How to Manage Pests

UC Pest Management Guidelines

SYMPTOMS

On lettuce, symptoms of beet western yellows rarely develop until plants reach rosette stage. At this point in crop development, the older leaves begin to turn yellow. This yellowing continues until all the oldest, lower leaves are bright yellow to sometimes almost white in color, with the main leaf veins remaining green. Yellowed leaves often have a thick, brittle texture. Yellowing can progress until the wrapper leaves adjacent to the head also turn yellow, and head color may be unacceptably light green. In most lettuce varieties, significant stunting or reduction in plant size does not occur. Overall symptoms of this yellows disease may resemble nutrient deficiencies, such as iron chlorosis. This distinctive yellowing of older leaves sets this disease apart from other lettuce virus diseases.

COMMENTS ON THE DISEASE

The disease is caused by Beet western yellows virus, which has an extensive host range, including over 150 documented plant species (see list below). Some isolates or strains of this virus have different abilities to infect certain plants; thus, not all strains of Beet western yellows virus may be able to infect all plant species, greatly complicating the etiology of this disease.

Beet western yellows virus is vectored by several aphid vectors, especially the green peach aphid. The virus is transmitted in a persistent manner by the aphid, meaning that once the aphid has acquired the virus by feeding on infected plants, that aphid can continue to transmit the virus for essentially the rest of its life. Unlike Lettuce mosaic virus, Beet western yellows virus is not seedborne in lettuce.

Partial List of Potential Host Reservoirs for Beet western yellows virus.

MANAGEMENT

In California this virus only occasionally causes significant economic damage. General virus disease management steps, such as those for lettuce mosaic virus, apply to beet western yellows virus as well. Aphids should be controlled, although insecticide programs will not prevent transmission of this virus and disease occurrence. Virus reservoirs (weeds, volunteer lettuce, and old lettuce fields) should be eliminated or reduced by herbicide programs and cultural practices. A few lettuce cultivars are reported to be resistant or tolerant to this virus.

Organically Acceptable Methods
Cultural controls are organically acceptable.

Chemical Control
There are no chemical controls for plant viruses.

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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