LETTUCE MOSIAC VIRUS (LMV) Family: Potyviridae

DISTRIBUTION: Worldwide, wherever lettuce is grown.

DESCRIPTION: LMV is a ssRNA positive-stranded virus with no DNA stage. It has flexuous (bending or winding alternately from side to side) thread-like particles approximately 750 X 13 µ m in length (Fig 1, virus particles from a purified preparation in phosphotungstate, back line represents 200 µ m).

HOSTS: Hosts include plants from nine families, including: lettuce, beets, safflower, spinach, pea and common weeds such as lambsquarter, bur clover, cheeseweed and shepherds’ purse.

DAMAGE: Symptoms of LMV 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 (Fig 2). 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. Early bolting may occur.

INSPECTION TIPS: Symptoms are most easily detected in young plants. First seen is an inward rolling of the leaves along the axis (Fig 3) and the first true leaf is irregularly shaped and slightly lobed. The field symptoms usually occur in patches (Fig 4) starting from the source of inoculum (infected seed). Leaf samples may be collected for an ELISA test.

LIFE CYCLE: Aphids and seeds transmit LMV from plant to plant. After the aphid deposits the virus on the host, it enters the cell and the coat protein is removed. The RNA is copied many times. Some of these copies remain as uncoated RNA molecules and move into adjacent cells to promote further infection. Aphids can obtain the virus after only brief contact with an infected plant and usually retain the virus for less than an hour, though a few viruses can survive up to 40 hours. Because of the brief retention period, aphids can normally only carry LMV for short distances unless they are wind blown. LMV owes its success to spread in seed, the abundance of available vectors and the ability to infect such a wide variety of alternate host plants, including weeds.

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