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How to Manage Pests

UC Pest Management Guidelines

| More pests | More crops | About guidelines |

Powdery mildew on melon leaves.

Cucurbits

Powdery Mildew

Pathogens: Sphaerotheca fuliginea and Erysiphe cichoracearum

(Reviewed 11/05, updated 6/08)

In this Guideline:

SYMPTOMS

All cucurbits are susceptible to powdery mildew, but the disease is less common on watermelon than on other cucurbits. Powdery mildew first appears as pale yellow spots on stems, petioles, and leaves. These spots enlarge as the white, fluffy mycelium grows over plant surfaces and produces spores, which give the lesions a powdery appearance. Affected leaves become dull, chlorotic, and may show some degree of wilting in the afternoon heat; eventually they become brown and papery.

COMMENTS ON THE DISEASE

Powdery mildew of cucurbits may occur at any time in coastal production areas but is more common in fall in the San Joaquin Valley, Sacramento Valley, and desert production areas. The disease may be caused by one of two species of mildew fungi, although E. cichoracearum is rare. Several races of each fungus also exist. The pathogens generally overwinter on weeds and their spores can be carried long distances by air currents. Infection is favored by water on the plant surface or high humidity (50 to 90%), and disease development is favored by vigorous plant growth and moderate temperatures.

MANAGEMENT

Plant resistant varieties, follow good sanitation practices, and control weeds. Carefully monitor fields, even those with powdery mildew resistant varieties, because there is recent evidence that plant resistance-breaking races are present in California. If multiple fungicide applications are needed to control powdery mildew, alternate materials with different modes of action especially if using fungicides with medium to high resistance potential (azoxystrobin-Quadris, myclobutanil-Rally, pyraclostrobin/boscalid-Pristine, trifloxystrobin-Flint, and trifumizole-Procure). Apply a treatment when disease symptoms first occur and repeat if symptoms reappear.

Common name Amount/Acre R.E.I.+ P.H.I.+
(trade name)   (hours) (days)
A. AZOXYSTROBIN
  (Quadris) 11–15.4 fl oz 4 1
  MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
  COMMENTS: Do not apply more than one application before rotating to a fungicide with a different mode of action (i.e., group number)
 
B. CINNAMALDEHYDE
  (Cinnacure) 0.25–1 gal 4 0
  MODE OF ACTION: A botanical fungicide.
  COMMENTS: Make no more than two consecutive applications before rotating to a fungicide with a different mode of action.
 
C. MICRONIZED SULFUR#
  (Thiolux, Microthiol) 4–6 lb 24 0
  MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M2)
  COMMENTS: Thiolux is not labeled for pumpkins but Microthiol is. Sulfur can injure plants, especially when temperatures reach 95°F. Do not use on sulfur-sensitive varieties.
 
D. MYCLOBUTANIL
  (Rally) 40W 2.5–5 oz 24 0
  MODE OF ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3)
  COMMENTS: Do not apply more than 1.5 lb/acre/season.
 
E. POTASSIUM BICARBONATE#
  (Kaligreen) 2.5–5 lb 4 1
  MODE OF ACTION: An inorganic salt.
  COMMENTS: Use the higher rate when disease pressure is severe. Direct contact with the fungus is required for control. Conditionally allowed in an organically certified crop; check with your certifier.
 
F. PYRACLOSTROBIN/BOSCALID
  (Pristine) 12.5–18.5 oz 12 0
  MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11) and Carboxamide (7)
  COMMENTS: Do not make more than one application before alternating to a fungicide with a different mode of action other than Group 11. Use allowed under a supplemental label.
 
G. QUINOXYFEN
  (Quintec) 4–6 fl oz 12 3
  MODE OF ACTION GROUP NAME (NUMBER1): Quinoline (13)
  COMMENTS: Registered for use on melons, including cantaloupe and watermelon. Do not apply more than 24 fl oz/acre/season.
 
H. TRIFLOXYSTROBIN
  (Flint) 1.5–2 oz 12 0
  MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
  COMMENTS: Begin applications when conditions are favorable for disease development and repeat, if needed, on a 7- to 14-day interval. Use higher rates and shorter intervals when disease pressure is severe.
 
I. TRIFLUMIZOLE
  (Procure) 50WS 4–8 oz 12 0
  MODE OF ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3)
 
+ 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. 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.
# Acceptable for use on organically grown produce.

[Precautions]

PUBLICATION

[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Cucurbits
UC ANR Publication 3445
Diseases
R. M. Davis, Plant Pathology, UC Davis
T. A. Turini, UC Cooperative Extension, Imperial County
B. J. Aegerter, UC Cooperative Extension, San Joaquin County
W. D. Gubler, Plant Pathology, UC Davis
J. J. Stapleton, UC IPM Program, Kearney Agricultural Center, Parlier

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