Dogwoods adorn many landscapes with their pink or white bracts in the springtime. They are also prevalent as an understory tree in forests. They are even prized for their colorful fall displays. Dogwoods, however, are susceptible to a number of diseases, most of which can be prevented with good maintenance, an annual fungicide program and diligence.
Dogwood anthracnose is caused by the Discula destructiva fungus. First signs of the disease are purple-edged spots on the leaves or larger patches of tan splotches especially along leaf margins. It appears similar to other leave spot diseases. Affected leaves will not fall in the autumn but remain on the tree. Twigs can become infected and the fungus can travel to the trunk where brown, elliptical cankers can develop annually. These cankers, if they become numerous enough, can severely damage or destroy the tree by girdling its branches or trunk. The disease is controlled through the use of fungicides and maintenance of the tree by pruning and discarding diseased branches and raking fallen leaves away from the tree.
Spot anthracnose is caused by the Elsinoe corni fungus. Only the young shoots, flower bracts and leaves are affected. Minuscule purplish spots, about 1/25 of an inch, appear on the bracts. As they multiply, they blend into larger patches of purple, deforming the petals and causing them to fall prematurely. Leaf spots are dark purple with yellow centers that disintegrate. The leaves become distorted and drop prematurely. Shoots and berries develop lesions that spread the spores. Control is achieved with a well-timed fungicidal spraying regimen that begins before buds open, is repeated as leaves open and again in early July and finally again in the fall before leaves drop.
Powdery mildew is a relatively harmless fungus caused by Microsphaera pennicillata. The telltale sign is a whitish or grayish growth, like mildew, growing over buds, young shoots and leaves. Leaves may become curled and may grow smaller than unaffected leaves. The disease can occur throughout the entire growing season. Treatment for powdery mildew is the same as for spot anthracnose.
Septoria Leaf Spot
Septoria leaf spot is a late summer disease caused by Septoria cornicola. In some instances, spring weather can produce severe symptoms earlier. Angular spots with veined borders start as purple but turn gray later on. Unlike spot anthracnose, the centers rarely drop out of the spots and are much larger. Treatment for septoria leaf spot is the same as for spot anthracnose.
Ascochyta blight is also caused by a fungus, Ascochyta cornicola. This disease is usually found after a wet spring when the gray-to-tan spots with dark borders are discovered in early June. If the spots multiply, the leaf may shrivel and turn black. Treatment for ascochyta blight is the same as for spot anthracnose.
Crown canker slowly develops near the base of the tree above the soil line. It is caused by Phytophthora cactorum. It most often occurs when the roots have been stressed, such as when a tree is transplanted or when the soil is poorly drained. The cankers can girdle and kill the tree. Often sprouts will emerge from the trunk below the canker. The disease can be prevented with bi- or tri-monthly applications of metalaxyl.
The origin of dogwood canker is unknown. It can produce two types of cankers: those that appear as sunken areas in the bark or swollen areas where the bark appears rougher than the surrounding area. The sunken areas can girdle the tree and kill it while the swollen area tends to invite insect infestation. Often trees with cankers will send up sprouts. There is no known treatment. The best prevention is to purchase canker-free plants.