Knowledge Gained

In the early 2000s, a new condition began plaguing watermelon crops in Florida in which fruit rinds would brown on the interior and vines would rapidly collapse and die. The disease was given the name watermelon vine decline (WVD) and was responsible for $60 million in yield losses in 2005.

Back then, little was known about WVD. But soon, scientists discovered the culprit. Scott Adkins, a plant pathologist with USDA’s Horticultural Research Lab in Ft. Pierce, identified an ipomovirus in the family of Potyviridae from an isolate originally collected by Susan Webb, entomologist with the University of Florida, in squash plants that caused the veins of leaves to yellow. Hence, it was called squash vein yellowing virus (SqVYV). Through collaboration with other researchers at USDA-ARS, FDACS, and UF/IFAS, it was confirmed this virus was the cause of WVD. It also has been established WVD is vectored by the sweetpotato whitefly (AKA silverleaf whitefly).
“This is very clearly transmitted by the whitefly, but it is a very different virus in terms of other whitefly viruses we are used to here in Florida like tomato yellow leaf curl virus (TYLCV),” says Adkins. “Over the past four or five years, we’ve been working to nail down some of the biology of the virus and its transmission. There is no silver bullet for WVD, but there are a number of things you can do to help manage the virus. If you do them all, you will be in better shape than you would be otherwise.”

Whitefly Priority

In addition to WVD, two other cucurbit viruses transmitted by the whitefly have been identified in Florida — cucurbit leaf crumple and cucurbit yellow stunting disorder viruses. The combination of all three has placed an emphasis on whitefly management in watermelon and other cucurbit crops.
 
Great care should be taken to avoid the development of whitefly resistance to insecticides. Control actions for the whitefly in cucurbits will be similar for TYLCV in tomatoes, although it is likely control recommendations in cucurbits will evolve over time.

Clean And Weed

Crop hygiene should play a critical role in a grower’s approach in helping keep whitefly numbers low. These practices will help delay the initial whitefly infestation and slow the introduction of viruses into the crop.
– Establish a minimum two-month crop free period during the summer, preferably from mid-June through mid-August or longer.
– Delay planting new fall crops as long as possible and remove spring crops as early as possible to increase the summer crop-free period and avoid carryover of disease and pests.
– Try to eliminate, as much as is practical, any cucurbit weeds (balsam apple, creeping cucumber, smellmellon) that could serve as a source of viruses and whiteflies for the crop.
– Separate fall and winter cucurbit crops in time and space. Do not plant new crops near or adjacent to old, infested crops.
– If the cucurbit crop is to be a double crop, especially following tomatoes, the previous or primary crop should be thoroughly destroyed to reduce the initial whitefly population. Promptly and efficiently destroy all vegetable crops within five days of final harvest to maximally decrease whitefly numbers and sources of plantviruses.
– Destroy crops block-by-block as harvest is completed rather than waiting and destroying the entire field at one time.

Chemical Control

Sweetpotato whitefly can develop resistance to important insecticides quickly. Particular care should be taken with the neonicotinoids because of their important role in crop care. Use soil applications of neonicotinoids at planting for longer season crops, such as watermelon, so there is less chance of affecting bees pollinating the crop.
 
For best control, use a neonicotinoid as a soil drench at transplanting/seeding, preferably in the transplant/establishment water. In order to preserve the neonicotinoid-free period, do not use split applications of soil drenches of neonicotinoid insecticides (i.e., do not apply at transplanting and then again later).
 
If foliar applications of a neonicotinoid insecticide (dinotefuran, acetamiprid, and thiamethoxam are labeled for cucurbits) are used instead of soil drenches at transplanting, foliar applications should be restricted to the period before flowering because of potential toxicity to bees.
 
Information for this article was extracted from the UF/IFAS report “Recommendations For Management Of Whiteflies, Whitefly-Transmitted Viruses, And Insecticide Resistance For Production Of Cucurbit Crops In Florida.” Learn more at www.ifas.ufl.edu.

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