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By Shari Romar
  • School of Integrative Plant Science
  • Horticulture Section
  • Plant Pathology and Plant-Microbe Biology Section
  • Plants
  • Horticulture
  • Pathology
Whether you grow it on your windowsill or buy it at the farmers market, this Cornell plant pathologist has spent more than a decade helping to protect your favorite summertime herb from this imported disease.

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.

Basil downy mildew

Meet Meg McGrath

For Meg McGrath, plant pathologist based at Cornell’s Long Island Horticultural Research and Extension Center, the battle against basil downy mildew gets personal, as her own garden crops are affected.  She grows some of her basil in pots so that she can move them inside on rainy days and dewy nights.

Photo: Provided.

Basil downy mildew

Urban crops vulnerable, too

Urban basil crops are as vulnerable to basil downy mildew as commercial plantings in rural areas. McGrath regularly receives reports of the disease on apartment balconies.

Photo: Shari  Romar

Basil downy mildew

Primo for pesto

Healthy basil plants have a deep green color.

Photo: Margery Daugherty

Basil downy mildew

Yellow spells trouble

Infected basil plants turn yellow between the veins on the top side of leaves.

Photo: Margery Daugherty

Basil downy mildew

Infected leaves

Leaves in McGrath’s lab showing signs of basil downy mildew, interveinal yellowing.

Photo: Margaret McGrath

Basil downy mildew

Why they call it downy mildew

The undersides of leaves infected with basil downy mildew produce dusty brown spores.

Photo: Margery Daugherty

Basil downy mildew

Spores up close

Dusty brown spores on the underside of a basil leaf in McGrath's lab.

Photo: Margaret McGrath

Basil downy mildew

Keep leaves dry

High humidity common during Northeast summers favors infection. Keep leaves as dry as possible when watering. Bring pots inside when it’s rainy or on dewy nights. .

Photo: Margaret McGrath

Head shot of meg mcgrath
Basil in a cute birdie pot with New York City skyline in the background
healthy green top leaves of basil plant
yellowish basil leaves
basil leaves with interveinal yellowing
Dusty dark brown spots along underside of basil leaves
dark brown spores on the underside of a basil leaf
drop of moisture at the tip of an infected basil leaf

How to keep your plants healthy

While there’s no vaccine for basil downy mildew, there are practical steps both commercial growers and gardeners can take to reduce the devastation.

The first line of defense is to grow varieties that are resistant to the disease.  Sweet basil is more susceptible than spice types such as lemon or cinnamon basil.

Sweet basil varieties that have shown good resistance in trials that McGrath has conducted on Long Island include Prospera and Amazel, as well as resistant varieties developed at Rutgers University: Devotion, Obsession, Passion and Thunderstruck. Other recently released varieties reported to be resistant include Pesto Besto and variants on Prospera with different leaf types.

The second line of defense is to keep foliage dry.  Basil prefers full sun and good air circulation. Try not to crowd plants and avoid getting the foliage wet when watering. Dew that forms on humid evenings is particularly troublesome because it creates especially favorable conditions, and there’s nothing you can do to prevent it.

To help home gardeners cope with that, McGrath suggests growing basil in containers that you can bring indoors on rainy days and humid nights. Be sure to check plants for symptoms before you purchase them, and at least weekly thereafter. Harvest healthy leaves on any plants that show symptoms, then seal the plant in an old plastic bag and put it in the trash (do not compost) so that spores don’t spread to healthy plants. You can reseed pots where you’ve removed sick plants.

Organic and conventional fungicides are also available. Growers should carefully monitor their crops to ensure downy mildew is present before applying fungicides. Greenhouse growers should take additional steps to reduce moisture on leaves, such as using fans directed at leaves to keep them moving and dehumidifying air blown up through the basil canopy.

Unfortunately, this pathogen shows signs of being able to quickly develop resistance to fungicides, as well as evolve to overcome the resistance bred into some basil varieties. “The big changes more recently have been the pathogen’s ability to overcome fungicides, confirmed elsewhere in the world — so far in Italy and Israel,” McGrath said. “Plus, there are a couple places where they detected a new race of the basil downy mildew pathogen that is able to overcome one of the resistant basil varieties.”

But predicting pathogen evolution is difficult.

“We could be very lucky and the pathogen might just evolve resistance to one type of fungicide chemistry and not be able to evolve resistance to others, which would be fabulous,” said McGrath. Currently, basil downy mildew doesn’t overwinter on dead leaf tissue or in the soil — which would greatly increase the annual recurrence of the disease. But that could also change. This pathogen has the ability to produce a resting type of spore that can remain dormant for years, like a seed. But that only happens when different mating types (the pathogen’s equivalent of gender) are present. And so far, only a single mating type is known to exist in the U.S.

Report outbreaks

In an effort to understand more about the disease and its spread, McGrath pioneered an effort to track the disease through online reporting in 2009.

“Gardeners and growers understand the value of monitoring so that they can see where and when the disease is occurring,” she said. “To those of us studying the disease, it’s valuable to know that same information on a larger scale. And we make it easy for people to report outbreaks.”

Early incarnations of McGrath’s reporting system were simple Google Docs forms. But the current platform is far more sophisticated and includes a map with yearly reports plus details such as photos and best methods to manage basil downy mildew. As many as 280 reports roll in each year from throughout the United States (only six western states have yet to have a confirmed report) and far beyond — including Argentina, Turkey and Australia.

McGrath counts growers, extension educators, and fellow researchers among her trusted “citizen scientists” and disease scouts. But most reports have surprisingly come from gardeners, many mourning their aborted pesto-making plans.

“Unfortunately, this disease is here to stay,” said McGrath.  “But at least these gardeners can know that they are helping us increase our collective knowledge of the disease and do what we can to reduce its impact. We hope that as a result they will be harvesting armloads of healthy basil in the years to come.”

Learn more:

Shari Romar is a communications specialist in the School of Integrative Plant Science, and managing editor of the Branching Out newsletter.

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