Hydrangeas make lovely ornamental shrubs, but are often regarded as difficult to grow. They do need plenty of water and shade in hot climates, but the most important consideration is selecting a variety appropriate for your area. 'PeeGee' and 'Annabelle' are cold hardy and suitable for northern climates. Big leaf varieties work well in warmer areas. Diseases are more prevalent in hot, humid climates and usually affect nurseries and greenhouses more than private plantings, according to Alabama Cooperative Extension.
The two main diseases that cause rusty flowers, according to Alabama Cooperative Extension, include anthracnose and botrytis blight, or gray mold. Both diseases are caused by fungi and are more prevalent in greenhouses and nurseries than in private home landscapes.
Anthracnose causes brown spots on leaves, usually followed by brown spots on the flowers. The spots may start small and spread across the entire leaf or flower surface. Gray mold causes irregular reddish-brown spots on the flowers. The blooms soon wither and may exhibit a fuzzy gray growth. Gray spores may be visible, as well.
Both fungi proliferate when temperatures are between 75 and 95 degrees F, especially after several days of foggy or moist conditions. Anthracnose overwinters in dead leaves and plant debris.
Good cultural practices can minimize the incidence and spread of disease. Watering plants by soaker hose rather than sprinkler keep leaves dry. Watering first thing in the morning, as opposed to evening, allows leaves to dry quickly. Removing diseased blooms and leaves prevents the spread of disease and cleaning up dead leaves in the fall minimizes the chance of overwintering.
Hydrangeas infected with anthracnose or gray mold can be treated with fungicides, such as chloronthalonil or thiophanate methyl every 10 to 14 days until symptoms subside. Fungicides may be applied more often on severely infected plants.