Cercospora leaf spot

Published August 30, 2019

Cercospora leaf spot is an infectious disease that affects smooth, panicle, oakleaf and bigleaf types of hydrangea in both landscapes and nurseries. This disease is caused by the fungus Cercospora hydrangea and is perhaps the most common disease seen on this ornamental during the months of July through October. The disease rarely kills the plant, but if it is severe, it can reduce overall plant vigor by repeated defoliation. For bigleaf-type hydrangeas in the landscape, Cercospora leaf spot tends to be less severe under shady conditions, but in nursery environments under shady conditions, frequent overhead irrigation can intensify disease activity and subsequent defoliation and loss of vigor.

Cercospora leaf spot can be more intense on hydrangea in a nursery environment, under shady conditions and with overhead irrigation.

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The guide to CERCOSPORA LEAF SPOT

Symptoms: Leaf spot symptoms can vary according to the type of hydrangea (panicle, smooth, etc.) that is infected.

In general, leaf spots are first visible on older leaves at the bottom of the plant then spread upward toward the top of the plant. Initial spots are purple and small with a circular shape. As spots enlarge, they often become irregular or angular in shape and develop a tan or gray center surrounded by a purple or brown border. Leaves that are severely spotted often become a yellow-green color.

Initial infection usually occurs during May, but symptoms don’t become apparent until later in the season. Midsummer environmental conditions contribute to disease severity. Rainfall and overhead irrigation are major factors that play a pivotal role in symptom expression and intensity. Late summer rainfall can be a major contributor to defoliation and decline.

The fungus produces numerous spores that are sometimes visible with the unaided eye as minute dark specks within the center of the spot.

Management: Sanitation is an important tool in disease management. Since the fungus can easily survive in infected leaves that fall and remain on the ground or that remain on the bush, removal of these leaves can help prevent future infections and disease outbreaks.

Spotted leaves should be removed any time during the growing season when they are present, especially before new leaves begin to form in the spring. If possible, irrigate plants using drip irrigation, since splashing water from an overhead sprinkler can spread the fungus from leaf to leaf and create an ideal environment for disease activity.

There are some fungicides available to help manage Cercospora leaf spot. Products containing chlorothalonil, myclobutanil or thiophanate-methyl are most effective when applied prior to or at the first sign of leaf spots. These fungicides work best to protect newly developing leaves, but they will not protect new growth that emerges after the application has been made. Consequently, to keep plants relatively free of the disease, multiple applications (usually every 10 to 14 days) may be necessary.

Source: University of Arkansas

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