Oakleaf hydrangeas are woodland shrubs featuring large clusters of blooms in the summer and deep mahogany foliage in the fall. Gardeners of these hydrangeas may experience a variety of problems. Under certain conditions, fungal diseases, pest attacks and chlorosis may affect oakleaf hydrangeas.
The fungal disease botrytis blight attacks oakleaf hydrangeas during cool, wet weather. Spots develop on the flowers and eventually merge, forming large, wet blotches. Remove and dispose of all diseased foliage. Disinfect the pruning tool between each cut. Apply a copper fungicide to the shrub to prevent the spread of the disease. To prevent future outbreaks, increase the air circulation between hydrangeas by spacing them at least 48 inches apart. Avoid watering the flowers and leaves and water directly into the soil.
Powdery mildew produces a white fungus on the undersides of leaves, causing the upper leaves to turn purple or brown. Apply a sulfur fungicide to the hydrangea and dispose of all fallen foliage in the fall. To prevent splashing rain from spreading the disease, add a layer of mulch around the hydrangea.
Armillaria Root Rot
Oakleaf hydrangeas affected by armillaria root rot experience a sudden wilting of leaves and may die quickly. White fungus develops on the roots and surrounding soil and in the fall, mushrooms sprout up around the plant. Fungicides will not control this disease, so prevention is the best defense. This disease tends to attack hydrangeas previously damaged by drought, so water oakleaf hydrangeas every four to seven days during periods of drought to prevent an outbreak. Avoid planting new hydrangeas in the same location as plants previously infected with this disease.
Aphids frequently attack new growth on oakleaf hydrangeas. These small green pests gather in clusters on the undersides of leaves, causing leaves to curl and turn yellow or brown. Ants often feed on the sticky secretions of aphids, so the presence of ants on hydrangeas may signal an aphid problem. Rid hydrangeas of aphids by applying a powerful spray of water every other day. Spray in the morning to allow time for the leaves to dry before evening. If necessary, apply insecticidal soaps to the plant.
Nearly impossible to see with the naked eye, mites typically attack oakleaf hydrangeas during dry weather. When under attack from mites, new growth appears distorted and leaves may look scorched along the edges. Water hydrangeas regularly during dry periods to prevent mite attacks. If mites appear, spray the leaves with a forceful jet of water every day for three days. Apply insecticidal soap if mites continue to attack the hydrangea.
If new leaves appear yellow, the hydrangea may be suffering from chlorosis. Chlorosis often occurs as a result of low iron levels in the soil and affects hydrangeas planted in highly alkaline soil. Purchase a chelated iron product at a garden center and add it to the soil to increase iron levels. Follow instructions on the product. For long term results, increase the acidity of your soil by adding sulfur or mulching hydrangeas with pine needles when planting.