The white cone-shaped blooms of oakleaf hydrangea (H. quercifolia) display brilliantly against deep green leaves shaped like those of an oak tree. This deciduous shrub is cold hardy in USDA zones 5 through 9 and can be grown in full shade to full sun. Without pruning to control its size, oakleaf hydrangea can reach a height and width of 6 feet. Oakleaf hydrangea is susceptible to several plant diseases.
Two different types of root rot, Phytophthora and Armillaria, may strike oakleaf hydrangea. Phytophthora, a fungus, is a form of root rot associated with oakleaf hydrangea grown in containers and may not display until after the shrub takes its place in the landscape. The condition can be perpetuated with excessive watering and poor drainage, resulting in leaf wilting, yellowing or shedding. A fungicide can help control this problem.
Armillaria root rot, also called mushroom root rot, is caused by the Armillaria fungus and can affect hydrangea during drought conditions. The disease can be recognized by wilting leaves that are not rehydrated when the shrub is watered. Oakleaf hydrangea can die from this threat as fan-shaped fungus forms around the base of the shrub. Armillaria root rot may be prevented with proper air flow and planting in an area away from other diseased plants, even if those plants have been removed. Fungicide cannot help. The shrub may need to be dug up and destroyed, and no new plants should be grown in the same area.
Leaf spot is a common fungal infection of all hydrangea shrubs. With oakleaf hydrangea, leaf spot appears in irregular-shaped blotches of brown or purple that can turn the leaf yellow. Leaf spot can be caused by excessive moisture from frequent rain, improper watering or lack of air circulation. Prune out and dispose of stems whose leaves have spots. A fungicide can be used. Water at ground level. If using a sprinkler, water in the morning to give the water time to evaporate from the leaves. Aggressive pruning to reduce the shrub's size or pruning of nearby vegetation could be performed to increase air flow.
Displaying as a white dusting on the leaves, powdery mildew is uncommon with oakleaf hydrangea though it may show in adverse situations, like improper watering or lack of air circulation around the shrub. Powdery mildew can be controlled through pruning, proper watering techniques, good air flow and fungicide preparations.