Clemson University researchers are noting that cooler-than-normal temperatures this spring have left South Carolina’s watermelon crop flat and more vulnerable to Fusarium infections.
During a Watermelon and Vegetable Field Day held recently at the Edisto Research and Education Center, Clemson Cooperative Extension Service Vegetable Specialist Gilbert Miller told growers the daily heat units necessary to grow, develop, and ripen watermelons were well below average this year.
“In South Carolina, watermelons need close to 800 heat units from female bloom to ripe fruit,” Miller said. “For 2023, we were about 200 heat units behind 2022.”
In addition, the South Carolina watermelon crop was harvested later than usual and there was a significant delay in soil temperatures reaching an optimal 81°F, resulting in a higher chance for plants to be infected with Fusarium.
Symptoms include yellowing, stunting and death of seedlings, as well as yellowing and stunting of older plants. Delayed transplanting, cultivar selection, and grafting can help manage Fusarium wilt.
“Delaying transplanting until the soil has warmed to about 81°F at a 4-inch depth can reduce the number of plants with Fusarium wilt,” says Tony Keinath, Clemson Research and Extension Vegetable Pathologist. “Delayed transplanting does not affect the sizes of seedless watermelon fruit or the net return per acre. Growers can use delayed transplanting by transplanting severely infested fields last, but make sure the desired market window is maintained.”
‘Eleanor’ and ‘Fascination’ are partially resistant cultivars that have been shown to perform better when Fusarium wilt is present. But grafting is more effective…and more expensive.
‘Carolina Strongback’ is a watermelon grafting rootstock developed by Clemson University and USDA scientists. It has resistance to Fusarium wilt races 1 and 2, as well as tolerance to root-knot nematodes. ‘Carolina Strongback’ was derived from two wild watermelon species, each with resistance to Fusarium wilt race 1 and 2.
“No Fusarium wilt developed on Fascination grafted with Carolina Strongback rootstock,” Keinath adds. “Yields were higher for grafted plants, but grafted plants can be twice as expensive to buy.”
Keinath suggests planting one-half of a crop in grafted plants and one-half in non-grafted plants. Hairy vetch grown as a winter cover crop may help reduce Fusarium wilt in the spring.
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