Curly top virus is widely distributed in Texas. It is most noted for destruction of tomatoes, peppers, and sugar beets, but watermelon, beans, spinach, and squash are also susceptible as well as many weeds and ornamentals.
Symptom expression changes from barely noticeable stunting and leaf puckering to death of the entire plant. Infected plant leaves become distorted through curling, twisting, and rolling. Tomato leaves become thickened and leathery. Branches bend down, leaves cup upward and twist on their petioles. Veins on the underside of leaves will turn purple in some varieties. Smaller tomato plants often turn yellow and die after expressing the other symptoms. Tomatoes, melons, and cucurbits in general appear to ripen prematurely, but have an odd taste.
Transmission is by the sugar beet leafhopper (Curculifer tennelus). The virus is not transmitted to leafhopper eggs. True seed transmission does not occur but persists in potato seed pieces. The leafhopper can retain the virus in as little as 1 minute, but maximum retention occurs after 2 days. Usually a minimum incubation of 21 hours is required within the leafhopper before it can be transmitted to another plant. Very high temperatures can reduce this time to 4 to 6 hours. Symptoms appear in 24 hours in high temperatures, 14 days in normal temperatures, and up to 30 days when cool. Severe infection is related to high light intensity, prolonged summer heat, and high evaporation. Relative humidity above 50% reduces curly top and below 35% makes it severe. It is thought that high humidity delays visits of leafhoppers.
Attempts to control leafhoppers with insecticides is not effective. Fine mesh mechanical barriers over tomato plants on ground or around tomatoes in wire cages is effective. Elimination of infected weeds to drive off leafhoppers before transplants are planted should be tried. Resistant varieties include Roza, Rowpac, Columbia, and Saladmaster. All four are also resistant to Verticillium and Fusarium. Seed of these varieties can be obtained by contacting Dr. Harold Kaufman, Texas Agricultural Extension Service, at Lubbock (806) 746-6101.