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Researchers Study Fusarium Wilt in Watermelons

Fusarium wilt is a devastating disease for many crops, but is manageable if the affected crop is rotated out of the field for several years and replaced with a crop that isn’t susceptible to the disease. Reduced land availability has made that option considerably more difficult, leading USDA-Agricultural Research Service scientists to find management techniques for watermelon crops that don’t involve rotation.

“Over the years, (fusarium wilt) has become one of the biggest constraints to yield that we have,” said Benny Bruton, research plant pathologist at the USDA-ARS South Central Agricultural Research Laboratory, also known as the Lane Research Center, near Lane, Okla. “If you keep planting watermelon to a field without rotating out, inevitably you’re going to develop a fusarium wilt problem.”

Studying fusarium wilt control in watermelon is an ongoing project at the center, dating back at least 25 years, when Bruton first arrived. Recent research has focused on studying the toxicity of the fungus, and researchers found there are several races of fusarium wilt that affect watermelon, termed race zero, race one and race two. Most watermelon varieties exhibit some level of resistance to race zero, but 75 percent to 80 percent of varieties currently grown in the United States are susceptible to the other races.

A virulent new race, termed race three, has recently been discovered in some watermelon fields. Race three is different because it is considerably more virulent than the other races. The good news is that the pathogen currently is found only in Maryland, but because the fungus can be seed-transmitted, researchers believe it could eventually spread across the United States. They aren’t waiting for it to spread and are studying it now to find control methods before it becomes a problem.

One thing that makes the races difficult to understand is that they can each produce different toxic compounds affecting the plants, and researchers are trying to understand how these compounds may be involved in the wilt process.

“What makes a race different than another race? Is it in the ability of the fungus to get into the root of a plant? Or in the release of toxic compounds and the intensity of their toxicity to the plants?” asked Wayne Fish, research chemist at Lane Research Center. “What we’re finding out with watermelon fusarium wilt is if you take two races and isolate the compounds toxic to watermelon plants, you don’t always find the same chemical.”

Scientists are studying the toxic compounds to find out how much of each kind of toxin the fungus releases and why strains tend to attack a particular type of plant instead of others.

Fusarium wilt strains affect individual crops, but those pathogens don’t affect other plants. Immunity to certain strains has led to the success of grafting in Europe and Asia, where watermelon plants are grafted onto rootstock of other cucurbits, such as gourd, that are unaffected by the fungus affecting the watermelon rootstock.

Grafting never gained popularity in the United States due to high costs associated with the process, along with availability of fungicides and cultivatable farmland, but decreased availability of land has led to an increased problem with fusarium wilt, leading researchers to revisit the practice.

The watermelons were grafted onto rootstock of squash and gourd, and researchers found that the grafting works well to control fusarium wilt.

“The plots held up very, very well,” Bruton said. “You can control a number of soilborne diseases by grafting. For fusarium wilt, it works beautifully.”

Previous research had indicated problems with grafting, namely that the taste seemed affected by the process, but Bruton believes that may not be a problem after all.

“(Grafting) delays harvest about a week,” he said. “If you take any watermelon fruit and harvest five to seven days too early, it’ll have an off-flavor. We don’t think grafting weakens any flavor; the fruit’s simply not ripe.”

Scientists looked at using Indian mustard as a method for fumigating the fungus. The plant produces a natural cyanide compound, but in a small enough concentration that it isn’t dangerous to humans. When mulched into the ground, it can have a negative impact on the fungus.

Researchers chopped the plant shortly after flowering, when the plant has the largest biomass, and plowed it into the soil. The plots were irrigated with sprinkler systems in an effort to concentrate the gas, but they didn’t get the results they were looking for. That may be because beneficial microbes antagonistic to fusarium wilt are fumigated along with the fungus. By adding beneficial microbes to the soil following fumigation, control was much higher because it increased the microbial population.

Soil additives, including humates, humic acid and molasses, also show benefits to fungal control, and by adding them to the soil, Bruton hopes to create an environment that is less conducive to fungal pathogens and more favorable to the beneficial microbes.

Researchers dissolved the humates in water and applied them through a combination of irrigation and spray rigs. They also used molasses, dissolving it in a mixture of 50 percent water and 50 percent molasses and applying it through spraying rigs.

“Some of these plots are looking great, and some of these plots are wilting,” Bruton said. “We don’t know if the plants will hold up to the end of the season, but what we’re seeing right now is pretty exciting.”

Originally posted Thursday, Aug. 20, 2009