This month’s Tree Tip was written and researched by Chris Hudson, Plant Healthcare Specialist and Certified Pesticide Applicator. He may be contacted directly via email at chris@woodlandtree.com with any questions concerning this article or any plant health care issues regarding stress, insects, or disease.
Powdery mildew is a fungus that can be commonly found on a large number of trees and shrubs in our area. However, dogwoods and crepe myrtles seem to be affected the most. The dry weather, warm days, and cool nights are ideal conditions for the fungus to thrive.
Powdery mildew mainly attacks and feeds on the leaf surface and is encouraged by crowding and poor circulation. The fungus generally originates on new tender growth and spreads to other foliage. From the leaves, powdery mildew can infect stems and small branches, killing the woody tissue. The disease is also spread by spores, so it is possible for other trees or shrubs in the landscape to be affected.
Left unchecked, powdery mildew can, at a minimum, reduce aesthetics and contribute to the decline of a tree or landscape plant, making the plant more susceptible to other insect and diseases. At a maximum, powdery mildew can overtake the tree or shrub and kill the plant. Dogwood trees are most susceptible to severe infestations and have been known to succumb to more aggressive strains of powdery mildew.
Diagnosing
Powdery mildew appears as white or grayish patches of fungal growth on the top and bottom of leaves. Leaves that remain untreated will begin to curl, wilt, and eventually fall. Infected stems turn dark brown and/or black and are no longer viable.
Prevention and Control
To prevent powdery mildew from appearing or spreading, several cultural steps can be taken. After the leaves have wilted and fallen off, they're still able to transmit spores even after chemical treatment, so fallen leaves should be raked and removed. Also, hosts and overhanging trees should be thinned to allow more sunlight and eliminate crowding of the leaves. When present, infected woody tissue should be removed. Finally, proper fertilization is required. Excessive nitrogen fertilizer can contribute to the spread of powdery mildew. Low rates or slow release nitrogen should be used on susceptible host plants in the spring.
Chemical prevention of powdery mildew is recommended for Dogwood and Crepe Mytles that have had issues with the fungus in previous years. Prevention consists of a foliar spray program, with three applications 45 days apart, starting in May.
Once powdery mildew has affected trees and shrubs, control treatments are required to keep damage at a minimum. Control treatments consist of two foliar spray applications approximately 10 to 14 days apart, depending the severity. After the initial treatments one additional application is recommended after about 45 days.
Keeping your trees and shrubs healthy and vigorous is our goal. The staff and arborists at Woodland Tree Service are more than happy to evaluate your trees and shrubs and help your plants meet their full potential.
If you need assistance with any of your trees or shrubs, please contact the experts at Woodland Tree Service. Email info@woodlandtree.com or call us at (901) 309-6779.