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Greenhouse Updates: May 7, 2015

Basil - Bacterial leaf spot
May 7, 2015

Symptoms of bacterial leaf spot caused by Pseudomonas cichorii on basil appear as brown and black spots on leaves and streaking on the stems. The leaf spots are angular or irregular or delineated by the small veins. The bacterium has a wide host range and infects chrysanthemum, geranium, and many other ornamental and foliage plants but is not known to be host specific. P. cichorii can be present on asymptomatic plants allowing long distance distribution of the bacterium via propagative material. On basil, seed and infected plugs are the most likely sources of infection. Disease development is highly favored by high relative humidity, extended periods of leaf wetness, and high nitrogen levels. Bacteria are easily spread by water splash and handling infected tissue.

Sanitation is the most important disease management strategy. Remove and destroy affected plants and infected plant debris. Minimize water splash, reduce leaf wetness duration by watering early in the day and use drip irrigation if possible. Lower relative humidity by improving air circulation with fans, venting and providing sufficient plant spacing. Avoid handling plants when foliage is wet. Bactericides are only marginally effective in controlling bacterial diseases. Sanitation and environmental control are extremely important disease management principles.

Photos: Bacterial leaf spot on basil

Fact sheet: Basil Diseases, Cornell University

Tina Smith, UMass Extension and Angie Madeiras, UMass Extension Plant Diagnostic Laboratory