Hydrangeas have few serious pest and disease problems that affect their health although there are several listed here that will cause cosmetic damage. For information on the selection, care, and pruning of hydrangea please visit our Hydrangea fact sheet.
Diseases
Botrytis blight (Botrytis cinerea). Small water-soaked spots on the petals quickly expand into brown irregular blotches. Brown, withered masses of flowers may be covered with the fuzzy gray growth of the fungus. Flower buds may also be affected and leaf spots can occur when infected petals have fallen on them. Disease development is favored by cool, humid, wet conditions. The causal fungus survives almost indefinitely in plant debris. Spores are spread to healthy tissue by wind. For prevention, use the methods described below for powdery mildew.
Leaf spots caused by a variety of fungi can occur on hydrangea. A common leaf spot disease in the northeast is caused by the fungus Cercospora. Tan spots with reddish brown borders form on the leaves. Infection can be reduced by minimizing leaf wetness. If needed, protectant fungicides can be applied prior to infection. This may be desirable for hydrangeas that had a lot of leaf spots the previous year and prior to wet weather. In addition, removal of infected leaves during or at the end of the season may help reduce inoculum the following season.
Powdery mildew (Microsphaera penicillata) can occur on all hydrangeas but is most serious on bigleaf hydrangea. In addition to the usual white, powdery fungal growth on the leaf surfaces, yellow or purple leaf blotches might be present. This disease doesn’t usually harm the plant but is aesthetically problematic for some gardeners. The problem will be most severe on plants in shady or crowded sites that promote high humidity. Clean up fallen leaves and dead tissues to remove the spores that may cause new infections. Chemical control is unnecessary except in severe cases. Products available to control powdery include potassium bicarbonate or horticultural oils.
Root rot (Armillaria spp.) can occur in landscape plantings, especially on oakleaf hydrangea (H. quercifolia). Plants stressed by drought or other factors are most susceptible. Symptoms first appear as wilting on one or more shoots. Watering will not return the leaves to normal. Within a few weeks, the remaining shoots will wilt and the hydrangea will die. The fungus produces white, fan-shaped mycelia mats under the bark at or near the soil line. In addition, black, shoestring-like rhizomorphs of Armillaria can usually be found on the surface of infected roots, in the surrounding soil, and under the bark. In the fall after a heavy rain, clusters of honey-colored mushrooms appear above the damaged roots.
Good growing conditions are the best defense against Armillaria root rot. Water during summer droughts and fertilize according to soil test results. There is no chemical control. Another root rot, caused by Phytophthora nicotian, is favored by poorly drained soils. Symptoms are similar to those caused by Armillaria sp. Do not plant hydrangea where flooding occurs after a heavy rain.
