Nothing spells disappointment more during pesto-making season than visiting your basil patch only to find your carefully tended crop ravaged. But that is exactly what happened to Meg McGrath, a plant pathologist based at Cornell’s Long Island Horticultural Research and Extension Center, in August 2008.
McGrath was attending a scientific meeting in Europe when a colleague forwarded a concerning email from a Long Island herb grower. By the time McGrath returned home, she discovered the basil in her own garden was infected. And the grower’s crop was totally unmarketable.
The culprit: basil downy mildew — an imported plant disease that had recently arrived in the U.S.
“It was sad to see what had been a beautiful basil crop rendered unusable by this new disease,” she said. “But I was scientifically intrigued by the quick spread of this new pathogen in the United States.
“And I also wondered about the batch of pesto my husband made and froze while I was away. The color did look a little brownish, likely due to spores he didn’t notice on the underside of the leaves.”
Wind speeds spread
Since that season, McGrath has been on a mission to help growers and gardeners fight the disease, enlisting their help to track and limit its spread.
“Knowledge about where and when a disease occurs is important for management,” said McGrath.
Basil downy mildew is caused by Peronospora belbahrii, a fungus-like oomycete, one of a group of microorganisms that includes some of the biggest threats to global food security and natural ecosystems. It was first noted in Uganda in 1932, but wasn’t reported again until 2001 in Switzerland. It hopscotched through Europe and then jumped to Florida in 2007 (likely through contaminated seeds) the fall before McGrath found it on Long Island.
Leaves on infected basil plants turn yellow between the veins. (See images in slideshow below.) This is sometimes misdiagnosed as a nutrient deficiency. While not harmful to eat, infected leaves lack the full flavor and deep green color of healthy leaves.
The leaves soon turn entirely brown and fall off. But before they do, the undersides become covered with copious amounts of dusty brown spores. What makes the disease so devastating is that these spores can quickly spread far and wide on the wind to infect plantings miles away.
“It’s not easy to escape basil downy mildew because the spores are so well dispersed by wind,” said McGrath. High humidity common during Northeast summers also favors infection.
Urban crops are not immune, as McGrath regularly receives reports of symptoms on basil grown high up on city apartment balconies.
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