Mildew-Resistant Basil Prospers in Maryland

By MG Betty Cichy

Herbs of all sorts will be in the spotlight this year as the University of Maryland Extension’s Grow It Eat It program celebrates the Year of Herbs. The winter Grow It Eat It meeting on March 13 will include information on resources MGs can use to educate the public about growing and using herbs. Two herbs in particular—clustered mountain mint and ‘Prospera’ basil—will be a focus of attention. Graphics and social media posts featuring the herbs are already online for sharing, and the state office will hand out free seed packets for MGs to use in their activities.

Clustered mountain mint (Pycnanthemum muticum) is a native perennial that’s a magnet for bees and other pollinators. It may be new to many gardeners, but no doubt everybody knows basil. Still, many home gardeners may not have yet tried ‘Prospera’, a downy mildew-resistant cultivar that until recently was rarely available at local garden centers.

Downy mildew, a fungus-like pathogen caused by the water mold Peronospora belbahrii, has been a problem for growers and gardeners in Maryland and nearby states since at least 2009, soon after the first infected plants were observed in Florida. Airborne mildew spores make their way from the South every year, and the first signs of infection—yellowing between the veins on top of the leaves, and fuzzy purple patches on the underside—typically show up on local basil in late July or early August. Infected plants don’t recover, and eventually stop growing and die.

Downy mildew on basil leaf underside
Photo: Maryland Grows Blog, Feb 21, 2020

But ‘Prospera’, a cultivar developed by plant breeders in Israel, proved highly resistant to downy mildew in field trials conducted by the University of Maryland in 2019 and 2020. A team led by Extension specialists Dave Clement and the late Stanton Gill planted five basil cultivars, including four bred for downy mildew resistance, at different sites around the state, and observed the plants throughout the summer and fall.

Only ‘Prospera’ and a cultivar called ‘Amazel’ showed no signs of infection by the time the trials ended in early October. But ‘Amazel’, a sterile variety propagated by cuttings and marketed under the Proven Winners brand, petered out early, while ‘Prospera’ continued producing healthy leaves for the duration of the trial.

Basil cultivars (l-r): ‘Amazel’, ‘Aroma,’ and ‘Prospera'
Photo: Maryland Grows Blog, Feb 21, 2020

I wrote about the new mildew-resistant basils for The Seed four years ago, and I recently checked in with Extension specialist Jon Traunfeld, director of the Home and Garden Information Center, for an update. Traunfeld noted that while the field trials in Maryland have ended, ‘Prospera’ has also been a top performer in trials at Cornell University, which conducts an extensive research and detection program on downy mildew in basil.

As the original pathogen evolves into new strains, researchers are still working to keep downy mildew at bay. After commercial growers in New York observed downy mildew on a few greenhouse-grown ‘Prospera’ plants in 2022, breeders introduced ‘Prospera’ cultivars with even higher levels of disease resistance. They’ve also come up with improvements that make ‘Prospera’ more appealing to some growers and gardeners, including a more compact growth habit. (The original ‘Prospera’ can grow up to five feet tall.) Thai, red-leaf, and Italian large-leaf ‘Prospera’ cultivars are other recent introductions.

So far ‘Prospera’ still seems to be performing well in home gardens. “We have not had any recent reports of this disease from Maryland gardeners via Ask Extension. I think many gardeners have switched to resistant cultivars,” Traunfeld noted, adding that last year’s dry summer may have also contributed to a reduction in downy mildew infections. 

 

Many Montgomery County MGs have been using ‘Prospera’ for years with great success. “We’ve grown ‘Prospera’ at our Giving Garden for two years and had vigorous growth with no fungal issues,” says MG Cat Kahn, who chose “Prospera’ as the herb to use when she led the former Growing Gardeners program. MGs Ann Claxton and Terri Valenti have each lost basil to downy mildew in the past, but say the ‘Prospera’ plants in their gardens have been mildew-free. MG Erica Smith describes ‘Prospera’ as “reliable and tasty,” and is looking forward to trying some of the new varieties.

Traunfeld says the state MG office will distribute around 5,000 packets of ‘Prospera’ basil to county and city MG coordinators to use in their activities this year. Seed for home gardens is available from many garden catalogs and online, while Johnny’s Selected Seeds and High Mowing Seeds offer a large selection of the new ‘Prospera’ cultivars.

You can learn more about the Year of Herbs and maybe find out how to snag a few packets of seed for your MG activities at the winter Grow It Eat It meeting on March 13, from 10 a.m. to 2 p.m. You can register to attend either virtually or in person.