Then my flowers became infected. Since I've watered, fertilized, and pulled them through some tough growing conditions, I'd like to keep my zinnias flowering as long as possible. The County Extension Office has received calls about this problem over the past week, and in checking with plant pathologists at Mississippi State University, we've confirmed our diagnosis of powdery mildew. The problem springs up on zinnias in the form of patches of fungus growth on the top surface of zinnia leaves. This particular powdery mildew fungus attacks not only the foliage, but generally spreads to flowers.
Infected flowers are unsightly, and wither away within a few days.
The only good news about this fungus is that it's host range is confined to zinnias. These conditions are ideal for development of zinnia powdery mildew. The disease is quite distinctive in its appearance, but if you're not sure about identification of the problem, bring several specimens by the County Extension Office for examination. Homeowners who want to extend the bloom period of their zinnia planting still have time to slow down the disease, provided appropriate control measures are followed. Apply to the plants to the point when spray just begins to run off the leaves.