Basil Downy Mildew

Basil’s fragrant leaves make it a garden favorite, but there is a new disease on the horizon: basil downy mildew. And warm, moist conditions are all basil downy mildew needs to set up housekeeping on your basil plants.

Basil downy mildew symptoms (Scot Nelson) Public Domain

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First seen in Africa, in the 1930’s, 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 and is now a global problem for everyone who enjoys basil and pesto.


Downy mildews

Like other downy mildews, basil downy mildew is caused by tiny, algae-like microbes called oomycetes. Oomycetes parasitize vascular plants to complete their life cycle. They do this by collecting on the underside of leaves. From there, these tiny one-celled creatures send out threads that enter the leaf through the stoma and begin propagating. Since the oomycetes cannot pass beyond leaf veins, the damage from each infection is usually contained between leaf veins. 


New spores are then released through the stoma, where they fall to soil, waiting to be splashed right back up by rain or irrigation water, or caught on the breeze for a ride to a new host plant. The party responsible for basil downy mildew travels on a variety of surfaces to reach your garden. 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?


Symptoms of basil downy mildew

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 is probably basil downy mildew. Those spores are reproductive bodies and each infected leaf is a disease factory.


Once a plant is infected, it is too late. Harvest any healthy leaves and bury the plant under soil or in the compost pile to prevent spores from spreading. Generally speaking, these pathogens will not survive in compost or through winter temperatures. We hope. For now, the basil downy mildew pathogen is under California State quarantine, which means infected plants must be destroyed.

Dark purple to gray spores of basil downy mildew on the underside of an infected leaf (Scot Nelson) Public Domain

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Preventing basil downy mildew

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.


Good air circulation goes a long way toward keeping leaves dry. Dry leaves are not hospitable to this disease, so keep irrigation water at ground level. Skip the watering can. Instead, 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. Do not leave them in place to harbor disease. This helps break the disease triangle and reduces the chance of things starting up again each 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. You can even use the Basil Downy Mildew Reporting Page to add you contribution to science!