Downy mildew resistant varieties of sweet basil now available

Four new varieties of sweet basil resistant to downy mildew are now available to growers.

Rutgers University-New Brunswick announced in June that collaborative research and variety selection had spawned seeds for new varieties resistant to the dreaded disease.

Sweet basil is used in a variety of dishes, but most people probably associate it with Italian pasta sauces and pesto. Other varieties of basil exist, including Thai, lemon and lime basil, but their flavors are not interchangeable with sweet basil. Imagine a spaghetti sauce with the spiciness of Thai basil, for instance.

Basil downy mildew was first reported in south Florida in October 2007. A year later the disease had spread to seven other states, including North Carolina and several Northeastern states. Scientists are not certain where the pathogen originated from but hypothesize that it may have come from Uganda. Besides the U.S., the disease is now in several African countries, the Middle East and Europe.

Until now basil growers had to begin treating before seeing symptoms of the disease and then treat frequently until there were no more reports of the disease in the area. To help farmers save money on expensive fungicides, Extension specialists recommended that farmers participate in a monitoring program for basil downy mildew, either a spreadsheet developed by Cornell University or the cucurbit downy mildew forecasting website.

Conventional growers could prevent the disease with a fungicide program as long as they treated before symptoms began, but organic growers had no effective solution. Once plants were infected, spores appeared on the underside of the leaves, making them inedible. The disease travels quickly through the crop, so growers who did not spray in time often lost their entire crop.

Although the pathogen is in the same family as cucurbit downy mildew, it has distinct differences. One major difference is that it tolerates warmer weather than the pathogen that causes cucurbit downy mildew. North Carolina extension pathologists began reporting instances of the disease in June.

As scientists searched for a resistant strain, they found that some of the “exotic” ornamental basils were immune to downy mildew. However, when crossed with sweet basil, they did not produce seed. Scientists began looking for crossed varieties that would be resistant rather than immune.

“We made thousands of crosses,” said James Simon, professor of plant biology at Rutgers University-New Brunswick. “We developed varieties that had resistance but originally didn’t have the aroma and/or flavor or that particular look we were after. Or, we got some that had flavor but lacked resistance to the disease.”

Eventually they developed four downy mildew resistant (DMR) varieties, each with a different appearance and flavor: Rutgers Obsession DMR, Devotion DMR, Passion DMR, and Thunderstruck DMR. VDF Specialty Seeds, which is selling the seed, is marketing each variety to different market slots to fulfill different needs.

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