Five different diseases that cause yellow leaves and brown spots commonly plague hydrangeas, according to Auburn University plant pathologists Austin Hagan and Jackie Mullen. Cases of root rot can also cause yellow leaves and brown spots in hydrangeas. Root rot occurs when hydrangeas are planted in poorly draining soils or are overwatered, according to the US National Arboretum. Yellow leaves may be a symptom of iron deficiency. The higher the pH of the soil (the more alkaline), the less available iron is to the hydrangea. You can add iron to the soil, or increase its acidity (lowering the pH) by adding aluminum sulfate or organic mulch.
Cercospora Leaf Spot
The Cercospora hydrangeae fungus can attack your hydrangeas. This fungus appears as circular brown or purple spots on the hydrangea leaves. The spots may turn tan or light gray in the center and are surrounded by a brown or purple ring. Removing diseased leaves, adding nitrogen and surface watering can slow the spread of Cercospora leafspot. Outdoors, rainfall and splashing water spread the fungus.
The Colletotrichum gloeosporioides fungus attacks hydrangea leaves and blooms. The fungus prefers heavily fertilized hydrangeas and hot, wet weather. Brown spots on the hydrangea leaves may appear like bulls-eyes. As these brown spots grow, they become angular blotches. This fungus comes from the spores of wet plant debris. Removing fallen and diseased leaves can help slow the spread of anthracnose, and protective fungicide treatments every two weeks may help.
Botrytis blight is a gray mold that sprouts after cloudy, humid and rainy weather. It usually affects flower petals but this reddish-brown spotting can affect hydrangea leaves. Clearing away debris often and treating with fungicides can help prevent the blight.
Powdery mildew spreads rapidly in greenhouse hydrangeas but isn't as obvious in landscape hydrangeas. The cottony, white fungus first appears as small white spots on the leaves. The E. polygoni fungus that causes powdery mildew may also lead to yellow spots and purple blotches on hydrangea leaves.
Phytophthora Root Rot
Excess water can cause phytophthora root rot in hydrangeas. Yellow leaves and wilting are signs of this root fungus. Plant hydrangeas on raised beds to prevent root rot. Fungicides can help prevent the disease but cannot cure it once it's established.