The following is information on Cercospora leaf spot of Hydrangea which is now present in the landscape.
Cercospora leaf spot on hydrangea is appearing in the landscape again. The irregular purple to purple- rimmed spots with gray centers infect both Hydrangea macrophylla and Hydrangea quercifolia. With lots of rain or overhead irrigation, this fungus disease can cause spotting that detracts from the overall appearance of the plants. If defoliation occurs, which does not happen often, plant health can be compromised. Most of the time fungicides are not warranted.
Fallen diseased leaves are the primary source of spores of the causal fungus Cercospora hydrangeae. These spores are spread to the healthy lower leaves by splashing water. Once C. hydrangeae is introduced into a planting of hydrangea, yearly outbreaks of this disease are likely to occur. 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.
In the landscape and container nursery, removing dead diseased leaves, applying enough nitrogen to maintain a moderate growth rate, and surface watering will help slow the development and spread of Cercospora leaf spot. Since the appearance of symptoms is usually delayed until late summer to early fall, protective fungicide sprays are rarely needed for the control of this disease on hydrangea in the landscape or nursery. For effective control of Cercospora leaf spot with a fungicide, begin applications when spotting of the leaves is first seen and continue applying that treatment as needed. Typically, protective fungicide treatments are suggested only on highly valued plants that suffer noticeable damage every year. Fungicides registered for the control of Cercospora leaf spot include azoxystrobin, chlorothalonil, myclobutanil, and thiophanate-methyl.
Cercospora on Hydrangea. Photo from the University of Arkansas.
Information from Bob Mulrooney, Extension Plant Pathologist, UD and "Diseases of Hydrangea" by Austin K. Hagan, Extension Plant Pathologist and Professor, and Jackie M. Mullen, Extension Plant Pathologist and Diagnostician, both in Entomology and Plant Pathology at Auburn University.