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Triple Threat for Cucurbits

 

Alicia Whidden, Extension Agent II, UF/IFAS, Hillsborough County Extension Service

Jane Polston, Professor, UF/IFAS, Plant Pathology Department

Phyllis Gilreath, Extension Agent IV, UF/IFAS, Manatee County Extension Service

Scott Adkins, Research Plant Pathologist, USDA/ARS, Ft. Pierce

 

 

Over the last couple of years, the number of whitefly-transmitted viruses in some cucurbit fields has increased to almost epidemic proportions. Growers and scientists are now dealing with 3 major viruses in cucurbits, all of which are transmitted by the silverleaf whitefly, Bemisia tabaci . The host range is similar (mostly cucurbits) but the symptoms differ.

Most growers are aware of Squash Vein Yellowing Virus (SqVYV) . Symptoms of this Ipomovirus were first seen in watermelon in Florida in the mid 1980's. It is widely distributed in SW and West Central Florida and has also been reported from southern Indiana . It is probable that this virus is native to Florida . Cucurbits are hosts, especially squash and watermelon, but Momordica charantia (balsam-apple) is also a known host and potentially an excellent reservoir of SqVYV. Symptoms of SqVYV in watermelon are death of young plants, death of vines of older plants and necrosis in the fruit, especially just inside the rind. This virus is the cause of watermelon vine decline (WVD) which Florida watermelon growers have been battling since 2003. Trials for resistance to SqVYV are being conducted by grafting watermelon germplasm onto gourd rootstock and evaluating the watermelon scions for symptoms. Several potential sources of resistance in wild type watermelons have been identified. Also being evaluated are insecticides and use of silver plastic mulch to manage SWF and thus WVD.

Cucurbit Leaf Crumple Virus (CuLCrV) is a begomovirus first seen in Florida in 2006 in squash. At the same time it was found in grafted watermelon transplants received in Georgia from the Western U.S. Known hosts include tobacco and bean. Like the other viruses, SqVYV and CYSDV (see below), CuLCrV is able to infect most cucurbits including watermelons, cucumbers, squash, and pumpkin. (Figures 1 and 2) Weed hosts are being investigated, but it is possible that balsam apple may be a host as it is in SqVYV. Initial symptoms include a chlorotic mottle pattern on foliage and crumpling of leaves. Plants which are infected early are stunted. In squash, leaves can be thickened and distorted as well as curled and crumpled. Fruit symptoms vary but severe color break was observed in yellow summer squash in 2006.

 

 

 

 

 

 

 

 

 

 

 

Cucurbit Yellow Stunting Disorder Virus (CYSDV) was not seen in Florida until 2007. It infects melons, cucumbers, gourds and winter and summer squash. Symptoms appear first on older leaves toward the center of the plant, progressing outward along vines toward growing points. Symptoms often mimic water stress. Then a yellowing between the leaf veins appears and the leaves later turn bright yellow. (Figure 3) On some, small green spots develop on leaves of certain varieties. Older leaves drop as the plant's internal transport system breaks down. This virus does affect fruit quality by reducing fruit size and sugar content, plus shortening the product's shelf life. It was first identified in cucumber and melon crops in the Middle East more than 15 years ago and in cucumbers and melons in Spain about 10 years ago. In 2003-04, it was identified in Central America and the Rio Grande Valley, Texas, and 2006 in Arizona and California where it and CuLCrV caused significant yield losses. It is not known if this virus infects wild cucurbits or other uncultivated hosts. As with some other viruses, it may cause symptomless infections in some hosts.

Management recommendations for these viruses are similar to recommendations for tomatoes and TYLCV. They include:

• Select the most vigorous and well adapted varieties
• When using transplants, use pathogen-free, whitefly-free transplants. Use caution when buying transplants that were produced in the western U.S.
• Use reflective mulches
• Treat prior to planting with nicotinoids to manage whiteflies in the field
• Apply appropriate insecticides for whitefly control during production in the field
• Don't plant in old established fields. Volunteers, especially cucurbits and balsam apple, can be a significant reservoir for these viruses.
• Post-production sanitation – pull up the plastic and plow fields under. Prevent growth of volunteers or remove all volunteers
• Maintain a host-free period between spring and fall crops

 

 

 

 

Figure 3. Melon plant infected with CYSDV showing typical symptoms on the older leaves. Photo courtesy of W. Wintermantel (USDA, Salinas , CA ).

 

 

 

 

 

References for this article and for additional information on these cucurbit viruses:

“On Guard Against Watermelon Vine Decline”, USDA/ARS, Nov/Dec 2007. Adkins, S. T., B. Bruton and S. Kousik. http://www.ars.usda.gov/is/AR/archive/nov07/vine1107.htm

“Proactive efforts underway to minimize CYSDV losses in fall desert cucurbit crops” Western Farm Press, Aug 20, 2007. http://westernfarmpress.com/vegetables/082107-cucurbit-virus/

“Whitefly-Transmitted Cucurbit Leaf Crumple Virus in Florida ”. S. E. Webb, F. Akad, T. W.Nyoike, O. E. Liburd, and J. E. Polston. UF/IFAS. EDIS ENY-477. March 2007. http://edis.ifas.ufl.edu/pdffiles/IN/IN71600.pdf

“New Virus Attacks Melons, Cucumbers and Squash”. University of Arizona , UA News. March 21, 2007. http://uanews.org/node/13185