How to Manage Pests

UC Pest Management Guidelines

Cucurbits

Powdery Mildew

Pathogens: Sphaerotheca fuliginea (=Podosphaera xanthii) and Erysiphe cichoracearum (=Golovinomyces cichoracearum)

(Reviewed 12/09, updated 6/12)

In this Guideline:

SYMPTOMS AND SIGNS

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 and desert production areas but is more common in fall in the San Joaquin Valley and Sacramento Valley. 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 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. Start monitoring for powdery mildew during the vegetative growth stage and continue through fruit development. Carefully monitor even those fields with powdery mildew resistant varieties, because there is evidence that plant resistance-breaking races are present in California. Strains resistant to strobilurins (group 11) have already been found throughout the state. 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 –Cabrio, pyraclostrobin/boscalid-Pristine, trifloxystrobin-Flint, and trifumizole-Procure). Apply a treatment when disease symptoms first occur and repeat if symptoms reappear.

Common name		Amount per acre	R.E.I.‡	P.H.I.‡
(example trade name)			(hours)	(days)
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 (PDF), and environmental impact. Not all registered pesticides are listed. Always read the label of the product being used.
A.	TRIFLUMIZOLE
	(Procure 480SC)	4-8 fl oz	12	0
	MODE OF ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3)
	COMMENTS: Do not apply more than 40 fl oz of Procure 480SC/acre/season.
B.	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.
C.	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.
D.	MICRONIZED SULFUR#
	(Microthiol)	4–6 lb	24	0
	MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M2)
	COMMENTS: Sulfur can injure plants, especially when temperatures reach 95°F. Do not use on sulfur-sensitive varieties.
E.	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.
F.	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)
G.	TRIFLOXYSTROBIN
	(Flint)	1.5–2 oz	12	0
	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)
H.	PYRACLOSTROBIN
	(Cabrio)	16 oz	12	0
	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).
I.	KRESOXIM-METHYL
	(Sovran)	3.2 -4.8 oz	12	0
	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).
J.	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.
K.	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. May not provide good control under all conditions.
‡	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.

PUBLICATION

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
• 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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