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From gardeningknowhow.com: This basil disease is caused by a bacteria called Pseudomonas cichorii. Symptoms of bacterial leaf spot are black or brown spots that appear on the leaves and streaking on the stems of the plant. Bacterial leaf spot occurs when infected soil is splashed onto the leaves of the basil plant.
Thank you. What do I do to get rid of it?
Caused by a bacteria known as Pseudomonas cichorii, this basil disease shows itself as brown or black spots on the leaves, as well as streaking on the stems. Pseudomonas cichorii does not only affect basil but also chrysanthemums, geraniums, and other ornamental plants. The bacteria is easily spread by water splashing the plant from infected soil. Bacterial leaf spot likes warm and wet conditionsHow to Control Bacterial Leaf Spot
There is no way to cure bacterial leaf spot, but there are measures you can take to prevent it and minimize the damage. Remove infected plants as soon as possible. Gently water the basil plants directly on soil so that water does not splash up on to the leaves. Use drip irrigation to keep your plants watered if possible. Lower relative humidity by improving circulation of the air. Try to avoid handling the plants when the leaves are wet, as this can spread the disease.