Beet Curly Top Virus (BCTV, curly top) is a very destructive tomato pathogen, widespread in multiple strains across the semi-arid regions of the Western United States. It can spell disaster not only for hopeful backyard tomato gardeners, but also small-scale farms that rely on income from high-value heirloom tomatoes. TomatoCulture is a case in point. In 2015, BCTV caused a 40% loss in potential heirloom tomato yields. A fellow New Mexico farmer reported pulling nearly 100 curly top-infected plants per day at one point during the season. Small-scale farmers are not the only ones impacted. A severe 2013 outbreak of BCTV in California’s San Joaquin Valley caused tomato losses that exceeded $100 million (CA Tomato Growers Association, UC Davis, 2014).
The beet leafhopper (Neoaliturus tenellus) is the only known vector of curly top. According to plant pathologists at New Mexico State University (NMSU), and confirmed by several other Western universities, BCTV is very difficult to control. Although there is ongoing research, there is still no known resistant tomato variety.
High BCTV percentages in crops are consistently related to high October and November rainfall in New Mexico (Romney 1939; R. Creamer, unpublished data). Adult leafhopper populations increase in mid-April, remain high through the early summer, and decrease towards fall (Creamer et al., 2003). Young tomato plants are highly susceptible and, if infected, bear no fruit and die prematurely. Stress factors later in the season, like blazing sun, heat and drought, can cause symptoms to emerge in older plants.
The telltale signs that a tomato plant is infected with curly top virus are: swollen purplish veins on the undersides of leaves that have turned pale and leathery, and curled inward; drooping branches; stunted growth; loss of vigor; and a sick-looking overall pallor to the plant.
Fruit that has developed on the infected plant will stop enlarging and ripen quickly, but will not be edible. Basically, curly top is a heinous tomato disease that will almost surely kill a young plant and severely stunt a mature plant. Once infected, there is nothing you can do. One bright spot with this disease is that it can only be spread by the beet leafhopper. The disease cannot move from one plant to another without transmission by the insect.
Management strategies for large-scale tomato growing have focused on spraying herbicides to control the various winter weeds on which the beet leafhoppers overwinter and feed, and using insecticides to control the leafhopper itself. Researchers suggest controlling the major host weeds (such as London Rocket) as the most sensible area-wide approach to managing beet leafhoppers because these weeds serve as virus reservoirs (Creamer et al. 2003). However, the beet leafhopper is a very mobile disease vector, making unclear the benefit of weed control efforts. Spraying for the leafhopper has also proven ineffective, although it may prevent some infield spread on large acreages (NMSU publication H-106).
Other recommended controls pertain mostly to home gardeners and include cultural practices that reduce the attractiveness of tomato plants to the leafhoppers. A publication from Oklahoma State University indicates that the beet leafhopper is most attracted to “widely spaced, vigorous plants grown in open areas where the plants sharply contrast with the surrounding soil.” Defensive strategies include:
- Control cool season weeds – Especially London rocket. Pull these as much as possible around your garden in the late winter and early spring.
- Plant densely to confuse the leaf hoppers. Tomatoes can be spaced 30 inches apart.
- Use Row covers – row covers are your best single strategy to keep the leaf hoppers from accessing your plants. Covering your plants or wrapping your tomato cages with row cover fabric or clear plastic creates a nice barrier and greatly reduces the risk of disease transmission.
- Provide partial shade for your plants. NMSU suggests that home gardeners plant tomatoes in light shade, as beet leafhoppers prefer to feed in bright sunshine. In addition to providing a deterrent to marauding leafhoppers, partial shade gives tomatoes a much-needed break from the hot afternoon sun. A conveniently located tree or a building is the easiest way to provide shade from the Western sun; alternatively you can build a small shade structure and use 40% shade cloth.
- Use living mulches and intercropping. Tomatoes can be interplanted with a number of “companion” plants that help increase biodiversity and confuse or deter pests like the leafhopper. Good companions for tomatoes include: basil, marigolds, sage, nasturtiums, sunflowers, onions, chives, garlic, parsley, thyme, borage, and carrots.
At the farm, TomatoCulture is in the second year of a two-year research project, funded by a grant from Western SARE, to test an integrated approach to beet leafhopper control and reduced incidence of BCTV in our tomato fields. Control strategies include: dense spacing; row covers; weed barrier; living mulches; insectary plantings for natural leafhopper enemies; and a shading system. We are testing and comparing two innovative shading strategies: 1) A fabric shade cloth, suspended above the rows using infrastructure already in place for trellising the tomatoes; and 2) Inter-planting with wild sunflowers (Helianthus annuus L.), which grow taller than the tomatoes, providing dappled shade. We will share our study results when the project is finished.