Tomato curly top disease is caused by the Beet Curly Top Virus (BCTV). Curly top disease, caused by viruses in the genus, Curtovirus, has impacted western US agriculture for over a century, and is a significant threat to tomato production. The two most abundant curtovirus species today are Beet curly top virus (BCTV) and Beet mild curly top virus (BMCTV) but other species exist as well. This post discusses the viruses that cause curly top disease.
Beet curly top virus is only spread from plant-to-plant by the beet leafhopper, Circulifer tenellus. The virus and the beet leafhopper have very wide host ranges. Once acquired by the leafhopper, beet curly top virus is carried for the rest of the leafhopper's life, and thus long distance spread is common. Infected plants are often widely scattered in a field; field margins are especially vulnerable because leafhoppers like to feed on plants that border bare soil areas. Beet curly top virus is limited to the phloem, the food-conducting tissues of the plant, and the leafhopper must feed on the phloem in order to acquire and/or inoculate the virus to plants. Only brief feeding periods (minutes) are required for the leafhopper to acquire the virus and transmit it to new plants. Plants begin to show symptoms about 7 to 14 days after they are first infected by a leafhopper. Tomato is not a preferred host for the beet leafhopper; however the leafhoppers transmit the virus to tomato while sampling it.
Leaves of infested plants are dwarfed, crinkled, rolled inward, and cupped upward. Veins on the underside of leaves usually have a purple discoloration, may be roughened, and often produce swellings or spine-like outgrowths. Roots are stunted and may exhibit a proliferation of secondary rootlets. Phloem tissues become necrotic and appear as dark rings when viewed in cross section.
Affected plants do not recover and die or remain stunted without setting additional fruit. For some time now, the problem has been attributed to feeding by the potato (or tomato) psyllid which causes psyllid yellows. In psyllid yellows, damage is caused by a toxin that the immature psyllids (nymphs) produce as they feed. Reported symptoms of psyllid yellows are very similar to those observed on the affected tomato plants.
Management of curly top disease is difficult. Efforts to breed resistance to curly top into tomatoes have been largely unsuccessful. All currently available tomato varieties are susceptible. Spraying tomatoes with insecticides does not control the disease because leafhoppers migrate from distant places and do not reproduce or remain in tomato fields. By the time migrating leafhoppers succumb to an insecticide, they have already transmitted the virus to the tomatoes. When symptoms of curly top become evident in tomatoes, the leafhoppers have long since moved away to other crops or weeds which they prefer. Removing symptomatic plants is probably a good idea, but since the vector does not remain in tomato fields, there probably is little secondary, or plant to plant spread within a field. Other management strategies have focused on using cultural practices that reduce the attractiveness of tomato to the leafhoppers. The beet leafhopper (and most other insect vectors) is attracted to widely spaced, vigorous plants grown in open areas where the plants sharply contrast with the surrounding soil. In areas where curly top is chronic, dense plant spacing, shading, row covers, and intercropping have been reported to reduce levels of curly top.
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