BEET CURLY TOP DISEASE IN TOMATOES

From the OSU Oklahoma Botanical Garden and Arboretum News, Summer 2003
By John Damicone and Richard Granthan,
OSU Entomology and Plant Pathology

Tomato Problem Identified in Oklahoma

Reports and samples of tomato plants with severely stunted, pale green and curled leaves with purple-colored veins have been pouring in from across the state. Levels of the problem are very high with over 50% in some plantings in western Oklahoma. About 30% of the tomato variety trial at Bixby is affected. This problem, which resembles a virus disease, has been observed almost every year in Oklahoma since the 1980s, but usually only isolated plants are affected. This year the problem is obviously more severe and widespread. 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, the immature psyllids or nymphs produce a toxin as they feed causing the damage. Reported symptoms of psyllid yellows are very similar to those observed on the affected tomato plants. In previous years, testing of symptomatic plants with a serological screen for several common viruses that affect tomato has been negative. The negative test results for the virus have supported the "psyllid yellows" theory. However, careful examination of affected plants this year has revealed no signs of psyllids or their prior presence.

The problem also closely resembles "curly top", caused by beet curly top virus (BCTV). BCTV has long been a problem in semi-arid regions of the Western U.S. BCTV is a Gemini virus for which available serological tests are not effective. This explains why previous virus testing has not detected BCTV. Using a commercial DNA hybridization test, and a PCR test conducted at University of California-Davis by Dr. Bob Gilbertson, we have recently confirmed the problem is indeed curly top, caused by BCTV.

Curly top disease has long been a severe problem on tomatoes in the states of New Mexico, Utah, California, Washington, and Oregon; and considered the most important disease of tomato in many of these areas. The disease has a wide host range, but is particularly sever on sugar beets, tomato, pepper, and spinach. Beet curly top virus (BCTV) causes curly top. The virus is in the Gemini virus group, which, are DNA rather than RNA-encoded viruses. Whiteflies spread most Gemini viruses, but BCTV is transmitted from plant to plant by the beet leafhopper, Circulifer tenellus. Both the virus and the beet leafhopper have very wide host ranges. Once acquired by the leafhopper, the leafhopper carries the BCTV for the remainder of its life, and thus long distance spread is common. Infected plants are usually scattered in a field. The beet leafhopper acquires the virus from infected crop plants or weeds such as wild mustards and Russian thistle. Only brief feeding periods (seconds) 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.

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 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. Widely spaced, vigorous plants grown in open areas where the plants sharply contrast with the surrounding soil attracts the beet leafhopper. In areas where curly top is chronic, dense plant spacing, shading, row covers, and intercrop-ping have been reported to reduce levels of curly top.

We are unsure as to what the future holds for this disease in Oklahoma. Even in states where curly top has been a problem for many years, levels of the disease vary greatly from year to year. It is suspected Oklahoma has had curly top for some time, but it has been misdiagnosed as psyllid yellows. The sporadic nature of this disease is thought to be the result of yearly variation in over wintering leafhopper populations and their migration pattern. It is currently unknown where the virus-carrying leafhoppers originated, or where over wintering virus sources are located affecting this year's tomato crop. #

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Asian Longhorned Beetle vs. Cottonwood Borer,

Many persons are confusing the Asian Longhorned beetle with the Cottonwood Borer. For a comparison of the two see BEETLES. For more information see Asian Longhorned Beetle, and Asian Longhorned Beetle. #

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WEST NILE VIRUS

A note from the Tulsa Master Gardeners:

Dedicated gardeners spend a considerable amount of time in their gardens. It behooves us to take precautions against this threat to our well-being --
USE AN INSECT REPELLANT WITH DEET!!

More information on West Nile virus can be found in a fact sheet published by the Tulsa City-County Health Department entitled West Nile Virus, and from the OK State Health Department "On Watch".

For more information on mosquitoes go to the Mosquitoes article on this web site, and to the entomology department at OSU Mosquitoes.

For more information about DEET go to the DEET web site. #