Basil Downy Mildew

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Sweet Basil affected by basil downy mildew
Bruce Watt, University of Maine,

by Colette Mancke, Tennessee Master Gardener.

What is basil downy mildew?
Basil downy mildew is a devastating disease that affects the leaves, branches and stems of many types of basil (ie.,plants in the genus Ocimum) commonly used for cooking. Green-leafed varieties of sweet basil are particularly susceptible to the disease, while purple-leafed varieties of basil, Thai basil, lemon basil and spice basil are less susceptible. Certain ornamental basils (e.g., hoary basil) appear to be highly resistant to the disease. Basil downy mildew was first reported in the US in 2007 and has since spread widely to wherever basil is grown.

What does basil downy mildew look like?

Symptoms of basil downy mildew typically develop first on lower leaves, but eventually an entire plant will show symptoms. Initial symptoms include leaf yellowing (which gardeners often think is due to a nitrogen deficiency) followed by leaf browning. Affected leaves also curl and wilt and on the undersides of the leaves, a gray-purple fuzzy material will develop.

Where does basil downy mildew come from?

Bruce Watt, University of Maine, 

Basil downy mildew is caused by the fungus-like organism Peronospora belbaharii. This pathogen can be easily introduced into a garden each year via contaminated seed, on infected transplants, or via wind-borne spores. Once introduced into a garden the pathogen can spread by wind, by rain splash, or via items (hands, clothing, garden tools)

How do I avoid problems with basil downy mildew?

Whatever type of basil you choose, try to grow your plants in a manner that will keep them as dry as possible, thus creating an environment that is less favorable for the downy mildew pathogen to develop and infect. Plant basil in a sunny location, space plants as far apart as possible and orient rows in the direction of prevailing winds to promote good airflow and rapid drying of plants when they get wet. Avoid overhead watering.

Lemon Basil

“Lemon Basil” by FarOutFlora is licensed under CC BY-NC-ND 2.0 

Use of fungicide treatments to control basil downy mildew IS NOT recommended. This disease also infects cucurbits. To find more information on this topic Cornell University has articles on their Downy mildew research. Or try this site which forecasts the spread of the disease in cucurbits.

Much of this information can be found at the University of Wisconsin -Extension.