How to Manage Pests

UC Pest Management Guidelines

SYMPTOMS

Symptoms of lettuce mosaic vary greatly. Leaves of plants that are infected at a young stage are stunted, deformed, and (in some varieties) show a mosaic or mottling pattern. Such plants rarely grow to full size; head lettuce varieties infected early fail to form heads. Plants that are infected later in the growth cycle will show a different set of symptoms. These plants may reach full size, but the older outer leaves will be yellow, twisted, and otherwise deformed. On head lettuce the wrapper leaves often will curve back away from the head. Developing heads may be deformed. In some cases brown, necrotic flecks occur on the wrapper leaves.

COMMENTS ON THE DISEASE

There are several sources of the Lettuce mosaic virus. Since the virus is seedborne in lettuce, infected seed is a primary way of introducing lettuce mosaic to fields. The virus can infect numerous crops and weeds, thereby creating reservoirs of the virus. Lettuce mosaic virus can also be vectored by aphids, which spread the virus within a lettuce field and introduce it into lettuce fields from infected weeds and crops outside the field.

MANAGEMENT

A lettuce mosaic control program is a good example of an integrated way of controlling a plant disease. Plant lettuce seed that has been tested for the virus and that contains no infected seed per 30,000 seed tested (Imperial and Monterey counties mandate via ordinance that only tested, clean seed can be planted in the county). Remove potential virus reservoirs (see lists below) by practicing good weed control within and outside lettuce fields, and by plowing down harvested lettuce fields in a timely manner. A lettuce-free period during winter months helps break the virus cycle (again, some counties mandate such periods via county ordinances). Good aphid management should be practiced. Some resistant varieties are available.

Organically Acceptable Methods
Cultural controls are organically acceptable.

Partial List of Potential Host Reservoirs of Lettuce mosaic virus.

PUBLICATION

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

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