Until a few years ago, Southwestern watermelon growers thought powdery mildew to be one problem they didn't have to worry about.
Just when they thought it was safe to produce melons without injury from the disease, it popped up almost simultaneously in the Southeast, Texas, Oklahoma and Arkansas.
“We think this fungus is a new mutation,” says Angela Davis, a U.S. Department of Agriculture-Agriculture Research Service geneticist at the Lane Ag Center, Lane, Oklahoma. “Two races have emerged,” she said.
Davis said the fungus has shown resistance to one of two fungicides farmers use on watermelons, leaving them to rely on the other almost exclusively. “This new powdery mildew has shown resistance to Quadris. Flint still works, but if growers don't use Flint properly, we could see resistance to it as well.”
One solution will be plant resistance, and that's what Davis and other university and USDA researchers are trying to find.
“We're looking for resistance in wild citron lines,” Davis said. “We've found a couple of lines with resistance to Race 1 and are screening for resistance to Race 2. We're working in cooperation with scientists at the USDA lab in Charleston and with scientists at Texas A&M.”
Identifying resistance in wild strains and incorporating it into marketable watermelon varieties takes time, Davis said. “We have two seed companies interested in working with these resistant wild strains and trying to get something that's marketable.”
Davis uses traditional breeding techniques to develop marketable varieties. “It will take multiple generations to get the wild citron genes out.”
She said researchers have identified wild cucurbits with resistance to all races of powdery mildew. “We want to isolate the genes and compare them with watermelons and see if we can find out why watermelons are susceptible to powdery mildew.”
She said after isolating the resistant genes in wild cucurbits, researchers will do some transgenic work to see if those genes can be transferred. “This work will give us a better understanding of how powdery mildew is controlled in resistant plants.”
She said researchers may take as long as 18 months to analyze data from the first round of transgenic work. “And those melons will be of inferior quality and not marketable. After the first round, we still have to get the resistant candidates into a breeding program.
For now, growers need to do all they can to preserve their fungicide options for as long as possible.
“Powdery mildew is carried by the wind, so even with a sound rotation program, growers are likely to have infection if conditions are right. They should check with a knowledgeable extension agent or plant pathologist when they see conditions favoring sporulation (warm and humid).”
She said early detection and treatment and alternating fungicides will improve control potential and delay resistance.
“Materials with different modes of activity may take out spores that somehow get by Flint,” she said.
Davis said research will combine traditional breeding methods with gene transfer to develop new varieties and new fungicides.
“It's a shame we had such a public relations backlash with bio-technology,” she said. Davis said resistant crops offer too much to society to be dismissed out of hand. “Long-term, we have to use bio-technology to meet the challenges of new pests and to remain competitive in a global economy. Watermelons are susceptible to so many pathogens that it's hard and expensive to grow them. The same holds for other crops as well. We've been manipulating genes for centuries in variety development, looking for mutants and breeding for those traits. Now, we do the same thing, one gene at a time, to develop resistant plants.
“Plant resistance is a superior approach to pest control. It is so much better for the environment. It saves predators and leaves no chemical residue.”