Powdery mildew is the most common problem for zinnias. To prevent it, be sure….

There is good air circulation around the plants and they are not too close together. Mildew likes cool, wet conditions. Water during daytime instead at night. Water carefully, directing the water at the base of the plant, into the soil, to prevent water from staying on leaves and stems. Some growers use preventative applications of fungicide before powdery mildew develops.