Rust, like leaf spot diseases, is caused by a fungal pathogen. Spores spread via wind or splashing water to hydrangea leaves. Rust appears on foliage as masses of dry, orangish spore pustules primarily on leaf undersides while the upper leaf surface turns yellow or brown. Infected hydrangea leaves may drop early.
Also question is, what causes brown spots on leaves of hydrangeas?
Leaf Spot Disease in Hydrangea. Two types of fungal diseases can infect hydrangeas -- cercospora leaf spot and anthracnose. Cercospora leaf spot (Cercospora hydrangea) begins as brown or purple spots on the leaves at or near the base of the plant. Anthracnose is caused by the fungus Colletotrichum gloeosporioides.
What causes hydrangea leaves to curl and turn brown?
If the margins of the leaves fade from green to grey and then turn brown, the plants were dry for too long. If the petals of the flowers turn brown at the tip, not enough water was applied. Too much sun exposure can cause your hydrangea shrubs to burn on its leaves and blooms.