Hydrangea Plant Diseases

Hydrangeas (Hydrangea spp.) are a genus of small trees, flowering shrubs or climbing plants native to Asia, North America and South America. Five varieties are most often cultivated in North America and thrive in U.S. Department of Agriculture hardiness zones 4 through 9, depending on the variety or cultivar. These plants yield large, attractive white, blue or pink blossoms. They generally grow best in partial shade, prefer moist, well-drained soil and do well in greenhouses or as container plants. Although generally healthy, hydrangeas are susceptible to a number of diseases, some of which can be life-threatening.

The Erysiphe polygoni fungus causes powdery mildew disease. A grayish-white powdery substance covers infected leaves, and the leaves may eventually turn yellow or fall from the plant. Although unattractive, powdery mildew is rarely fatal. Avoid problems by spacing your plants far enough apart to allow good air circulation. If a fungicide is needed, add 1 tablespoon of a thiophanate-methyl fungicide to a gallon of water. Spray plants as soon as the first signs of powdery mildew appear and then reapply every 10 to 14 days as needed.

The Botrytis cinerea fungus attacks new hydrangea buds and petals during periods of humid, wet, cool weather, causing botrytis blight. Water-soaked spots appear on the blossoms, and the spots soon turn into brownish-red blotches. The flowers deteriorate, and a grayish fungus covers the surface. Avoid botrytis blight by making sure your plants have adequate sunlight and air circulation. Providing appropriate amounts of fertilizer, avoiding chemical or mechanical injuries and watering plants regularly will help prevent disease. You can treat infected plants with thiophanate-methyl. Add 1 tablespoon of the fungicide to a gallon of water and spray every seven to 14 days or as long as the cool, overcast weather lasts.

Anthracnose is most common in hot, humid, wet weather. The Colletotrichum gloeosporioides fungus creates large, round or irregular spots on the leaves and blossoms on the upper part of the plant canopy. If anthracnose occurs, remove diseased leaves and treat the plant with thiophanate methyl, or add 2 teaspoons of chlorothalonil to a gallon of water, and spray the plant every 10 days to two weeks.

Rounded tan spots ranging from 1/8 to 1/4 inch appear on hydrangea leaves when the plant has cercospora leaf spot. These spots usually have a brown or purple halo. Fungicides are rarely necessary. Avoid wetting the plant’s leaves when watering, and remove diseased or dead leaves to prevent the fungus from spreading to healthy foliage.

The Phytophthora nicotiana fungus attacks the roots of poorly drained plants. Roots are usually off-white in color, but diseased hydrangea roots are brown and break off easily when handled. Above the surface, the plant may appear stunted, and the leaves often turn yellow or fall from the plant. Adequate drainage and appropriate amounts of water will help prevent phytophthora root rot, but infected hydrangeas cannot be cured with fungicides.

The Armillaria mellea and Armillaria tabescens fungi live in the soil and infect drought-stressed hydrangeas. The shoots wilt, and the hydrangea may die. Stringy black fungi appear on the roots, and honey-colored mushrooms form around the plant base. This fungus does not respond well to fungicides. Avoid problems by watering plants every four to seven days during drought conditions. If plants become infected, remove and destroy them.

Light-colored rings develop on the foliage of hydrangea with ring-spot virus. Some types of hydrangeas may also have stunted or distorted leaves. There is no cure or treatment for this type of virus. The Texas Plant Disease Handbook recommends removing and destroying diseased plants.