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
"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
In the meantime, farmers’ basil crops are at risk, and some have endured
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
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, sweetwatergrowers.com.
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.