How to Manage Pests

UC Pest Management Guidelines

SYMPTOMS AND SIGNS

The powdery mildew fungus appears as a white, powdery growth on both upper and lower sides of lettuce leaves. Such infections may cause deformity and buckling of the leaves. In advanced stages, leaves may begin to dry out and turn brown. On occasion, the small, brown, spherical structures of the pathogen's perfect stage (cleistothecia) may be seen on the leaves.

COMMENTS ON THE DISEASE

Powdery mildew on lettuce is favored by warmer, dryer conditions and occurs sporadically during the summer months. Spores are dispersed in the air via wind currents. Chains of spores, which are visible with a hand lens, distinguish powdery mildew from downy mildew, which has branched fruiting bodies. Powdery mildew is usually of minor economic importance in coastal areas but is more of a concern in desert growing areas.

MANAGEMENT

Begin treatments when powdery mildew is first observed on plants.

Common name		Amount/Acre	R.E.I.+	P.H.I.+
(trade name)			(hours)	(days)
When choosing a pesticide, consider the general properties of the fungicide as well as information relating to environmental quality.
A.	SULFUR#
	(Microthiol Disperss)	5–6 lb	24	0
	MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M2)
	COMMENTS: Do not apply sulfur if temperatures will exceed 90°F within 3 days of application. Thorough coverage is required.
B.	AZOXYSTROBIN
	(Quadris)	12.3–15.4 fl oz	4	0
	MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
	COMMENTS: Alternate with a fungicide, such as sulfur, that has a different mode of action Group number. See label for special phytotoxicity precautions.
	+  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. # 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 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.

PUBLICATION

UC IPM Pest Management Guidelines: Lettuce
UC ANR Publication 3450
Diseases
S. T. Koike, UC Cooperative Extension, Monterey County
R. M. Davis, Plant Pathology, UC Davis

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