What Is Wrong With My Basil?

— Written By
Basil plant infected with downy mildew

Look for fuzzy grey spores growing on the back of basil leaves to confirm basil downy mildew. Image by Debbie Roos.

Do your basil plants look strange? Are leaves turning yellow or brown and do you see dark, fuzzy stuff growing on the backside? If so, your basil plants have basil downy mildew, a fungal disease that proliferates during warm, wet weather. While late summer outbreaks of basil downy mildew have occurred on farms and in home gardens every year since this disease was first recorded in NC in 2008, this year’s outbreak appears to be one of the worst.

The good news about basil downy mildew is that it only infects basil plants and will not spread to other types of plants in your garden. Also, downy mildew requires a living host and does not survive in plant debris so you do not have to worry about it persisting in the soil. The bad news is there is little home gardeners can do to save infected plants. During warm, wet weather this disease spreads rapidly by wind dispersed spores (the black fuzzy stuff growing on the back of the leaves). Our recent rainy weather has created the perfect conditions for this disease to multiply.

Infected plants should be removed from the garden to prevent spores from spreading to healthy basil plants growing on nearby farms or in gardens. Infected plants can be tilled into the soil or composted. Do not overwinter infected plants in a greenhouse or on a windowsill as this will provide a living host for the disease to persist in our area. Basil downy mildew can be passed on through infected seeds so do not save seed from plants showing symptoms. Remove any volunteer seedlings that come up where infected plants were growing as these likely also have the disease.

If your plants are still healthy, you can reduce the chance of them getting basil downy mildew by minimizing leaf wetness. While you can’t prevent the rain from wetting your plants, you can help them dry off quicker by thinning and pruning plants to increase air flow around them. Also, when watering plants, avoid wetting the leaves by applying water at ground level by using a soaker hose.

Sweet basil is particularly susceptible to this disease. In the most recent issue of the NC Pest News, Extension Plant Pathologist Lina Quesada-Ocampo reported that “while resistant varieties are not yet available, some basil varieties have been found to be less susceptible. Red basil (‘Red Leaf’ and ‘Red Rubin’), Thai basil (Queenette’), lemon basil (‘Lemon’, ‘Lemon Mrs. Burns’, ‘Sweet Dani Lemon Basil’), lime basil (‘Lime’), and spice basil (‘Spice’, ‘Blue Spice’, ‘Blue Spice Fil’, ‘Cinnamon’), presented less severe downy mildew symptoms in a previous study.” Chatham Extension sustainable agriculture agent Debbie Roos reports widespread infection of many varieties of basil in this year’s outbreak, which you can read more about in her recent post.

Learn more about basil downy mildew from this Cornell University fact sheet

Use Extension Search to find research based information from Cooperative Extension systems across the U.S.

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