WESLACO — Agricultural researchers suspect the pesky whitefly is picking up a virus from weeds and transferring it to South Texas fall watermelon and squash crops.
Squash leaf curl virus (SLCV) has become such a problem in the Falfurrias and Premont areas that many watermelon growers there have elected to sit out the season.
“That is what California growers did when they were first confronted with serious whitefly-transmitted diseases,” said Dr. Tom Isakeit, a plant pathologist at the Texas A&M Agricultural Research and Extension Center in Weslaco.
The disease was first confirmed in South Texas in the fall of 1993 on watermelons and other cucurbits, and has been blamed for large losses of watermelons and squash in California and Arizona dating back to 1977. It affects only squash and watermelon.
Symptoms of SLCV, says Isakeit, are easy to spot: Leaves are blistered and curled with yellow blotches and vines grow upwards instead of flat on the ground. Plants are stunted and fruit development is poor.
“Squash leaf curl virus doesn’t affect the taste or quality of the fruit, but it does severely reduce yields,” said Isakeit, “Yield losses of 75 percent to 100 percent have been reported.”
Isakeit suspects fall watermelon crops are harder hit than spring watermelons because there are few whiteflies around to infect the young plants during planting in winter, as opposed to high populations in August when fall crops are planted.
“Some growers wondered if this disease wasn’t being carried to watermelons by whiteflies coming off of defoliated cotton, but there’s no evidence to support this idea. I’m beginning to think that SLCV is present in some of the native plants or weeds,” he said.
Isakeit’s theory is bolstered by the fact that a pie melon weed showing the distinctive virus symptoms in Starr County last spring was tested and found to have the virus thought responsible for the development of SLCV.
“That’s one possible source,” he said, “but there could be other native plants out there serving as virus reservoirs that need to be checked.”
Once a squash or watermelon plant is infected, there is no treatment.
“The systemic insecticide, Admire, may do a good job of controlling whiteflies,” said Isakeit, “but the whiteflies transmit the virus before the insecticide can kill the whiteflies, so that is not a solution.”
Nor is there any currently available variety of watermelon or squash that shows resistance, according to variety trials conducted at the Weslaco experiment station.
“We’re trying to determine,” he said, “the feasibility of using transplants to manage this disease. Transplants may provide a virus- free head start that is needed to produce a good crop. But even if we determine that transplants offer an advantage, growers will have to decide if the added cost of transplanting versus direct seed is worth the effort.”
All of which leaves growers with only one, but very important, preventative measure to take. Isakeit urges growers to plow down crop residues and any volunteer watermelon or squash plants that may emerge after harvest.
“My greatest concern now,” he said, “is preventing the appearance of this problem in the spring crop. So it’s important for growers to destroy all crop residues and volunteers after the fall harvest to rid themselves of any sources of whitefly and virus that might carry over into the spring. This is especially important if we have a mild winter that doesn’t kill whiteflies naturally.”
Isakeit says grower meetings will be held in January to alert growers to the virus situation, current research, and the importance of crop destruction.
-30-