Lettuce Mosaic Virus Background

Overview:

Lettuce Mosaic Virus (LMV), a seed-borne potyvirus, is a major disease of lettuce, with considerable economic impact. The worldwide exchange of seeds has contributed to the spread of this virus to virtually any country where lettuce is grown.

LMV can infect all lettuce types (Crisphead, Cos, Boston, Bibb, leaf, etc) as well as escarole and endive. Other susceptible hosts include pea, sweet pea, marigolds and wide range of weeds found near production fields.

In the field, the primary means of introducing LMV, are infected seed. LMV is seed-borne in all lettuce types, (not in endive). Aphids, on the other hand, can efficiently transmit this virus, therefore, contributing to the secondary spread of LMV in and between fields; from either infected lettuce plants or from weeds.

Seed-borne infected plants show mosaic and stunting and will never develop marketable heads. Later infections (secondary spread) could result in mosaic, leaf puckering, and deep or accentuated serration of the leaf margins in most lettuce types. Endive heads may be stunted, but generally show only mild mosaic symptoms. The diagnosis of this disease in the field may be at time difficult due to the presence of other viruses on the plants.

Control:

Disease-free assayed stock seed, cultural practices and resistant varieties are the best ways to keep this disease under control.

1. Seed Assays for LMV

Testing seeds/seedlings, by the enzyme-linked immunosorbent assay (ELISA), is by preference the worldwide protocol used by commercial seed companies, to ensure disease-free stock.

Infection thresholds for seed lots, based on ELISA testing, differ according to the epidemiological aspects of any particular area (how secondary infection progresses).

In the USA, due mainly to a high population of aphis in the fields, the threshold is zero in 30,000 seed infection. In Europe the testing threshold is zero in 2,000 seeds The nature of the lettuce acreage and production system, plus lower population of aphids, allows the industry there to tolerate much more LMV-infected seed than the growers in the US.

2. Cultural Practices

a. Weed control: Weeds can be a significant reservoir of the virus and the source from which aphids obtain the virus, therefore, weeds must be regularly controlled and removed in the lettuce production areas.

b. Plow-down of old lettuce plantings: Old, infected lettuce plants, like weeds, are a source of virus. Aphids can pick up the virus from these old plants and readily transport it to younger, nearby lettuce plantings. It is important to plow down, in a timely manner, old plantings once they are harvested.

c. Lettuce host-free period: In places like California, where more than one Lettuce crop per season is possible, a host-free period is enforced for two weeks in December. This step is effective because LMV is an obligate pathogen and cannot survive in nature without living plant hosts. This ban on lettuce production reduces the amount of virus that would "bridge" over from one season to the next.

d. Aphid control: Aphid control slows down the spread, but does not prevent transmission, because the aphids can transmit the virus before the pesticide acts to kill it.

3. Resistant Lettuce Cultivars

Commercial cultivars resistant to LMV are available. Check with your local seed dealer to determine if there is one suitable for your area.


All these management approaches are based on the epidemiological factors controlling the spread of the disease and integrating them into industry and grower’s standards will keep this disease manageable and under control. A combination of all or most these approaches may be suitable for a particular lettuce growing area. Check with local agricultural authorities before implementation.

The QualiPlate Kit for Lettuce Mosaic Virus screens for the presence of LMV in seed and leaf extracts obtained by different grinding methods. The antibody used in the kit has been shown to be reactive to at least six geographical isolates of the virus. In studies on seed lots determined to be LMV positive by other test methods, this kit was able to consistently detect the presence of the virus (using minimum sample sizes of 30,000 seeds and minimum sub-sample sizes of 500 seeds).

Cultural practices, resistant varieties and disease-free stock seed are the best ways to keep this disease under control.



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References:

U.C. Davis, Central Coast Agriculture Highlights newsletter, February 1998

Cornell University Cooperative Extension, “Virus Diseases of Leafy Vegetables and Celery” Fact Sheet Page: 737.00

USDA – Agricultural Research Service, Agricultural Research magazine, February 2007