Powdery mildews, while they look alike, often have specific host ranges. The disease is easily recognizable as a white to gray powdery growth on leaves and sometimes stems and flowers. Some verbena varieties are susceptible to the powdery mildew caused by the fungus Podosphaera xanthii. Infected leaves may develop a purplish discoloration that could be confused with a nitrogen deficiency. Look on the underside of leaves for the thin strands of powdery mildew. As many growers have noticed, there is wide variation in the susceptibility of verbena cultivars to powdery mildew. Many varieties are much less prone to the disease.
Powdery Mildew on Ornamental Plants
Powdery Mildew in Greenhouses on the UConn IPM Web site