Sci s y 537 E.M. 3132 July 1969 W35. no. 3) 3~ c.2

CURLY TOP OF

Curly top of tomato, also known as western yellow blight, is a virus disease that almost eliminates to­ matoes as a field crop in centra_l and eastern washington. The disease is also of concern to the horne gar­ dener.

The disease can affect more than 300 broad-leaved plants, but tomato, , squash, , melons, , table beet, and pepper, as well as some flowering plants are the most common cultivated plants affected in Washington. Plants which are infected but not serious­ ly hurt by the curly top virus serve as reservoirs of the virus. Weeds are the main source of virus Fig. 1-- Pronounced rolling and purplish leaf during the spring and sugar beets veins is typical of curly top on to­ are the main source during the rna to. summer.

The symptoms vary somewhat with the plant involved. In tomato plants, there is usually a pronounced upward rolling of fully developed leaves. These leaves become yellowish with purple veins, and the foliage becomes stiff and brittle. The growing tips are stunted, and the small, youngest leaves curl and turn yellowish. Infected tomato plants decline rapidly and die. Symptoms on many plants, beets for instance, are charact~rized by stunting, leaf curling, vein swelling and vein clearing.

The curly top virus is transmitted by the beet which is found through­ out eastern washington. The leafhopper multiplies particularly well on sugar beets and on weeds that serve as reservoirs of the virus. The leafhopper can pick up enough virus in just a few minutes feeding on a diseased plant to remain in­ fective for the remainder of its life. A leafhopper can acquire virus only by feeding on a diseased plant.

COOPERATIVE EXTENSION SERVICE • COLLEGE OF AGRICULTURE • WASHINGTON STATE UNIVERSITY- • PULLMAN In cooperation with the United States Department of Agriculture Issued in furtherance of the Acts of May 8 and June 30, 1914, by the Washington State Univ.ersiiy Cooperative Extension Service, John P. Miller, Director · E.M. 3132 Page 2

After a virus-laden leafhopper feeds on a healthy plant, symptoms begin to appear in a week or two under normal temperatures. The virus multiplies rapidly in a plant. Under ideal conditions the plant may contain enough virus within 5 hours to be picked up by a feeding leafhopper but usually takes longer under most field conditions.

The disease occurs wherever the beet leafhopper is found, and environmental conditions that favor the generally favor the disease. multiply most rapidly when temperature and light intensity are high and days are long. These are the typical summer conditions in eastern Washington.

Tomato plants are most susceptible under these same conditions. Shading minimizes the disease because tomatoes are more resistant in the shade and because leafhoppers tend to avoid the shade.

Control of curly top is accomplished in some crops such as sugar beet, , and squash by growing disease resistant or tolerant varieties. Sugar oeets were once nearly eliminated as a crop in the desert regions of the United States before resistant varieties were developed. As yet, there are no suitable tomato varieties with curly top resistance.

Spraying tomatoes to control leafhoppers does not control the disease because the leafhoppers migrate from distant places and do not reproduce or remain in tomato fields. By the time the migrating leafhoppers succumb to an insecticide, they have already transmitted the virus to the tomatoes. Large scale spray programs are sometimes conducted to eliminate the leafhoppers in their breeding grounds on wild host plants, but such a program has not been attempted in Washington.

Losses from curly top in the horne garden can best be minimized by direct seeding tomatoes and by growing them in the shade of taller crops such as corn. Direct seeded plants are more resistant than transplants. The direct seeded plants should not be thinned since the disease will usually accomplish this and the unthinned plants will shade one another. Avoid planting table beets or spinach near tomatoes since these crops will be sources of both leafhoppers and virus.

Otis C. Maloy, Extension Plant Pathologist, Washington State University, Pullman, Washington and P. E. Thomas, Plant Pathologist and Mark Martin, Horticulturist, United States Department of Agriculture, Prosser, Washington.

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