Elizabeth Little/Extension Specialist
Department of Plant Pathology, University of Georgia
Many areas in Georgia are experiencing warm, humid weather with frequent
thunderstorms this month. This is the perfect weather for fungal leaf spot
development in the landscape. Most leaf spots are benign and will not damage
the plant. Cercospora Leaf Spot of Hydrangea is an example of a leaf spot
disease that commonly occurs during prolonged wet weather or under sprinkler
irrigation. As with most leaf spots in the landscape, chemical treatment is rarely
needed. Some leaf spotting and leaf drop will not the harm the plant.
Management of leaf spot diseases involves removing sources of the disease and
protecting the plant. Start by selecting plants that will thrive where they are sited.
The fungi that cause these diseases mainly survive on the infected leaves that
fall to the ground, so removing and destroying diseased leaves can help lessen
the amount of disease next year. Stressed plants are more susceptible to
disease, so check the cultural conditions and optimize them with fertility
management and mulching. Good air circulation around plants will lower the
humidly and leaf wetness and reduce disease. Avoid wetting the leaves when
irrigating landscape plants.
If the plant is repeatedly defoliated each year or appears to be dying back
because of the disease, prune out the affected stems and use fungicides
preventatively (before the symptoms) to protect new growth.