Basil’s fragrant leaves make it a Gilroy gardener’s favorite, but there is a new disease on the horizon: basil downy mildew. And warm, moist conditions are all it needs to set up housekeeping on your basil plants.
First seen in Africa, in the 1930s, basil downy mildew (Peronospora belbahrii) came to the U.S. in 2004 on infected seeds from Italy.
By 2008, it had made its way to California. It’s now a global problem for everyone who enjoys basil and pesto.
All types of downy mildews look like fungal disease, but they are something else. Tiny, algae-like microbes, called oomycetes, cause downy mildews.
The oomycetes that cause basil downy mildew are parasites that collect on the underside of leaves. From there, they send out threads that enter the leaf through tiny openings called stoma. Since the oomycetes cannot pass beyond leaf veins, the damage from each infection is usually contained between leaf veins.
Once inside, reproduction begins and new spores are released through the stoma.
These spores fall to the soil, waiting to be splashed back up by rain or irrigation water, or caught on the breeze for a ride to a new host plant. In addition to water and wind, spores can be carried on garden tools, clothing, transplants, and infected seeds.
So how do you know if your basil plants are infected?
Unfortunately, the earliest sign of infection, yellowing leaves, looks a lot like nutritional deficiencies. If you see yellowing between the major leaf veins with dark blotchy areas, take a closer look on the underside of those leaves.
If you see purple or gray powdery spores, it’s probably basil downy mildew.
Once a plant is infected, it is too late. Harvest any healthy leaves and throw the plant in the trash.
For now, the basil downy mildew pathogen is under California state quarantine, which means infected plants must be destroyed.
To avoid being part of the problem, be sure to buy only certified disease-free seeds and seedlings, place all new plants in quarantine, and monitor plants closely.
You can reduce the odds of your delicious basil plants becoming infected by providing good air circulation and keeping leaves dry. Avoid overhead watering and use a soaker hose or drip system that will prevent spores from splashing up onto the underside of leaves.
At the end of the growing season, cut basil plants off at ground level and compost them completely. This helps break the disease triangle and reduces the chance of it occurring next spring.
Some research is being done on the effectiveness of spraying basil plants with fixed copper as a preventative, but the results are not yet in. If you think basil downy mildew has appeared in your garden, please notify your local County Extension Office or Department of Agriculture.
Kate Russell is a UCCE Master Gardener. For more information, visit mgsantaclara.ucanr.edu or call (408) 282-3105 between 9:30 a.m. and 12:30 p.m., Monday through Friday.