Agriculture: Lettuce Pest Management Guidelines

Lettuce Mosaic

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