Verbenas (Fig. 1) are the spring crop most often afflicted by powdery mildew. Crops will become unsaleable if the powdery mildew, caused by Podosphaera xanthii, becomes an established epidemic before the disease is noticed. The powdery mildew colonies may appear as typical white spots on the upper leaf surfaces (Fig. 2A). In other cases the mildew forms thin white patches on the undersurface of yellowed lower leaves. Sometimes purplish spots form on verbena that growers don't recognize as powdery mildew (Fig. 2B).
Source: • American Floral Endowment Special Research Reports #100