Question: Why do zinnias in the garden develop leaf blight or mildew, and decline and die?
Answer: There are two important zinnia leaf spots in Indiana, one bacterial, caused by Xanthomonas zinniae, and the other fungal, caused by Alternaria zinniae. Bacterial leaf spots are small, angular, brown spots often surrounded by a yellow margin. Alternaria leaf spots are larger and reddish brown with grayish centers. Both diseases result in leaf blighting, and death of foliage, and eventuallly plant death. Powdery mildew, covering leaves with grayish white superficial growth also causes foliage to die. Some of these zinnia diseases are seed-borne, so purchase seeds from a reliable source. Fungicides may be beneficial for powdery mildew and Alternaria leaf spot, but will not control bacterial leaf spot. Avoid wetting foliage while watering. Fall garden clean-up will help to reduce carry over of inoculum for next year's plantings.
-Dr. John Hartman, Extension Plant Pathologist, University of Kentucky, Lexington
Edited for Indiana by Gail Ruhl, Purdue University