Question: My zinnias have white dust all over their leaves. What will stop it? It seems to be hurting the plants.
Answer: That’s powdery mildew, and it does weaken zinnias. More than that, however, it’s also disfiguring. The plants look messy in the garden. Plant zinnias in full sunlight, in areas that receive good air circulation. Avoid overhead irrigation whenever possible, or at least water early in the morning, so the plants can dry quickly. Fungicides can help slow the disease, and it’s also less of a problem for zinnias that are set out in mid- to late summer, for fall blooming.