Downy mildew

What downy mildew looks like on leaves.

Last year I grew a new sweet basil named Dolce Fresca and loved its self-branching, compact habit. Its flavor was just like Genovese, the standard for all large-leafed basils.

But in late August, the leaves suddenly turned brown and the plants looked like they were dying. Diagnosis: downy mildew, something I’d never seen before.

Here are some notes on preventing this disease, and what to do if you get it.

Basil downy mildew is caused by the fungus-like pathogen Peronospora belbahrii. This disease can survive the winter only in warm weather areas; but the spores can ride long distances on moist air currents, and that’s how it gets to North Carolina. Year-round greenhouses and un-treated seeds may also be sources for infection.

Downy mildew causes lower leaves to turn yellow, after which dark spots appear. The disease moves up the plant and will eventually kill it, if not treated.

Downy mildew likes high humidity and warm temperatures, so most of the damage typically occurs in late summer. No resistant varieties of sweet basil are available. Some flavored and dark-leaf basils have been found to be less susceptible, but they don’t necessarily make good-tasting pesto.

To help prevent the disease, water the base of the plant instead of from overhead, and water only in the morning so the foliage dries quickly. Spacing plants to allow good air circulation will also help.

The most effective prevention is a spray program starting in June, making sure to get good coverage on both sides of the leaves. Because downy mildew isn’t truly a fungus, many popular fungicides will not control it.

Two organic options that work are copper sulfate and bacillus subtilus (like Serenade) and chlorothalonil-containing products (like Daconil) are also effective. Once an infection occurs, the only remedy is to regularly spray one of these products per the instructions on the label.

In my case, almost all the leaves were brown before I realized what was happening. I removed the infected foliage and sprayed the plants thoroughly according to the directions on the label (including the time interval between application and harvest.)

After that, good green leaves re-appeared, but that took several weeks, and during that time the plants were, as they say in France, “très uglée.”

So next year I’ll monitor them more closely and start a spray program at the first sign of trouble.

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