Answer: I'm not sure what might be feeding on the foliage of your zinnias unless there are beetles in the soil and they wander and dusk and feed. You may have good results if you put a mulch over the bare soil around your plants. It might be just enough of a deterrent to keep the leaves from being eaten. I wouldn't use anything to prevent disease unless the plants are beginning to show symptoms of disease. The best ways to prevent diseases are to grow the plants in an open position in full sun with plenty of air circulation around them, avoid overhead watering, select resistant varieties, and keep them generally healthy by providing them with good soil and regular watering if needed. Some of the more disease resistant varieties are Profusion Cherry and Profusion Orange, the Oklahoma Series and finally, the Pinwheel Series, which is particularly resistant to mildew.
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