Garden Q&A: Sweet basil susceptible to basil downy mildew

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Q. My basil plants seem to be dying. The leaves started getting yellow spots that turned brown, and then the entire leaf died. I have been growing basil for many years and have never had a problem. Can you tell me what is going on?

A. The sample submitted with this question had all the signs and symptoms of basil downy mildew. This is a relatively new disease in the United States, first identified in Florida in 2007. It has been reported in a handful of states every year since then, from the East Coast to Western states. It has also been identified in Canada and Argentina.

Affected plants first show angular yellow leaf spots that are delineated by the major veins. The entire leaf often turns yellow, with brown (necrotic) areas and irregular black spots. Affected leaves die and drop prematurely, and affected plants look sickly. These symptoms may be mistaken for sunburn or a nutrient deficiency. What sets basil downy mildew apart from other problems are the purplish-gray spores that develop on the underside of affected leaves. The disease starts low on the plant and works its way up.

It is caused by Peronospora belbahrii, a fungus-like organism classified as a water mold or oomycete. Some of the most virulent plant pathogens belong to this group of organisms, including late blight of potatoes and tomatoes and sudden oak death. The causal organism is carried as spores by wind and on seeds, transplants and fresh leaves, although it may not be visible under cool, dry conditions. The disease flourishes in warm, humid conditions and can spread through a planting rapidly.

Monitor basil plants frequently and immediately remove any with suspicious symptoms. Be sure to grow basil in full sun, and allow space around individual plants to permit good air circulation. If possible, use drip irrigation rather than overhead watering to keep leaves dry. Home gardeners can also use potassium bicarbonate as a preventative fungicide application. This is a relatively non-toxic fungicide sold under trade names such as Bi-Carb, Green Cure and Remedy. Be sure to follow label directions regarding frequency of applications and use the shortest recommended intervals during wet weather. Potassium bicarbonate is unlikely to cure severely infected plants. They should be removed from the garden ASAP.

Unfortunately, all sweet basil (Ocimum basilicum) varieties seem to be particularly susceptible to basil downy mildew. Other types of basil have more resistance, including lime, lemon (‘Lemon,’ ‘Lemon Mrs. Burns’ and ‘Sweet Dani Lemon Basil’), Thai (‘Queenette’) and spice types (‘Spice,’ ‘Blue Spice’ and ‘Cinnamon’). Red-leaf varieties also seem more resistant, including ‘Red Leaf’ and ‘Red Rubin.’

Be sure to purchase fresh seed rather than saving your own, and carefully examine all transplants before putting them out in the garden. Although basil downy mildew spores may not overwinter in our climate, be sure to rotate crops and avoid growing basil in the same spot where you had trouble with it this year.

Since I just wrote about impatiens downy mildew, I want to point out that the downy mildew that affects impatiens does not infect basil, nor does basil downy mildew infect impatiens.

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