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The disease called powdery mildew is common on many ornamental trees and shrubs including dogwoods. Warm, dry days and cool, damp nights with high humidity favor disease development. This fungus grows on fresh succulent stems, leaves, buds and flowers. Powdery mildew occurs in open landscapes as well as plants growing in dense, shady areas where air circulation is poor.
Powdery mildew gets its name from the fine white, powdery-like growth that is on the surfaces of the new leaves and the young shoots. This white material consists of the fungus body, the mycelium, and the numerous spores that it produces that are easily spread by the wind. Young leaves with severe infection can be twisted, stunted and deformed. Older leaves may only have a reddish or purplish discoloration on the upper leaf surface. With severe infection dead spots can be seen on the leaves or some marginal browning may be found. Scorching of the foliage may occur and leaves may also drop early from the tree. Young plants and new growth are usually more severely damaged than older plants.
There are many powdery mildew diseases on many of the different plants that we grow but they are generally host specific. They do not go back and forth across the different species. In the southeast, two different fungal organisms are known to live on leaf surfaces and tender shoots: Microsphaera pulchra and Phyllactinia guttata. Spores are spread by wind to surrounding dogwood plants.
Most powdery mildews of landscape trees occur late in the summer causing aesthetic damage. Powdery mildew probably weakens trees but it is not expected to be lethal. Some decrease in flower production can occur the year following times of high disease pressure. Winter hardiness can be reduced with infection. Powdery mildew can reduce plant photosynthesis and increase water loss from leaves. Over time this could weaken trees making them more prone to dogwood borer or infection by other pathogens.
Cultural controls should be the first line of defense. Begin by raking up and destroying all fallen leaves. Prune out dead and infected branches and twigs. Improve air circulation and sunlight penetration around the tree by removing overhanging branches and crowding vegetation.
Resistant species and cultivars are available and should be considered for new plantings. Cultivars of the oriental dogwood Cornus kousa (such as ‘Milky Way’, ‘Milky Way Select’, and ‘National’) and many of the Cornus florida x Cornus kousa hybrids (such as ‘Aurora’, ‘Constellation’, ‘Celestial’, and ‘Stellar Pink’) are generally resistant to powdery mildew. The flowering dogwood (Cornus florida) cultivars ‘Appalachian Joy’, ‘Appalachian Blush ‘, ‘Appalachian Snow’ and ‘Appalachian Mist’ are very resistant to powdery mildew. ‘Cherokee Brave’, ‘Springtime’, and ‘Pygmy’ have partial resistance. All other flowering dogwoods (C. florida) are susceptible.
If disease is severe enough to warrant the use of fungicides, be sure that the dogwood is a valuable specimen and that available spray equipment can provide good coverage. Timing of application is very important for fungicides to be effective. Products must be applied as soon as symptoms are noticed or when the flower buds begin to open. A second spray should be made as new leaves are unfolding. Two more applications may be needed at the end of July and before leaf drop in the fall. Fungicides effective for dogwood powdery mildew control include myclobutanil and propiconazole. Product labels provide information on how often to spray. If repeated sprays are used alternating fungicides is recommended to decrease the chance of fungi developing resistance.