Hydrangeas are deciduous flowering shrubs that bloom from midsummer to fall. Although the plants are noted for their easy care, there are several insects and diseases to which they are susceptible. Hydrangeas are known to acquire diseases such as leaf spots, blights, wilts, and powdery mildews. With early detection, most of these maladies can be eliminated.
Caused by the fungus Cercospora hydrangeae, leaf spots first appear as small circular brown or purple spots near the plant’s base. Cercospora leaf spots thrive in warm wet weather. The fungus lays dormant on afflicted fallen leaves through the winter and becomes a source for new disease in the next year. They can be treated by removing the diseased leaves and adding nitrogen to the soil.
TIP: Our expert gardening adviser adds, "Leaf spots are spread to healthy leaves through splashing water. To prevent this, water your hydrangea only at the surface, being careful not to get the leaves wet. Also, water only during the daytime so that the leaves can dry off before nightfall. Provide good plant spacing so air movement and sunlight can dry the leaves quickly. In extreme cases where plants are incurring noticeable damage each year, fungicide applications are suggested. The applications need to begin just before or just as the spots begin to appear, and repeat applications may be necessary."
Caused by the fungus Botrytis cinerea, blight appears on the flower buds and blossoms. The petals turn reddish-brown and eventually cover in mold. Blights normally occur during wet, humid seasons and are treated with fungicides.
TIP: Rachel suggests, "Blight can survive on infected plant debris indefinitely, so be sure to prune away these afflicted areas immediately, or it will spread. Avoid overhead watering or misting of your hydrangeas, particularly if blight has been a problem in previous years. Fungicides are most helpful as a preventative measure against blight, so if yours are planted in especially wet or humid areas, you might want to consider treatment before any symptoms actually arise."
Wilting can be a simple matter of lack of water, or a more serious disease such as mushroom root rot or phytophthora root rot. The plant will wilt and turn yellow if it is infected with one of these diseases. When root rot is the culprit, the best option is to ventilate the soil or try to restart the plant.
TIP: Rachel says, "If ventilation does not repair the damage, the hydrangea should be removed and destroyed. Fungicides will not control root rot. The best method to combat root rot is prevention. Ensure your hydrangeas have strong root systems by avoiding poorly draining soils and feeding twice a year (once in early spring and again in early fall) with a balanced fertilizer such as 10-10-10."
Powdery mildew starts with small circular white or light gray powdery patches on the hydrangea leaves. It favors high humidity and low air circulation. If left untreated, the white patches will become large yellow or purple blotches.
TIP: Rachel adds, "Good plant spacing and frequent "scouting" and inspecting of the leaf surfaces are good preventative measures against mildew. Typically, fungicide treatments are only suggested for those hydrangeas whose foliage was heavily afflicted by powdery mildew in previous years. Treatment should be continued at the interval listed on the product label."
These tips for spotting pests and diseases on your hydrangeas will ensure you can enjoy their beauty for years to come. It will also keep them from spreading to other plants in your garden.
Photo (c) bootandall, 2005 davesgarden.com/members/bootandall/