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Downy mildew fungus threatens U.S. basil crop

by Christina DiMartino | August 01, 2010
A fungus is threatening the U.S. basil crop for both home growers and commercial operations.

"In the U.S., downy mildew on basil first reared its ugly head on a commercial farm in Florida in 2007," Margaret McGrath, associate professor in the department of plant pathology and plant-microbe biology at Cornell University in Ithaca, NY, told The Produce News. “It appears that other commercial growers also had it, but because it's new to the U.S., they didn’t know what it was when they first detected it.”

Since then, the fungus has surfaced in both commercial farms and backyard gardens in several states including Georgia, Pennsylvania, New York, Virginia, Wisconsin, Ohio and New Jersey.

“In 2008, it was also reported in Canada and in a greenhouse in Argentina,” added Ms. McGrath. “In 2009, it was reported in California.”

Ms. McGrath said that the fungus is believed to have originated in Uganda in 1923, but it quickly slid off the radar with no reports until 2001, when it showed up in Europe.

“It is believed that it moved from Uganda to Europe, where it spread quickly and widely,” she said. “It’s a pretty big problem in Europe today. Florida growers are dealing with it year round now, and occasional outbreaks are reported in other areas.”

The pathogen makes basil plants unmarketable because it attacks the leaves, which is the part of the plant that is consumed. It is host-specific in that other plants near infected basil plants are not affected. Similar to other downy mildew fungus, it spreads by seed and by air. Ms. McGrath said that it is believed the fungus entered the United States with seeds from Europe, but there is no precise evidence to prove this.

Downy mildew fungus cannot be eliminated, “but it can be controlled,” said Ms. McGrath. “There are some fungicides that help, and there is research underway for other products both at Cornell University and at the University of Florida. Hopefully, we will see some new controlling agents in the near future.”

In the meantime, farmers’ basil crops are at risk, and some have endured major losses.

James Dault, owner of Sweetwater Growers, a greenhouse operation in Canton, GA, that produces microgreens, watercress, arugula and basil, is one company that suffered major losses from downy mildew. In business since 2003 and with about two acres of greenhouses, the fungus first showed up at the farm in late 2008.

“The problem continued into early 2009, so we sent samples to a laboratory for testing,” Mr. Dault told The Produce News. “At the time, I had about one acre of basil. I lost the entire crop. We replanted another crop in 2009, but that, too, became infected, so we lost it. In all, we had a couple hundred thousand dollars in losses.”

Nearby farm Neva Hydroponics in Bowersville, GA, also had problems with downy mildew, but it was using a few new processes to combat the fungus, Mr. Dault said.

“We followed their processes and it worked,” he said. “We increased our ventilation, which gives the plants more air, and we use grow lights, which increases the heat. We switched to natural gas heating and raised the temperature of the greenhouse to 75 degrees. We also use fans, which, in addition to the grow lights, help to keep the plants dry. We haven’t changed our chemical application, but we are using some phosphite treatments as a preventative measure.”

In June 2010, the two companies merged to create Sweeva Corp., and Mr. Dault is its chief financial officer.

“I would be happy to answer questions that other growers have about the methods we used to get the downy mildew problem under control,” offered Mr. Dault, who can be reached through his web site,

Ms. McGrath said that unlike Mr. Dault, one of the problems with growers who produce basil along with other herbs or vegetables is that they just throw in the proverbial towel on their basil crop when faced with the fungus rather than try to fight it. “If they need a separate — and costly — chemical to control it, some feel it’s just not worth the expense or the investment of time and labor compared to what their other plants can yield,” she said.

In the field, plants fare better against downy mildew when they are spread out so that sunshine and air can better reach them. But that creates a space issue for many growers. Also, downy mildew thrives on humidity, so extra attention has to be placed on planting in low-humidity areas or with special equipment. “You really need to be in a prevention mode with this fungus,” said Ms. McGrath.