Your watermelon crop is a week or two from harvest when vines and leaves begin to yellow, crumple and die in a matter of days. Or you’ve shipped produce cross-country, where unsuspecting customers cut into fruit that looks fine on the outside, but brown and gummy inside.

Both scenarios have been real for Florida watermelon farmers hit by a fast-moving disease called vine decline, which has ruined an estimated $60 million worth of crops since it first appeared in southwest Florida in the spring of 2003.

But University of Florida scientists who’ve been studying vine decline since then say they believe they’ve discovered what causes it — and can now begin working on ways to control it.

Pam Roberts, an Institute of Food and Agricultural Sciences plant pathologist, says researchers now think the silverleaf whitefly spreads a virus to the watermelon by feeding on infected plants and moving on to new healthy ones.

“There’s still some confirmation work to be done,” she says. “We’re not 100 percent sure — but we’ve got pretty conclusive field evidence that has confirmed the role of the virus.”

The whitefly is an old enemy in agriculture, but one that’s traditionally plagued tomato farmers, she said.

“There’s been some kind of a shift, where now they’re really shifting to watermelons,” says Roberts, who is based at UF’s Southwest Florida Research and Education Center in Immokalee.

Now that they’ve zeroed in on the likely culprit, Roberts says, researchers can begin looking at ways farmers can control the virus-spreading flies, whether by using things like reflective mulch or different pesticides.

To determine the cause, researchers put squash inoculated with the virus in the field with healthy watermelons. Some of the watermelons were in cages that protected them from the whitefly, while others were exposed.

Of the exposed watermelon, about 98 percent suffered vine decline, Roberts says, while the caged watermelon stayed healthy. Now they’re testing the unhealthy plants to be sure they have the virus, and to ensure the disease hasn’t been transmitted to the protected plants.

“But just by looking at it, it’s fairly obvious what happened,” she says.

Roberts is part of a team that included Scott Adkins, Benny Bruton and Chandrasekar Kousik of the U.S. Department of Agriculture, Carlye Baker of the Florida Department of Agriculture and Consumer Services, UF Extension agents Phyllis Gilreath and Gene McAvoy, UF researchers Rosa Muchovej and Diann Achor, and UF nematology and entomology experts Phil Stansly and Susan Webb.

Bob Morrissey, executive director of the Plant City, Fla.,-based National Watermelon Association, says vine decline has been so devastating that some farmers have bailed out of farming altogether.

“It has cost the Florida growers alone over $60 million in the last four years,” he says. “If that doesn’t state what the problem is, then nothing really will. Sixty million to the watermelon industry is absolutely monstrous.”

David Coates, who owns Coates Farm Produce, Inc., is a major watermelon producer and shipper, farming in both Florida and Indiana. His crops have been hit by vine decline about six times.

“The biggest problem so far is that it doesn’t manifest until a week or two before harvest, then it takes no time at all, it sweeps right through a field and you’re done,” he says. “It definitely is not a lot of fun.”