Several diseases attack the oakleaf hydrangea. Each has its own characteristics, so you can tell which disease your plant might have. Start by looking at the leaves of the oakleaf hydrangea.
Powdery mildew attacks many greenhouse-grown oakleaf hydrangeas. Early signs of this disease are small circular white spots with feathery edges scattered on the lower surface of the leaves. This disease will increase until it covers all the leaves and shoots.
Cercospora Leaf Spot
Cercospora leaf spot occurs in oakleaf hydrangeas that are landscape planted. The early signs of this disease are small, brown or purple spots that begin on the leaves near the base of the plant. As the season progresses, the centres of these spots will change to a light grey colour.
Anthracnose occurs in oakleaf hydrangeas planted in the field and landscape. Unlike Cercospora leaf spot, the spots of the anthracnose can reach an inch or more in diameter. The shapes of the rings are angular when they cover the midvein or a major vein of the leaves.
Botrytis blight occurs in the flowers of greenhouse-grown oakleaf hydrangeas. The small, water-soaked spots that occur on the petals quickly grow into reddish-brown irregular blotches. The bright flowers fade into a brown colour and a fuzzy grey growth of fungus covers them.
Mushroom Root Rot
Mushroom root rot occurs in landscape oakleaf hydrangeas. This disease causes a sudden withering of shoots that looked healthy. After a rain, honey-coloured mushrooms appear on the roots.
Phytophthora Root Rot
Phytophthora root rot occurs in container-grown oakleaf hydrangeas. Early signs are wilting of the foliage. The main feeder root will be brittle and brown.