There are two important zinnia leaf spots: 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. Fungal leaf spots are larger and reddish-brown with grayish centers. Both diseases result in leaf blighting and death of foliage and eventually plant death.
Powdery mildew, caused by the fungus Erysiphe cichoracearum, covers leaves with grayish-white superficial growth but can also cause foliage to die.
Some of these zinnia diseases are seed-borne, so purchase seeds and transplants from a reliable source. Look for seed that is certified disease-free. Fungicides may be beneficial in preventing powdery mildew and Alternaria leaf spot but will not control bacterial leaf spot.
Other preventative measures: avoid wetting foliage while watering and clean up plant debris to reduce carry-over of inoculum for next year's plantings.
Contact your local Cooperative Extension office for specific fungicide recommendations.
