These are the cleistothecia or overwintering bodies of the fungus. The disease is most commonly observed on the upper sides of the leaves. It also affects the bottom sides of leaves, young stems, buds, flowers and young fruit. Infected leaves may become distorted, turn yellow with small patches of green, and fall prematurely. Infected buds may fail to open.
Powdery mildews are severe in warm, dry climates.
This is because the fungus does not need the presence of water on the leaf surface for infection to occur. However, the relative humidity of the air does need to be high for spore germination. Therefore, the disease is common in crowded plantings where air circulation is poor and in damp, shaded areas. Young, succulent growth usually is more susceptible than older plant tissues. For example, the species Uncinula necator, which causes powdery mildew on grape and linden, does not attack lilac.