   Curly Top Virus - August 1, 2001 Jeff Schalau, County Director, Agent, Agriculture & Natural Resources Arizona Cooperative Extension, Yavapai County As many Verde Valley tomato growers are aware, this summer has been tough going. I have visited several farms and gardens and seen many dead and dying tomato plants. I also collected plant samples from two locations in Camp Verde and sent them to the University of Arizona Cooperative Extension Plant Pathologist, Mary Olsen. We think it's beet curly top virus, but we are trying to confirm it. It will take some time, but we should soon know. Regardless, many Verde Valley farmers have pulled up 20 to 80 percent of their tomato plants this year. Viruses are very difficult to control and often result in severe crop losses. It is also hard to predict severity and incidence from season to season due to complex interactions that exist among the pathogen (virus), host (plant attacked), vector (organism that transmitted the virus), virus source, and environment. Because of these uncertainties, it is difficult for growers to adopt control strategies and apply them year-to-year. Viruses are submicroscopic parasites that must infect a host cell to reproduce themselves. In essence, a virus is a protein covered (and sometimes lipid membrane bound) package containing DNA or RNA. They do not grow once formed. Instead, they invade their host's cells and replicate. After replication, they often kill the host cell and are released to infect new cells. Measles, mumps, yellow fever, poliomyelitis, influenza, and the common cold are all viruses that affect humans. Curly top virus (CTV) is widely distributed in Arizona. CTV is transmitted by the beet leafhopper (Curculifer tennelus) and it attacks many crops including tomatoes, beets, peppers, squash, beans, cucurbits (squash, melons, and cucumbers), spinach, potatoes, and other crops. Weeds and ornamentals are also susceptible. As the plant becomes infected, leaves become puckered and stunted. Tomato leaves curl and roll upward and the main leaf petiole curves downward. The leaves also become leathery and turn yellow. Eventually the plant stops growing and dies. The infected tomatoes are also bitter tasting. Am I painting a familiar picture? CTV is not transmitted in plant seeds, but can be spread in potato seed pieces. Likewise, the virus is not spread from generation to generation in leafhopper eggs. The leafhopper ingests cell sap, which contains the virus. The virus incubates in the leafhopper for 4 to 21 hours before it can be transmitted. It is faster in hotter temperatures. Once incubated, the virus is transmitted to a host plant by the leafhopper. Symptoms appear after 24 hours in hot temperatures and more slowly in cooler temperatures. Once you see the symptoms, it is best to pull the plant out and destroy it. Spraying insecticides on tomato plants is not an effective leafhopper control strategy. In fact, leafhoppers do not prefer tomatoes as a food source. They inadvertently land on the plant and feed then move on. Their preferred food source is often weeds. As the weeds dry up, the leafhoppers move into cropped areas to forage. There is little secondary spread of CTV from one tomato plant to the next within a field. Random infection patterns observed in most situations support this idea. The best control strategy is to control weeds adjacent to cropped areas before transplants are planted. Tumbleweeds (Russian thistle) are a favorite food source of the leafhoppers. Fine mesh barriers are another strategy to prevent leafhoppers from feeding. There are also four CTV resistant tomato varieties. These are: Roza, Rowpac, Columbia, and Saladmaster. These four varieties are also resistant to Verticillium and Fusarium. Interestingly, in other parts of the state, chiles are the most seriously affected crop. In Prescott Valley and Dewey, I have seen beans, tomatoes, squash, and peppers that had symptoms resembling CTV. I will notify the world with the results of the University of Arizona CTV tests when they are available. Until then, enjoy the crops that are not diseased. The University of Arizona Cooperative Extension has publications and information on gardening and pest control. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 or E-mail us at mgardener@kachina.net and be sure to include your address and phone number. Find past Backyard Gardener columns or submit column ideas at the Backyard Gardener web site: http://ag.arizona.edu/yavapai/anr/hort/byg/. |