Answer: There are no pesticides registered for disease or insect control on basil. Prevention of disease through good cultural practices is the most effective means for healthy crop production. To help prevent foliar fungal diseases, keep foliage as dry as possible by watering early in the day so foliage dries quickly, or by using drip irrigation. To reduce soil borne diseases, rotate herbs to different parts of the garden each year. I would remove the affected leaves or cut the stems way down to remove the black spots on the leaves. Hope it resprouts with healthy new foliage!
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