How to Manage Pests

UC Pest Management Guidelines

SYMPTOMS AND SIGNS

Initial symptoms of Verticillium wilt appear at rosette stage when the lower leaves wilt. As an infected plant develops, the outer whorl of leaves turns yellow, wilts, and dies. Those leaves closest to the lettuce head can yellow, die, and remain closely appressed to the head. Discolored streaks occur in the internal vascular tissues of the taproot and crown, and can be a combination of green, brown, or black.

Symptoms may appear similar to those caused by Fusarium wilt, or by severe corky root. The internal root discoloration of Verticillium wilt may also be confused with ammonia toxicity damage, though excess ammonia usually causes a brick-red discoloration of the central root core and not the black vascular streaking caused by V. dahliae.

COMMENTS ON THE DISEASE

This disease was discovered for the first time in a few fields in California's Central Coast in 1995. The causal organism, Verticillium dahliae, is a well-known pathogenic fungus that infects and damages numerous crops. As a soil inhabitant, V. dahliae can persist in the soil for an almost indefinite period of time. The pathogen produces small, black resting structures (microsclerotia) that resist weathering and drying.

MANAGEMENT

Control measures are similar to those suggested for Verticillium diseases of tomato, strawberry, cauliflower, and other crops. If resistant varieties become available, they will be a primary choice for managing the disease. Otherwise, crop rotation for 3 to 4 years away from lettuce and other susceptible hosts to nonhosts like small grains or fumigation will be necessary.

Organically Acceptable Methods
Cultural controls are organically acceptable.

Chemical Control
Soil fumigation with effective materials is the only chemical control available for reducing soilborne populations of the pathogen.

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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