 19.1 lb/acre per season.
  (Ridomil Gold Bravo SC) 2.5 lb 48 0
  MODE-OF-ACTION GROUP NAME (NUMBER1): Phenylamide (4) and Multi-site contact (M5)
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.



[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Cucurbits
UC ANR Publication 3445


R. M. Davis, Plant Pathology, UC Davis
T. A. Turini, UC Cooperative Extension, Imperial County
B. J. Aegerter, UC Cooperative Extension, San Joaquin County
J. J. Stapleton, UC IPM Program, Kearney Agricultural Center, Parlier

Acknowledgment for contributions to Diseases:
W. D. Gubler, Plant Pathology, UC Davis

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