By Robert Mattera III, MS
If you’ve ever grown basil at home, whether in the garden, a pot on the patio, or a kitchen windowsill, you know how rewarding it is to pick fresh, aromatic basil leaves for your favorite dishes. Unfortunately, for the past decade, a devastating disease called basil downy mildew has swept into gardens around the world. Basil downy mildew can turn healthy basil into a yellowed, necrotic mess overnight and there are few to no options for control, especially for the home grower. Luckily, plant breeders are working hard to develop new basil varieties that can resist this disease!
What Is Basil Downy Mildew?
A close-up of ‘Prospera’ grown in a field at Rutgers University in 2019 showing the classic signs and symptoms of basil downy mildew.
Basil downy mildew (Peronospora belbahrii) is a fungus-like pathogen (called an oomycete) first identified in Uganda in 1932 and later rediscovered in Europe in the early 2000s. It spread to the United States around 2007. Since then, it has caused disease around the world, making it extremely difficult to grow basil. The pathogen thrives in mild, humid environments and spreads through airborne spores. Symptoms include yellowing, brown spots, and a fuzzy gray-purple growth on the underside of leaves. Infected plants decline quickly, becoming unharvestable. Because the spores can travel long distances via wind, even careful gardeners can find themselves battling an outbreak.
Downy mildew is especially virulent on sweet basil (Ocimum basilicum). Until 2017, no resistant sweet basil varieties existed. It can wipe out entire crops, especially if the weather is warm and wet. Unfortunately, traditional chemical fungicides aren’t always effective, and many home gardeners prefer to avoid heavy chemical use anyway.
What makes this disease even trickier is that the pathogen is evolving.
The Different ‘Faces’ of Downy Mildew
Just like the flu virus changes every year, the downy mildew pathogen has different “races”—genetic variations that can overcome different types of plant resistance. When breeders develop a basil variety that resists one race of the pathogen, a new race may emerge that breaks through that resistance. It’s an ongoing game of evolutionary cat and mouse.
Originally, it was believed that only one race was dominant throughout the globe, but in recent years, researchers have identified multiple distinct races. Some varieties that once offered good resistance—like ‘Prospera’ or ‘Pesto Besto’—have become ineffective as new races emerge.
Rutgers ‘Obsession DMR’ grown next to the sweet basil cultivar ‘Aroma 2’. ‘Obsession’ can be seen as healthy with minimal leaf yellowing, while ‘Aroma 2’ has defoliated heavily and the remaining leaves are chlorotic and covered in spores.
How Breeders are Fighting Back
Plant breeders first responded to this disease with ‘generation one’ resistant lines, including varieties from Rutgers University (‘Devotion DMR’, ‘Obsession DMR’ and ‘Passion DMR’), Bar Ilan University (Prospera series) and the University of Florida (‘Pesto Besto’). The Rutgers varieties have ‘durable’ resistance consisting of multiple genes. These lines do develop symptoms from basil downy mildew but typically much later in the season and only under extremely high disease pressure. Both the Prospera series and ‘Pesto Besto’ have strong resistance to race 0 of the pathogen, showing no signs of disease.
Eventually, Race 1 became the dominant race of basil downy mildew. This race interacts with ‘Prospera’ and ‘Pesto Besto’ as if they have no resistance. In response to this, Bar Ilan breeders recently released a ‘generation 2’ resistant sweet basil: the ‘Prospera Active’ series. However, there is already strong evidence to support that this resistance has been overcome.
Plant breeders are using traditional breeding methods to develop new and improved varieties. There is a specific focus to cross sweet basil with more resistant wild basil relatives, selecting offspring that retain the flavor and appearance gardeners love, but with improved disease resistance. Breeding basil is especially tricky because sweet basil is prized for its specific flavor, aroma, and leaf shape. Crossing it with other species can introduce unwanted traits, like a bitter taste or hairy leaves. Breeders must carefully select for plants that retain that desired aroma while still having strong resistance.
This field displays the diversity of basil, with over 450 different basil varieties. This experiment was to screen all of the accessions for new sources of basil downy mildew resistance.
This field displays the diversity of basil, with over 450 different basil varieties. This experiment was to screen all of the accessions for new sources of basil downy mildew resistance. What Home Gardeners Can Do
While breeders continue to improve disease resistance, home gardeners can take some simple steps to protect their basil plants:
- Choose a Variety of Resistant Varieties: Look for newer basil cultivars labeled “downy mildew resistant,” such as ‘Prospera Active’ or ‘Rutgers Devotion DMR’. By selecting and growing basil with different resistance sources, you can set yourself up to not lose your entire basil crop. The Rutgers varieties, while not immune, provide solid performance against all the known races of basil downy mildew.
- Improve Air Circulation: Plant basil with plenty of space between plants, and avoid overhead watering to reduce humidity around the leaves.
- Remove Infected Plants: If you spot signs of downy mildew, remove infected plants quickly to limit the spread.
The Future of Basil
Due to the work of plant breeders and researchers, the future of basil is looking brighter. With improved resistant varieties, home gardeners get one step closer to enjoying a full season of healthy, flavorful basil, without the heartbreak of losing an entire crop to downy mildew.
So the next time you’re planting basil, know that a lot of science has gone into making sure that perfect leaf makes it to your plate.
Robert Mattera presented a fascinating webinar on Breeding Better Downy Mildew Resistant Basils on Tuesday, May 13, 2025. Our webinars are free to The Herb Society of America members and are available for viewing after signing into the Member Login page. Webinars are $20.00 for guests. Become a member today, and enjoy all of our webinars for free along with access to the webinar library with over fifty program titles.
Photo Credits: 1) Ocimum sp. (Chrissy Moore); 2) Downy mildew on basil leaves (courtesy of the author); 3) Resistant and non-resistant basil varieties (courtesy of the author); 4) Basil research field (Micah Seidel)