When looking at the tip of dogwood tree branches, the younger leaves may look distorted and covered with a white powdery-looking material.
This material is actually part of the fungus, which is called mycelium. The common name of the disease is powdery mildew.
The white, dusty fungal growth is the typical sign of powdery mildew but is not always as obvious on dogwood as powdery mildews of other plants in the landscape. Some affected dogwood trees have older leaves that are yellow with a faint tan area on them. In addition, some may show yellow mottling like a nutrient deficiency in the leaves.
Another symptom of powdery mildew on older leaves is small, dark red splotches, which appear on upper surfaces of otherwise healthy-looking leaves. These splotches develop into dark brown to tan dead areas on the leaf. The white powder is usually not found on fully expanded leaves where the dead patches appear.
It only takes humid conditions for this disease to develop. Germination of the fungal spores and infection of the leaves occurs in about 6 hours, then new spores are produced in five or six days to infect more leaf area.
Powdery mildew most likely reduces plant photosynthesis and increases leaf water loss partly through the fungal mycelium, which is on the surface of the leaf. In the long run, this could make it more susceptible to insects and diseases. Under high disease pressure, a reduction in flowers could occur the next year.
The susceptibility of individual dogwood trees to powdery mildew varies greatly. Appalachian Joy, Cherokee Brave, Jean's Appalachian Snow, Karen's Appalachian Blush and Kay's Appalachian Mist are cultivars resistant/tolerant to powdery mildew.
To manage powdery mildew in dogwoods, avoid cultural practices such as applying nitrogen fertilizer now, pruning heavily and irrigating excessively, which stimulate succulent growth and encourage powdery mildew. Continue good cultural practices, such as mulching over the root system with only 2 to 2.5 inches of mulch, pruning out dead branches, and providing good air movement and light penetration by judicious pruning of nearby vegetation. If you see yellow mottling on the leaves, which looks like a nutrient deficiency, you may want to test the soil around the tree for phosphorus, potassium, and pH levels to determine if these are deficient in the soil.
When determining whether to use a fungicide to manage powdery mildew, consider the following questions:
Is this a valuable dogwood tree? Time and money are needed to use a fungicide.
Has the disease been properly identified? There are several reasons why the dogwood leaves may be curled, deformed, light yellow in color or turning brown. Check the trunk of the tree for injuries including borer damage. Consider the history of the tree and what may have happened to it. Possibly the soil should be tested to make sure the pH level is appropriate and phosphorous and potassium are at adequate levels.
Has there been a history of powdery mildew on this dogwood tree? The use of a fungicide might be considered when powdery mildew recurs from year to year on the same dogwood tree or in extreme cases of infection.
How many applications of fungicide will it take? The use of a fungicide needs to be started at the end of May and applied at three-week intervals during June, July and August.
Another essential question is do you have the equipment to apply the fungicide? Complete coverage of the upper and lower surface of all the leaves on the tree is needed. Reaching the top of tall dogwoods is difficult without a high-pressure sprayer.
Are effective and legal fungicides available for dogwood powdery mildew? The fungicides to choose from have the active ingredient of myclobutanil or propiconazol. Read the label of the product and make sure that dogwood or ornamental trees are listed on it. Follow label directions for application.
Have cultural practices been used? Following the cultural practices listed above is best for the health of the dogwood tree, is easy to do and usually less expensive than fungicides.
If you answered no to any of these questions, then fungicides probably should not be used. Most fungicides are capable of stopping the progress of powdery mildew infections fairly quickly during early stages of the disease but none will restore already discolored or damaged leaf tissue.
For more information about powdery mildew on dogwoods, contact the Daviess County Cooperative Extension Service at 270-685-8480.
Spot anthracnose may also appear on dogwood leaves. Spots on the leaves are circular to angular, dark purple and very small. The diseased leaf tissue often drops out, leaving holes or ragged edges, according to Dr. Nicole Ward Gauthier, University of Kentucky extension specialist in plant pathology. Severely infected leaves may be smaller in size or killed. The disease is managed with cultural practices, as listed above. If spot anthracnose was severe the previous year, and the tree is valuable, then fungicides applied early in the spring may prevent or reduce reinfection.
There's still time to sign up for the Extension Master Gardener class, which will meet at the Daviess County Extension Office from 9 a.m. to noon every Thursday, beginning Sept. 5 through Dec.r 19, except for Thanksgiving Day. The Extension Master Gardener Program is designed to expand horticultural knowledge among the community through the use of trained volunteers. Registration is required by Aug. 16. For more information and an application, please call 270-685-8480.
Annette Meyer Heisdorffer is the Daviess County extension agent for horticulture. Her column runs weekly on the Home & Garden page in Lifestyle. Email her at firstname.lastname@example.org.