Curly Top of Tomatoes

Jesús Valencia, Farm Advisor, Fresno County

Curly top virus is an insect-vectored disease affecting many vegetable crops but is especially troublesome in tomatoes and sugarbeets. Infections often result in poor plant stands and yield loss. Beans, cucumber, melons, peppers, spinach, squash, plus other crops and weeds, can be affected by curly top virus. Symptoms and virulence levels are different among the agronomic and weed species. Some plants carry the virus yet do not show symptoms or negative impact. Curly top virus does not survive in the soil or decomposed plant debris. It can only survive on live plant material, and it must be picked up by the leafhopper in order to be spread from plant to plant.

Sugarbeet Leafhoppers the Vector: The beet leaf hopper (Circulifer tenellus) is the vector of curly top virus. It is small (1/8 inch long) and pale green to gray in color. The insect overwinters as an adult feeding on biennial weeds and perennial plants native to the western foothills of the San Joaquin Valley. In the spring, females lay eggs and the first generation soon hatches. New hatches do not come inoculated with the virus and must feed on infected plants to acquire the disease. However, once the virus is acquired, it can survive in the insect for life.

Rising temperatures eventually cause the host plants in the foothills to dry up, so leafhoppers migrate to the valley floor where they feed and infect crops with the virus. Leafhoppers are strong flyers and travel long distances in search of food. They efficiently transmit the curly top virus to susceptible crops from plant to plant and from field to field across the valley floor. The insect can only transmit the virus by feeding. When it probes and feeds in the plant phloem, the virus is passed on and distributed into the entire plant with carbohydrates moving in the phloem tissue. Adult sugarbeet leafhoppers are generally better transmitters of the virus than young nymphs because of their ability to fly long distances in search of food.

Curly Top Disease Symptoms: Once the leafhopper infects plants, the virus multiplies rapidly and symptoms appear 5 to 10 days after initial infection. The most recently developed leaflets start twisting and turn a yellowish color. Older leaves thicken, roll upward and become leathery with a pronounced purple venation. Plant growth declines and the vine is severely stunted by the virus. Fruit ripens prematurely leading to 100% yield loss of infected plants.

Insect Control and Disease Management: Controlling leafhoppers in the crop may not reduce the incidence of the virus, since the insects do not colonize tomatoes and new waves of leafhoppers keep coming into the field. Tomatoes are just a "pit stop" for leafhoppers on their search for more suitable hosts. Unfortunately, as they probe into the tomato to find out if they like it or not, virus transmission takes place. However, insecticide sprays can be important in minimizing in-field virus transmission. Also, since the insects seem to prefer single, isolated plants, close spacing or multiple plants in the transplant plug may be a good control strategy.

Last year, transplanted tomato fields were more affected by virus than direct seeded fields. The reason for the difference between the two planting methods could lie in the total number of plants per acre. Direct seeded fields have more plants per acre. Closer plant spacing distracts leafhoppers from landing. When they do land and transmit the virus, there are more plants to survive the virus attack causing minimal impact on yield.

Increasing plant population in transplanted fields from April on could be a good strategy in reducing losses from the virus. However, control measures are only beneficial in years with high leafhopper/virus activity. Keep in mind that leafhopper populations build up during dry and mild winter weather conditions and are cyclical. Until we learn more about leafhopper behavior, it is difficult to predict how long cycles last or when they'll repeat again.

Editor's note: CDFA sponsors the Curly Top Virus Control Program which monitors leafhopper populations, treats the western foothills, and tracks leafhopper movement in the valley. A dry winter and spring rains are ideal for leafhopper and curly top virus. Movement to the valley floor is early this year. Informational bulletins are available and mailed to growers throughout the season. Call the CTVCP in Fresno (559) 445-5472 for information and to get on the mailing list.

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