Basil Downy Mildew

Basil’s fragrant leaves make it a garden favorite. But there is a new disease on the horizon: basil downy mildew. 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

First seen in Africa in the 1930s, basil downy mildew (Peronospora belbahrii) came to the U.S. in 2007, probably on infected seeds. It is now a global problem for everyone who enjoys basil and pesto.

Downy mildews

Like other downy mildews, basil downy mildew is an infection 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 the leaf veins, the damage from each infection stays between the veins.

But when new spores emerge from the stoma, they 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 travel 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 like nutritional deficiencies. If you see yellowing between the primary leaf veins, with dark blotchy areas, look at the underside of those leaves. If you see purple or gray powdery spores, it is probably basil downy mildew. Those spores are reproductive bodies. Every infected leaf is a disease factory.

Once a plant is infected, it is too late. Harvest any leaves that still look healthy and bury the plant under the 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. Or, you can bag those diseased plants and throw them in the garbage.

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

Preventing basil downy mildew

To avoid being part of the problem, buy only certified disease-free seeds and seedlings, place all new plants in quarantine, and monitor plants closely. As tempting as it may be, do not replant grocery-store basil plants in your garden. They are guaranteed safe to eat. They are not guaranteed safe to grow.

Wet leaves are prone to infection, so provide your basil plants with good air circulation and keep irrigation water at ground level. Skip the watering can. Instead, use a soaker hose or drip system to prevent spores from splashing up onto the underside of leaves.

Cut basil plants off at ground level at the end of the growing season and compost them to help break this disease triangle and reduce the chance of things starting again each spring.

If you think basil downy mildew has appeared in your garden, please notify your local County Extension Office or Department of Agriculture. You can also use the Basil Downy Mildew Reporting Page to add your contribution to science!