Curly top virus is spread to tomatoes by the beet leafhopper, an insect that picks up the virus by feeding on infected winter annual weeds. The tomato is infected as soon as the leafhopper bites the plant, so even if you spray the tomato with insecticides, the plant will be infected before the insect dies. The only effective way to prevent the disease is to keep the insect from feeding on your tomatoes in the first place. Covering the plants with row covers (lightweight fabric), similar to what is used on strawberries in California or tobacco in the south, has been successful in preventing the spread of curly top virus. Control weed populations around your tomato patch, since beet leafhoppers will most likely spend little time feeding on tomatoes before moving on to weeds and vegetables more to their liking. If curly top is a problem in your area, consider planting resistant varieties. The symptoms of tomato spotted wilt virus are often confused with curly top. Thus, a sample of the infected plants should be sent to a diagnostic laboratory for identification. Contact your local Cooperative Extension office for instructions on submitting plant samples to a diagnostic lab.

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