Powdery mildew is disease that is easy to diagnose. In Master Gardener training sessions, I often teach this because it is one of the most obvious diseases in the landscape. A powdery white growth on the surface of leaves, often in feltlike patches on the leaves and occasionally on stem tips or fruit, is a typical symptom. Some plants (such as rose or apple) may show some distortion of leaf growth.
The symptoms on dogwood are different than the typical powdery mildew and may be passed off as another problem. On dogwood, this disease may cause a white powdery growth on the leaf surface, but it may also cause scorch on the edges of leaves, dead patches on leaves, yellowing, leaves with a reddish color, and premature leaf drop.
In most cases, we do not consider powdery mildew a major problem because it occurs late in the season and does not affect overall plant health. On most ornamental species, it is considered more of an aesthetic problem than a growth problem. Dogwoods may be more severely affected by this disease because it occurs earlier in the season. There has been an increase in levels of powdery mildew in Kentucky over the last several years, and we have had reports of dogwood powdery mildew in Indiana.
Although the Plant Clinic has not been inundated with samples of powdery mildew, it has been identified in Illinois and may have been overlooked in many situations. Look for powdery mildew on dogwoods in exposed locations or partially shaded spots in the landscape. Dogwoods have many problems in our state, including winter injury, cankers, leaf spots, anthracnose, and borers, to name a few. Now powdery mildew can be added to the list.
According to University of Kentucky plant pathologist Dr. John Hartman, the oriental dogwoods (Cornus kousa) are resistant to dogwood powdery mildew; the C. florida (flowering dogwood) cultivar ‘Cherokee Brave’ has partial resistance; and all other C. florida dogwoods are susceptible.
Prune out dead wood, water the plants in periods of drought, and prune surrounding plants to promote better air flow in the landscape. It is also a good idea to mulch dogwoods to avoid trunk injuries from mowers or trimmers.
Fungicides may be used to control this disease, but make certain that it will be worth the expense, that the tree is a valuable dogwood specimen, that a correct diagnosis has been made, and that cultural practices are also employed. Dr. Hartman states that effective fungicides include Banner MAXX (propiconazole), Bayleton (triadimefon), Cleary 3336 (thiophanate-methyl), Eagle (myclobutanil), Immunex (the homeowner-use formulation of propiconazole), Immunox (the homeowner-use formulation of myclobutanil), and Rubigan (fenarimol). First sprays should be applied in early June and continued once every two to three weeks until mid-August. Be certain that your spray equipment provides good coverage of the fungicide.