Coming Again to a Garden Near You: Basil Downy Mildew

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From Caprese salad to pesto to fresh basil on a burger, sweet basil (Ocimum sp.) is one of summer’s culinary delights. But for many gardeners, a sneaky fungal-like organism known as basil downy mildew (BDM) attacks mid-summer, turning basil leaves yellow then brown. ‘Genovese’ basil, the most commonly grown culinary variety, is particularly vulnerable to this disease.

basil with browning leaves

Leaves of basil turn yellow then brown when infected by Basil Downy Mildew. Photo by Bruce Watt, University of Maine, Bugwood.org

The good news is that plant breeders have developed new, BDM-resistant cultivars of basil. With help from Extension Master Gardener℠ volunteers, by summer’s end, we will know which of these new cultivars perform well in different parts of North Carolina. This information will help home gardeners and commercial growers to produce a healthy basil harvest all season long.

Two women with plants

Dale Reece and Marcia Tate, EMGVs of Haywood County laid out the cultivars according to the experimental design. Photo by Sam Marshall

How Does It Get Here?

While BDM has affected basil grown in other parts of the world for some time, it was first discovered in the U.S. in 2007 in Florida. Since then, gardeners in states up and down the eastern seaboard and commercial growers in the Midwest and on the west coast have reported BDM damage each year.

BDM is caused by the oomycete Peronospora belbahrii, a fungal-like organism that reproduces via spores that travel on the breeze. It loves warm, humid temperatures (especially warm nights), and spreads when spores are picked up by the wind and land on nearby plants. BDM spores don’t like cold weather, and don’t survive over the winter in states like North Carolina where winter temperatures drop below freezing. Instead, it spends the winter further south in frost-free areas or in greenhouses. When the weather warms, spores catch a ride on the wind and are able to travel long distances across state lines, usually arriving mid to late summer, just as your basil is really thriving.

a basil leaf with yellowing between the veins

Leaves begin to yellow when infected by Basil Downy Mildew. Photo by Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org

How Will I Know It’s Arrived?

The first sign a gardener will notice is yellowing of the leaves, particularly in between the leaf veins. While yellowing, known as chlorosis, can be mistaken for a nutritional deficiency, a look at the underside of the leaves can help confirm the presence of BDM. The presence of lots of purple-brown spores on the underside of the leaves is a sure sign of infection. Eventually, the yellow parts of the leaves turn brown, ultimately leading to the death of the entire plant. 

close up of the underside of a basil leaf with black spores

Spores of Basil Downy Mildew on the underside of basil leaves. Photo by Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org

Once a spore lands on a leaf and infects the plant, symptoms appear within 10 days. There is no easy fix for BDM; fungal spray programs are labor-intensive, depend heavily on getting the timing just right, and there are no products available to home gardeners that are effective at controlling BDM. Growing BDM-resistant cultivars is a more sustainable way to ensure a season-long basil harvest. 

How Are Master Gardener Volunteers Helping?

two people working outside planting small basil plants in a raised bed

Jim Kwasnik, EMGV of Onslow County and Emilee Morrison, N.C. Cooperative Extension consumer horticulture agent plant the trial bed in the Discovery Gardens at the Onslow County Extension office. Photo credit: Emoni Burgess

Five basil cultivars bred to be BDM-resistant are being compared with the highly susceptible ‘Genovese’ cultivar. Cultivars in the trial are ‘Rutgers Obsession-DMR’, ‘Rutgers Devotion-DMR,’ ‘Rutgers Thunderstruck-DMR,’ ‘Rutgers Passion-DMR’ and ‘Prospera’. Extension Master Gardener volunteers in four counties (Haywood, Chatham, Pender, and Onslow) that span the mountains to the seas planted test plots in early June. 

colored plant tags on a countertop

Six different basil cultivars are being trialed for their resistance to Basil Downy Mildew. Photo by Pat Burton

Once a week throughout the summer, volunteers will examine each plant and rate it from 0 to 3, based on a disease severity scale. Plants with no symptoms will receive a rating of zero, while severely affected plants will receive a rating of 3. As they record their data, we will see in real-time when BDM arrives and how it affects the different cultivars. 

Woman with potted plants

Carole Keller, EMGV of Haywood County, planted her basil in grow bags. Photo by Sam Marshall

Of course, resistance to BDM is a desirable trait, but most importantly gardeners and consumers want basil to taste good! To gauge the flavor of resistant varieties, volunteers will conduct taste tests and comparing BDM resistant cultivars to ‘Genovese’ basil, the culinary favorite. Stay tuned for updates on Facebook and Instagram and a report at the end of the summer, providing locally tested recommendations on the best basil for your backyard. To stay abreast of reports of basil downy mildew in your county check out the Ag Pest Monitor. For more information on basil downy mildew check out this fact sheet.

We extend a special thanks to Johnny’s Selected Seed for donating seed of the BDM resistant cultivars for this trial.