Fungus threatens basil plants for this year and beyond

By Adrian Higgins
Wednesday, June 16, 2010

Herb grower Steve Hershfeld first noticed his basil plants ailing in early May and thought the symptoms signaled a nutrient deficiency. Francesco DeBaggio spotted it around the same time and raced to contain its spread, but to no avail. He has lost his entire, unsold 6,000-plant inventory.

Sweet basil, the mainstay herb of summer, is under attack from an aggressive fungal disease named downy mildew. Long an affliction of other plants, the disease is new to basil in the United States and threatens to become a permanent headache for grower, gardener and cook alike.

"It's here. And it's here to stay," said Meg McGrath, a vegetable disease expert at Cornell University's research farm on Long Island. "It was something we didn't have a couple of years ago, but we now have it."

The disease was first found in Uganda in the 1930s but didn't appear again until nine years ago, in Europe. The first cases in the United States were in south Florida in 2007; it has spread since along the East Coast and in Canada, parts of the Midwest and California.

In the home garden, leaves showing early stages of the disease can be harvested for fresh use. The disease is not toxic to humans, but a leaf in decline soon becomes unappetizing to the eye, McGrath said. Infected leaves can be removed to keep the plant going, but such a labor-intensive approach is not feasible for commercial growers, who cannot sell basil disfigured by the disease. When basil is grown in massed settings -- a production greenhouse or as a field crop -- failure of the entire crop is likely. But the outbreaks are sporadic enough that consumers should not see widespread shortages, McGrath said.

Garden plants that are now healthy might become infected. The disease spores are carried in the air and also survive in seeds.

Organic fungicides are limited in effectiveness; the better preventive, McGrath said, is to grow basil in an open, breezy location.

The effects first appear as faint yellow bands on the upper surfaces of the leaves. The lower leaf surfaces become dotted with tiny gray specks. "It takes about two weeks for the disease to show up after initial infection," said DeBaggio, who runs DeBaggio's Herb Farm & Nursery in Chantilly. The greenhouse and nursery, founded in Arlington by his father, Thomas DeBaggio, is known for its broad range of culinary herb varieties and is a spring magnet for local cooks and gardeners.

Sweet basil varieties are susceptible, along with Thai types. His lemon and purple basils seemed more resistant, DeBaggio said, and two varieties, pepper basil and spice basil, "didn't seem to be affected at all."

"We have sent some scouts out to farmers markets, and several of the vendors there have it as well," he said. For his nursery, DeBaggio said, "it's a pretty major hit, and a hit to our reputation."

Hershfeld's organic wholesale company, Hillcrest Nursery in Millers, Md., grows basil year-round. The infected crop was destroyed, and he is now growing basil afresh, vigorously culling symptomatic plants and spraying the crop with an organic fungicide. "It's still around," he said. "It's not nearly as prevalent as it was."

McGrath said gardeners can expect to see seemingly healthy plants come down with the disease this summer, although environmental conditions have a major effect on whether plants get sick. She saw the mildew appear last August in her home garden, but plants raised in open, sunny areas are far less troubled than those grown in shadier beds with poor air circulation.

"I think the individual grower is going to be hit the hardest," she said. "And it may hit the organic growers harder."

Jim Simon of Rutgers University said basil farmers in New Jersey saw widespread losses last season as the disease spread northward through the Garden State. Another grim bout of mildew is expected this season, but even earlier due to heavy rains and high humidity.

Cut basil might be scarce and costlier this year in Washington area supermarkets, he said. Simon is hybridizing basil species and varieties to bestow more resistance in sweet basil types, an effort that will take a while.

"Every year it's going to get worse before it gets better," he said.


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