Powdery mildew is common seen in greenhouse grown big leaf hydrangea (Hydrangea macrophylla). However, it is rarely seen on oakleaf hydrangea (H. quercifolia)

The symptoms start as small white patches scattered on the leaves. The pathogen forms a white hyphae (threadlike) that can be seen in the infected areas using a hand lens.

Yellow blotches can be seen on areas infected with the pathogen mainly on the upper side of the leaves.

Purple blotches can also be seen on the upper leaf surfaces.

Under severe conditions, the fungus completely covers the plant surface, and can lead to stunting of the plant and general discoloration of the plant.