Agriculture: Lettuce Pest Management Guidelines

Lettuce Mosaic

  • Lettuce mosaic virus
  • Symptoms and Signs

    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 that are 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 (Monterey County mandates such a period via county ordinance).

    Good aphid management should be practiced. Some resistant varieties are available.

    Organically Acceptable Methods

    Use cultural controls in an organically certified crop.

    Partial List of Potential Host Reservoirs of Lettuce mosaic virus.
    Weeds and Other Nonagronomic Plants
    Common Name Scientific Name
    scarlet pimpernel Anagallis arvensis
    shepherd's-purse Capsella bursa-pastoris
    Carduus broteroi
    Italian thistle Carduus pycnocephalus
    lambsquarters Chenopodium album
    Chenopodium amaranticolor
    Mexican tea Chenopodium ambrosioides
    strawberry blite Chenopodium capitatum
    nettleleaf goosefoot Chenopodium murale
    Chenopodium quinoa
    city goosefoot Chenopodium urbicum
    chicory Cichorium intybus
    Cicer yamashitae
    bull thistle Cirsium vulgare
    redstem filaree Erodium cicutarium
    Lactuca livida
    willowleaf lettuce Lactuca saligna
    prickly lettuce Lactuca serriola
    Lactuca virosa
    henbit Lamium amplexicaule
    little mallow Malva parviflora
    burclover Medicago polymorpha
    Nicotiana benthamiana
    Nicotiana clevelandii
    bristly oxtongue Picris echioides
    Rumex britannica (= R. orbiculatus)
    common groundsel Senecio vulgaris
    milkthistle Silybum marianum
    spiny sowthistle (prickly sowthistle) Sonchus asper
    common chickweed Stellaria media
    Urospermum picroides
    — = no common name

    Agronomic Plants
    Common Name Scientific Name
    safflower Carthamus tinctorius
    chickpea, garbanzo bean Cicer arietinum
    escarole Cichorium endivia
    endive Cichorium endivia
    witloof chicory Cichorium intybus
    lettuce Lactuca sativa
    pea Pisum sativum
    spinach Spinacia oleracea
    New Zealand spinach Tetragonia expansa

    Ornamental Plants
    Common Name Scientific Name
    love-lies-bleeding Amaranthus caudatus
    aster Aster spp.
    China aster Callistephus chinensis
    Shasta daisy Chrysanthemum maximum
    lisianthus Eustoma grandiflorum
    gazania Gazania spp.
    globe amaranth Gomphrena globosa
    sweet pea Lathyrus odoratus
    trailing African daisy Osteospermum fructicosum
    cineraria Senecio cruentus
    African marigold Tagetes erecta
    zinnia Zinnia elegans
    Text Updated: 04/17