See:
Hazelnut (Corylus avellana) - Powdery Mildew
Notes Phyllactinia guttata and Podosphaera clandestina have been reported and are fungi that overwinter on fallen leaves and fallen infected twigs. The conidia are readily wind disseminated. For Phyllactinia, chasmothecia develop within the external hyphae and form appendages late in their development. The uneven wall thickness and bulbous nature of the appendages allows the entire cleistocarp to be dislodged from the host. When dry, the appendages flex like a spider's legs and allow the cleistocarp to be dislodged and carried by wind. Symptoms are common for powdery mildew as white powdery growth appearing on leaves. There have been no samples diagnosed with this disease by the OSU Plant Clinic.