   Curly Top Virus…Again - August 20, 2008 Jeff Schalau, Associate Agent, Agriculture & Natural Resources University of Arizona Cooperative Extension, Yavapai County It appears that curly top virus (CTV) has again impacted vegetable crops across Yavapai County this summer. I have visited gardens or received reports of CTV from Prescott, Chino Valley, Camp Verde, and Cottonwood. The virus has been reported primarily on tomatoes, but it can affect other crops as well. For years, CTV has been an 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 2004 when many Verde Valley farmers lost 20 to 80 percent of their tomato plants. Viral diseases are difficult to manage complex interactions that exist between the pathogen (virus), hosts (plants fed upon), vector (organism that transmitted the virus), and environment. Because of these uncertainties, it is difficult for vegetable growers to adopt CTV management strategies and know how and when to apply them. CTV is found across across the western U.S. and is transmitted by the beet leafhopper (Circulifer tenellus). 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. After the beet leafhopper ingests plant cell sap containing 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 other host plants by the leafhopper. The disease is transported within the plant through the phloem tissue and symptoms usually begin to appear after 24 hours in hot temperatures and more slowly in cooler temperatures. When a susceptible plant becomes infected, leaves become puckered and stunted. Tomato leaves curl and roll upward and the main leaf petiole curves downward. In time, the leaves also become leathery, veins turn a purplish color and the interveinal leaf area turns yellowish. Infected plants will not recover and eventually the plant stops growing and dies. Infected tomatoes may ripen even when immature however, edible size fruit are likely to be bitter. Once you observe definite symptoms, it is best to pull the plant out and destroy it. CTV cannot overwinter in the soil; it must remain in a living plant. Plants cannot be infected by planting them in soil where infected plants grew last year or by use of compost from infected plants. CTV is not transmitted in plant seeds, but can be spread in potato “seed” pieces. Similarly, the virus is not spread from generation to generation in leafhopper eggs. 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, and then move on. Their preferred food source is often weeds – mustards in the spring and tumbleweed (Russian thistle) in the summer. As the weeds dry up, the leafhoppers migrate looking for green vegetation. These are often areas where vegetable crops are grown. 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. This may be of little value since beet leafhoppers are known to fly long distances. Fine mesh barriers (floating row cover or other horticultural fabric) could prove to be a viable management strategy to prevent leafhoppers from feeding. The most critical time to have the plants covered would be early to mid-season. There are also four CTV resistant tomato varieties. These are: Roza, Rowpac, Columbia, and Saladmaster. In 2005, Utah State University grew seedlings of these varieties, distributed them to Master Gardeners and found that Columbia and Roza showed the greatest potential for marketing and home production. These seeds are available, but you must scan the seed catalogs and web sites to find them and grow your own starts. 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 ext. 14 or E-mail us at cottonwoodmg@yahoo.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/. |