In November 2006, Cucurbit leaf crumple virus (CuLCrV), a virus new to Florida, was found in squash (Cucurbita pepo L.) fields in north central and northeast Florida. Leaves of yellow straightneck squash and zucchini were thickened and distorted, as well as curled and crumpled. The symptoms on infected yellow straightneck squash (Figure 1) were slightly different from those on zucchini (Figure 2 and 3). The leaves of yellow straightneck squash plants were rounded on the edges while leaves of zucchini plants were not. Zucchini fruit did not show obvious symptoms, but the fruit from infected yellow straightneck squash were streaked with green, making them unmarketable (Figure 4). Feeding by whitefly nymphs causes silvering of leaves of squash and blanching of yellow-fruited squash and yellow blotchiness of green-fruited squashes. The leaf silvering shown in Figure 2 and 3 is distinct from cucurbit leaf crumple disease and should be not be confused with it.
In 2007, the virus was found in watermelon and squash in five counties in southwest and west-central Florida. It continues to occur in the fall in northeast and north central Florida. Symptoms in watermelon include yellowing and crumpling of the leaves (Figure 5). Unlike squash, which is severely damaged, watermelon plants seem to recover from infection.
CuLCrV is a begomovirus first found in the western United States (Arizona, Texas and California) and northern Mexico (Brown et al. 2000; Brown et al. 2002) (CuLCrV has been known in some locations as Cucurbit leaf curl virus). CuLCrV is able to infect most cucurbits including cucumber, muskmelon, squash, pumpkin, and watermelon, and has been reported to infect bean. In fact, the virus was found in fresh market green beans in Hendry County in December of 2007 and February of 2008 (Adkins et al. 2009). Symptoms include rugosity and leaf deformation and a mild mottle (Figure 6). Honeydew, Crenshaw, and casaba melons appear to be immune (Natwick 2003). As with other begomoviruses, this virus is transmitted in a persistent manner by various biotypes of the whitefly, Bemisia tabaci (Genn.), including the silverleaf whitefly (B. tabaci biotype B = B. argentifolii Bellows & Perring). The adult whitefly must feed for a minimum of 30 minutes on the infected plant and can only transmit the virus after a delay of 6–8 hours. Once the whitefly is able to transmit the virus, it can continue to do so for days. The virus cannot be transmitted mechanically and is unlikely to be transmitted through seed.
Whitefly populations were exceptionally high in fall 2006 in north central and northeast Florida, possibly because of the drought conditions during the summer. Heavy summer rains can reduce populations under normal conditions, but whiteflies may also have become concentrated on irrigated crop plants when weed hosts wilted. These high populations may have been responsible for the high incidences of CuLCrV-infected plants in both locations, following the introduction of the virus from an unknown source.
Management of this new virus will not be much different than management of other begomoviruses in vegetable crops such as Tomato yellow leaf curl virus in tomato (Schuster and Polston 1999; Momol et al. 2001). Virus- and whitefly-free transplants should be used, and transplants produced in states where this virus is known to be a problem (Texas, Arizona, California) should not be planted in Florida. Where whiteflies are a problem, a soil-applied neonicotinoid insecticide such as imidacloprid (Admire®), thiamethoxam (Platinum®), or dinotefuran (Venom®) should be used at planting for longer season cucurbits, such as watermelon and calabaza, and possibly for green beans (imidacloprid only). If a foliar application of a neonicotinoid insecticide such as acetamiprid (Assail®), dinotefuran, or thiamethoxam (Actara®), is used instead of a soil application, it is best to apply it in the first 30 days of the crop, before flowering. In addition to protecting bees, it also will help limit the exposure of the whitefly population to neonicotinoids during the latter part of the crop cycle. Spiromesifen (Oberon®) is effective against immature stages of the whitefly as is buprofezin (Courier®), and pyriproxyfen (Knack®), both insect growth regulators. Although spiromesifen, pyriproxyfen, and buprofezin affect mostly reproduction and survival of immatures, they can help reduce secondary spread within and between fields by slowing the increase of the whitefly population. Because of concerns about insecticide resistance in whiteflies, it is critically important to observe the restrictions on the number of applications, to rotate insecticide applications among chemicals in different classes, and never follow a soil application of any neonicotinoid with a foliar application of another neonicotinoid. Further information on management of both begomoviruses and resistance to neonicotinoids can be found on the Whitefly MoA poster at the IRAC (Insecticide Resistance Action Committee) website http://www.irac-online.org.
In addition to starting with virus- and whitefly-free transplants, other effective cultural controls include avoiding planting next to older, whitefly-infested crops (virus and whitefly hosts such as beans and other cucurbits, but also whitefly hosts such as cabbage, collards, peanuts, tomato, cotton, and soybeans). Use UV-reflective mulches, which repel migrating whiteflies in the first few weeks of the crop (until canopy closure), thus delaying the introduction of virus. These mulches also repel aphids and will give additional benefits by reducing early spread of aphid-transmitted viruses. Remove weeds from fields, as they can be hosts for whiteflies, and can interfere with thorough coverage with insecticides applied for whitefly control.
Adkins, S., S. E. Webb, D. Achor, P. D. Roberts, and C. A. Baker. 2007. Identification and characterization of a novel whitefly-transmitted member of the family Potyviridae isolated from cucurbits in Florida. Phytopathology 97:145–154.
Adkins, S., J. E. Polston, and W. W. Turechek. 2009. Cucurbit leaf crumple virus identified in common bean in Florida. Plant Dis. 93: 320.
Brown, J. K., A. M. Idris, C. Alteri and D. C. Stenger. 2002. Emergence of a new cucurbit- infecting begomovirus species capable of forming viable reassortants with related viruses in the Squash leaf curl virus cluster. Phytopathology 92: 734–742.
Brown, J. K., A. M. Idris, M. W. Olsen, M. E. Miller, T. Isakeit, and J. Anciso. 2000. Cucurbit leaf curl virus, a new whitefly transmitted geminivirus in Arizona, Texas, and Mexico. Plant Dis. 84: 809.
Momol, T., S. Olson, J. Funderburk, and R. Sprenkel. 2001. Management of Tomato yellow leaf curl virus (TYLCV) in tomato in north Florida. PP-184, 2 pp. Gainesville: University of Florida Institute of Food and Agricultural Sciences.
Natwick, E. T. 2003. New virus disease in melons and squash is identified and described. http://ucanr.org/delivers/impactview.cfm?impactnum=140
Schuster, D. J. and J. E. Polston. 1999. Whitefly Management Guide: Tomato yellow leaf curl virus. Supplement in Citrus and Vegetable Magazine/The Grower, Summer 1999, pp. A6–A7.