Symptoms and Signs
Typical leaf symptoms on plants with alfalfa mosaic include bright yellow blotches with some mottle. Leaves eventually develop a bronze discoloration. Internally, phloem tissue becomes necrotic, including the phloem in the roots. The disease usually causes plant death. One of the most striking symptoms is the necrotic rings and spots on the fruit. Some fruit may develop a solid brown necrosis over the surface.
Comments on the Disease
Alfalfa mosaic virus infections of tomatoes generally occur when tomatoes are grown near alfalfa. The disease is most prevalent a few rows into a tomato field near alfalfa. Alfalfa mosaic virus is seedborne in alfalfa and most alfalfa fields are infected and provide a good inoculum source. The virus is transmitted by several species of aphids; spread from alfalfa to surrounding crops is common. Aphids transmit Alfalfa mosaic virus only when probing leaf tissues and not during feeding on plants. Once an aphid acquires Alfalfa mosaic virus, it retains the ability to transmit the virus for only a short period of time (minutes to hours) and spread is localized.
Outbreaks of virus diseases are unpredictable from year to year and for various geographic locations. Use of silver reflective mulches may delay the infection by aphid-borne viruses and reduce the incidence and severity of these diseases by repelling aphids that transmit them. Place reflective polyethylene mulches on planting beds before seeding or transplanting to reduce aphid landing and virus transmission. The mulches lose their effectiveness when more than 60% of the surface is covered by tomato plant foliage.
In general alfalfa mosaic is not a major problem of California tomatoes, although it occurs in localized areas each year. The best way to control alfalfa mosaic is to avoid planting tomatoes near alfalfa fields and to avoid use of insectary plantings that contain alfalfa near tomato fields. No resistance to this virus is currently available in commercial tomato cultivars. No effective chemical control strategies are currently available. Insecticides aimed at controlling the aphid vectors are ineffective.