Lacecap Hydrangeas & Fungus

Rust

• Rust spots can appear underneath the leaves and are yellow, brown or bright orange pustules. The spots spread from splashing rain or water and are worse in moist weather, according to Backyard Gardner. To prevent rust, remove all debris from around the plants. Do not water from above the plants and make sure there is enough drying time for the plants before night. A fungicide specifically for rust may be used to treat the disease.

Leaf Spots

• The United States National Arboretum indicates that leaf spot organisms are often found on hydrangeas. Caused by fungi or bacteria, they are characterized by lesions of brown, gray or black, with a surrounding edge of purple or yellow. It can be spread by rain, insects or garden tools that are dirty. Infected leaves should be removed when the hydrangea is dry. Fallen leaves around the base should be cleared as well. Water the plant at base rather than from above. A fungicide such as chlorothalonil, mancozeb or myclobutanil may also be necessary.

Powdery Mildew

• If hydrangeas do not have enough light or air circulation, they can develop a powdery mildew, according to Backyard Gardner. A gray or white powdery fungus appears on the top surface of the leaves. The leaves may also turn brown or yellow, curl and fall off. Subsequent leaves will be deformed. To prevent this problem, always water at the base of the plant and use nitrogen fertilizer cautiously. Remove debris from around the plant in the fall and apply a fungicide such as fenarimol, paraffinic oil or azoxystrobin, according to the instructions on the product.

Anthracnose

• Anthracnose is a fungus that can affect the leaves as well as the blooms, according to the Alabama Cooperative Extension System. This disease is encouraged by hot, moist weather and plants that are heavily fertilized. It starts with brown circular spots on the leaves that eventually turn to lighter brown or tan, forming bull's-eye type spots that can spread to the flowers. Fungicides that may be used to treat this fungus include chlorothalonil and thiophanate-methyl.

Botrytis Blight

• Botrytis blight is a gray mold that usually appears on the buds and the petals of hydrangea. It occurs in cool, wet, humid conditions and appears as reddish-brown blotches in an irregular shape. The petals fade to brown with a gray fuzzy growth of fungus. Fungicide treatment includes iprodione, mancozeb and thiophanate-methyl.