   Curly Top Virus - July 28, 2004 Jeff Schalau, County Director, Agent, Agriculture & Natural Resources Arizona Cooperative Extension, Yavapai County It appears that curly top virus (CTV) is impacting local vegetable crops this growing season. I have visited gardens or received reports of CTV from Sedona, Chino Valley, Camp Verde, and Cottonwood. The virus has been reported primarily on tomatoes, but it can affect other crops as well. Curly top virus has been an ongoing issue in the Verde Valley. The first records of CTV in our area come from Camp Verde growers in the 1860s and it probably impacted Native American farmers prior to that time. The last serious outbreak recorded locally was in 2001 when many Verde Valley farmers lost 20 to 80 percent of their tomato plants. Virus diseases in crops are very difficult to control and can result in severe 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. CTV is widely distributed across the west and is transmitted by the beet leafhopper. Beet leafhoppers feed on 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 yellowish. Eventually the plant stops growing and dies. The infected tomatoes turn red even when immature and edible size fruit may be bitter tasting. 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. Once incubated, the virus is transmitted to a host plant by the leafhopper. The disease is transported through the phloem tissue and symptoms usually begin to 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, feed, then move on. Their preferred food source is often weeds. As the weeds dry up, the leafhoppers move into cropped areas to forage. As far as we know, there is little secondary spread of CTV from one tomato plant to the next within a field. Random infection patterns support this idea. One preventative strategy is to control weeds adjacent to cropped areas before transplants are planted. Tumbleweed (Russian thistle) is a favorite food source of beet leafhoppers. Fine mesh barriers (ramie or other horticultural fabric) 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. I would be interested in hearing from other growers as to how seriously they are being impacted by CTV, on what crops, or if they have tried any of the CTV resistant tomato varieties. My phone number is 928-445-6590 ext 251 and my E-mail is jschalau@ag.arizona.edu. The University of Arizona Cooperative Extension has publications and information on gardening and pest management. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 ext. 14 or E-mail us at mgardener@verdeonline.com 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://cals.arizona.edu/yavapai/anr/hort/byg/. |