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Garden Q&A: Crop rotation won’t eradicate downy mildew on basil

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jessica walliser
Disease-free sweet basil

Question: I’m pretty sure I had downy mildew on my basil this year. The basil has been planted in the same garden for perhaps 15 years. Would rotation help? Is it possible that the apple mint I have growing nearby could also have it? Would solarization rid the soil of downy mildew? Thank you.

Answer: Basil downy mildew is a recently introduced disease that’s incredibly destructive to its namesake herb. First found in Florida in 2007, basil downy mildew has since been noted in dozens of states, including Hawaii. It’s also found in Europe, Central America and Asia, as well as on the African continent where this aggressive fungus is thought to have evolved.

The fungus is airborne, seed-borne, and tissue-borne, making it extremely easy for the disease to spread via contaminated seed, plants or air currents. The spores also are transported by moving the plants themselves or through infected leaves and stems purchased at garden centers.

Because this disease is so new and it looks at first like a nutritional deficiency or spider mite infestation, it’s difficult to diagnose. Basil downy mildew first appears as pale yellow mottling between the veins on the upper leaf surface and minuscule purplish-gray spores splattered on the lower leaf surface. The disease eventually progresses to dark brown to black irregular splotches and mottling on the leaves. You’ll also see the powdery, dark gray spore clusters on the leaf undersides.

Start battling this disease by planting only basil varieties with resistance to it. Studies have shown that all sweet basil varieties are highly susceptible. Lemon, Thai, “Red Rubin,” “Spice” and “Red Leaf” basils were noted in some studies to be the least susceptible. Breeders are trying to breed resistance into new varieties.

Also, be sure to properly space the plants. Good air circulation around the plants allows the foliage to dry faster and deters fungal diseases of all sorts. When watering the plants, try to keep the foliage dry, and water in the morning so the foliage is dry by nightfall. Remove any plants with a suspected infestation immediately. Dispose of them by burying them or tossing them into the garbage.

Organic fungicides also are an option, though they must be applied early and often, and as soon as symptoms are noticed. Potassium bicarbonate-based fungicides such as Green Cure, MilStop, Remedy and Bi-Carb, and to a lesser extent neem oil, have been shown to be effective against the fungus. Be sure to follow label instructions when applying them.

This disease does not live in the soil, nor do scientists think it is able overwinter in northern climates like ours, so solarizing the soil or drenching it with a fungicide will not control basil downy mildew. It is spread only by wind, infected seed and plant tissues. Though crop rotation is always a good idea, doing so will not necessarily help control the spread of this fungus.

Basil downy mildew is host-specific and will not affect any other plants.

Horticulturist Jessica Walliser co-hosts “The Organic Gardeners” at 7 a.m. Sundays on KDKA Radio. Her website is www.jessicawalliser.com.

Send gardening or landscaping questions to [email protected] or The Good Earth, 503 Martindale St., 3rd Floor, D.L. Clark Building, Pittsburgh, PA 15212.

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