How to Manage Pests

UC Pest Management Guidelines

SYMPTOMS

Varnish spot affects only the inner leaves of head lettuce varieties. To see the symptoms, remove the outer leaves of the head. Varnish spot appears as dark brown, shiny, firm, necrotic lesions on these inner leaves. Lesion size can range from small to very large, and lesion borders are not delimited by veins, but areas along veins are most often affected. In severe cases, an infected leaf may have 90% or more of its surface area diseased. A notable feature of these lesions is that affected leaf tissues are intact; the lesions are not soft or broken down. This is in contrast to secondary soft rots and slime of lettuce in which tissues are disintegrated.

COMMENTS ON THE DISEASE

This disease often occurs in places where Pseudomonas cichorii contaminates water in reservoirs. When such water is used to sprinkle irrigate head lettuce crops at the rosette stage, the bacteria are introduced into the developing head. There is some evidence that P. cichorii also survives for brief periods in the soil and is splashed up onto plants via sprinkler irrigation or rain. Varnish spot does not develop on cultivars that form an open head.

MANAGEMENT

Avoid using contaminated reservoir water when sprinkler irrigating head lettuce at susceptible stages. Rotate away from susceptible crops for 1 year. Other hosts include chicory, endive, cabbage, cauliflower, chrysanthemum, and celery.

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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