Lettuce Mosaic Virus

(Adapted taken from DPI Plant Path Circular No. 275 by Gail Wisler)

Dr. Carlye Baker (DPI)

Division of Plant Industry (DPI)

Lettuce mosaic virus (LMV) is member of the group of plant viruses called potyviruses. These viruses are transmitted from infected plants to uninfected plants by aphids. Unfortunately for lettuce growers around the world it is also one of a number of plant viruses that can be transmitted by seed. Research has shown that even with only 1-3% infected seed, the subsequent spread by aphids can lead to 100% infection by harvest time. Although this virus also infects common weeds such as groundsel, lambsquarters, and prickly sow thistle, infected seeds are the main source of this virus in lettuce fields. Consequently control of this virus depends primarily on the use of virus-free seed.

SYMPTOMS: Symptoms of LMV are most easily detected in young plants. First seen is an inward rolling of the leaves along the long axis and the first true leaf is irregularly shaped and slightly lobed. A mottling or mosaic pattern then develops, often with vein clearing and bronzing. As plants mature, these symptoms are absent and other symptoms must be relied on for detection. In the field this can be seen as severe stunting, yellow coloration, failure to head normally, downward curling of outer leaves(click on picture at the left) and early bolting. These field symptoms usually appear in patches originating from the source of inoculum (the infected seed) and the aphid spread from that point.

CONTROL: If the primary inoculum from seed is eliminated, even with the presence of aphids, there will be little problem with LMV in commercial production. Over the last 20 years a seed indexing program has been used to assure that the seeds planted by lettuce growers are virus free. Subsamples of all lettuce seed lots sold are tested using a sensitive serological technique called ELISA (acronym for enzyme linked immunosorbent assay). If one seed is infected the whole seed lot is rejected (See illustration at the left - the yellow wells are positive). This indexing program plus maintaining a period of lettuce-free cultivation and avoiding the planting of lettuce next to older plants has kept lettuce growing areas of the the United States virtually lettuce mosaic free.