Clemson Vegetable Pathologist Cautions Growers About Potential Diseases

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Pictured is downy mildew disease’s impact on vegetables.

By Clint Thompson

One Clemson Extension vegetable pathologist has observed downy mildew disease and fusarium oxysporum f. sp. niveneum disease on his research plots and cautions vegetable growers in South Carolina to be on the lookout for both.

“This is strictly based on observations from my research plots here in Charleston. I have not been out to any commercial fields,” Tony Keinath said.

Keinath reported downy mildew, a cool-season pathogen, on brassicas and can especially impact kale and collard.

“For downy mildew on collard, the pathogen prefers to grow in cool temperatures. Once the temperature reaches 85 degrees F, the pathogen basically goes dormant until late fall. And it probably survives in crop debris. It’s dormant, it’s not active at all,” Keinath said.

South Carolina has experienced cooler temperatures this spring, especially the timeframe between the end of April and first two weeks in May. Downy mildew mainly affects the lower leaves but will sometimes move to the larger leaves in the middle of the plant.

“Because it causes leaf spots on the leaves and the leaves are what the growers are harvesting … even a small amount of downy mildew means the leaves can not be harvested,” Keinath said.

Fusarium’s Impact

Fusarium is also active in cool soil temperatures. It infects roots when the soil temperatures are below 82 degrees F. Unfortunately, if growers observe fusarium damage, there isn’t anything they can do to treat the disease. Fungicides need to be applied during transplanting.

“If you spray them at this stage it’s too late because all the fungicides do is prevent infection. Once the fungus has gotten into the plant, then the fungicide does not work anymore,” Keinath said. “If they want to use grafted plants because the rootstocks are resistant to fusarium, that’s obviously something that has to be done at transplanting.

“They should make note of how widespread it is in a given field this year. Then if it’s (bad) enough to cause yield loss, they need to plan to use a control measure the next time they plant watermelons in that same field.”

There are different control measures available. One could be the watermelon variety and if it’s tolerant to the disease. Growers can also utilize varieties that produce medium to large size fruit. If you plant a variety that was bred to produce small fruit and you plant that in a fusarium-infested field, then a lot of those fruit will not reach marketable size.

Growers can also utilize hairy vetch as a winter cover. It will provide some suppressive effect on fusarium. Producers can also apply fungicides to the row bed before they lay plastic. Or they can do it through drip irrigation the same day as transplant.