How To Prevent Or Treat & Control Leaf Spots On Hydrangea Leaves

It's not unusual during hot and very humid summers for hydrangea to develop spots on their leaves. 

The first thing to know is that leaf spots on Hydrangea rarely if ever cause serious injury or death to the plant. Light spotting is unsightly and heavy spotting may temporarily reduce plant vigor and flower bud set. 

In most cases these leaf spots are indicative of Cercospora leaf spot, a temporary condition caused by the fungus Cercospora hydrangea, which is perhaps the most common disease seen on hydrangeas during the months of July through October. The spots form due to water drops from rain or irrigation that sit on leaves during hot and humid conditions. This is especially true in large plant nurseries and residential or commercial landscapes that use overhead irrigation frequently. When using overhead irrigation it is best to water in early to mid morning so the sun can dry the water from foliage.

Read on to learn more about leaf spot on hydrangeas and how you can prevent or treat plants if necessary.

Two types of fungal diseases can infect hydrangeas - Cercospora leaf spot and Anthracnose. 

Cercospora leaf spot is a common disease that usually infects when leaves are regularly wet by rain or irrigation. It begins as brown or purple spots on the leaves at or near the base of the plant. The circular spots are often small and scattered and usually measure 1/8 to 1/4 inches in diameter. Spots on the bigleaf hydrangea develop tan or gray centers surrounded by brown or purple halos. Leaves may eventually turn yellow-green and fall off. 

While Cercospora leaf spot rarely if ever kills the target plant, heavy spotting of the leaves and premature leaf shed is unsightly and may reduce plant vigor and flower bud set during the current season. 

In the landscape, there are several things you can do to prevent or slow the development and spread of Cercospora leaf spot: 

Anthracnose occurs sporadically in the landscape and only effects bigleaf hydrangea (Hydrangea macrophylla), what some folks call "mophead" or "French" hydrangea. These typically produce large snowball-shape flower clusters in shades of blue, pink, red or white. Since this disease appears to be more prevalent in large greenhouse or field plantings of intensively managed hydrangea, damaging outbreaks of this disease in residential landscapes are unlikely, but possible. 

The causal fungus Colletotrichum gloeosporioides can attack both the leaves and the blooms of hydrangea. Hot, wet weather conditions appear to favor disease development. Heavily fertilized plants seem to be more susceptible.

At first, the brown spots are circular or slightly irregular in shape and somewhat sunken on fleshy leaves of hydrangea. The center of these spots may reach 1 inch or more in diameter and turn light brown to tan in color. Alternating dark and slightly lighter rings of dead tissue often give the spots a bull’s-eye or a target-spot appearance. When larger spots border midvein or other major veins in the leaf, they become distinctly more angular in shape. Under ideal conditions for disease development, large, dark brown, irregular blotches may spread across the leaves and flower petals. Unlike Cercospora leaf spot, symptoms of anthracnose may appear almost simultaneously on leaves and blooms in the lower and upper regions of the plant.

Few options are available for controlling anthracnose on Hydrangea. 

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