Sid Mullis: Leaf spot is the plague of hydrangeas

Go outside and take a walk around your yard and look at the leaves of your ornamental trees and shrubs.


What I bet you’ll see is lots of leaf spot disease on the leaves. Leaf spots are worse when we have a lot of rain, which we’ve had since the beginning of August. Leaf spots will continue to get worse and cause premature leaf drop as we go into the fall.

Hydrangeas, one of the most popular plants in the landscape, are very susceptible to leaf spot diseases.

The most common diseases on the leaves of hydrangeas are cercospora leaf spot and anthracnose. The symptoms of cercospora leaf spot are scattered, small, circular brown or purple spots on the leaves. On the bigleaf hydrangea, the centers of these spots eventually turn tan to light gray and are surrounded by a brown or purple halo.

The spots are usually about 1/8- to 1/4-inch diameter. This combination of a pale center and dark margin is usually called a frogeye leaf spot pattern. In contrast, the leaf spots on oakleaf hydrangea appear angular and are dark brown to purple. Quite often heavily spotted leaves turn yellow green and might fall to the ground.

Typically, the spotting begins on the leaves at the base of the plant and then gradually spreads upward through the canopy. Spotting of the leaves, which usually starts in midsummer, is most noticeable by early fall.

Fallen diseased leaves are the primary source of spores of the causal fungus. These spores are spread to the healthy lower leaves by splashing water. Once cercospora is introduced into a planting of hydrangeas, you normally have it every year to some extent. Frequent late summer rain showers will not only greatly increase the rate of disease spread, but also intensify the level of leaf spotting and defoliation. Extended periods of drought will usually suppress disease development and spread.

To help prevent, or at least cut down, on the severity of the disease, apply just enough nitrogen to maintain a moderate growth rate. When you have to water them, directing the spray at the base of the plant will help slow the development and spread of cercospora leaf spot. Because the appearance of symptoms is usually delayed until late summer to early fall, it is basically too late to apply protective fungicides.

For effective control with a fungicide, you must begin applications when spotting of the leaves is first seen and continue applying that treatment as needed. Examples of fungicides registered for the control of cercospora leaf spot are Daconil 2787 (chlorothalonil) and Immunox (miclobutanil).

The other disease I mentioned earlier is anthracnose, and this one can attack both the leaves and the blooms of the hydrangea. Hot, wet weather conditions appear to favor disease development. Heavily fertilized hydrangeas can be the most sensitive to attack by anthracnose.

Unlike cercospora leaf spot, symptoms of anthracnose might appear almost simultaneously on leaves and blooms in the lower and upper region of the plant canopy.

There are few options available for controlling anthracnose. Daconil 2787 is the best fungicide for this disease, but like cercospora leaf spot, it is too late in the season to spray.

Keep in mind that these diseases rarely kill a plant. Heavy spotting and premature leaf shed are unsightly and can reduce plant vigor and flower bud set.