Basil Downy Mildew Confirmed in Iowa

Basil Downy Mildew is a relatively new disease in the United States. The Plant and Insect Diagnostic Clinic has confirmed basil downy mildew in Story and Polk Counties within the past two weeks.

The first noticeable symptoms are yellowing between the veins possibly imitating a nutritional deficiency.  If you flip over the yellowed leaves, you may see gray fuzzy growth on the underneath side of the leaf.  The leaves will then blacken and die.

Downy mildew is caused by a fungus-like organism called Plamospora belbahrii. This pathogen thrives under warm wet weather conditions. Downy mildew is spread through infected seed, transplants, or fresh leaves. Once in your garden, downy mildew can spread through rain splash, overhead irrigation, and wind. It also produces a type of spore (called oospore) that can remain inside infected plant material for several years.

If you suspect downy mildew on your basil or find fuzzy growth on leaf undersides, bag and remove the plants immediately. Currently, there are no resistant varieties of basil. To reduce the risk of infection, avoid overcrowding and promote air circulation among plants. Also, avoid overhead irrigation and watering at night. For more information on basil downy mildew and the efforts to monitor this disease please see this website from Cornell University.

Questions or think your plant has downy mildew? Please contact the Plant and Insect Diagnostic Clinic at 515-294-0581 or at  Visit us online at or on Facebook.

Basil downy mildew produces a gray, fuzzy growth on the leaf undersides.Basil downy mildew produces purplish-gray spores on the leaf undersides.

Basil downy mildew symptoms may resemble nutrient deficiency.Basil downy mildew symptoms may resemble nutrient deficiency.

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