The flowering dogwood tree is a popular addition to landscapes throughout North America. There are dozens of species and they come in a range of colors and sizes. The deciduous dogwood can appear as a small accent shrub, in the form of the Cornus sericea (Isanti dogwood) in a partially shaded yard or as a towering 60-foot centerpiece draped in sunshine in the form of the Cornus nuttallii, the Pacific dogwood. Diseases of the ubiquitous dogwood tree are, unfortunately, quite common.
Caused by a fungus, dogwood anthracnose has destroyed dogwoods throughout U.S. forests. Recognized by the tan and purple splotches it leaves on the leaves of an infected tree, anthracnose works from the leaf to stem, progressing to cankers that can kill twigs and branches. Left untreated, the disease will rise upward to damage the bark and eventually (within one to three years) kill the tree. The disease is more likely to occur in damp, forested conditions than on landscaped properties. Fungicides containing chlorothalonil, myclobutanil or propiconazole can be applied in the spring to combat the disease.
More common among low-lying dogwoods, another fungal disease is ascochyta blight. The rains of spring are a breeding ground for Ascochyta cornicola, which attacks dogwood foliage, causing the leaves to discolor, shrivel and turn black. Fungicides applied upon leaf emergence are recommended for treatment, with additional applications being used during wet summers.
Septoria Leaf Spot
A summertime disease, Septoria leaf spot appears as large purple spots on the leaves of the dogwood. These angular spots spread and turn gray as the disease progresses. Although the fungus can damage the leaves and is aesthetically displeasing, because the disease occurs so late in the season, it often coincides with fall's defoliation. Rather than treat Septoria leaf spot, it is recommended that fallen leaves are removed and discarded, thus reducing the likelihood of the fungus overwintering.
Caused by Microsphaera penicillata, powdery mildew attacks a variety of ornamental dogwoods. The fungus appears as a fine white coating on leaves, buds and young shoots, and can hamper leaf growth. Affected trees may have stunted root growth and may appear water-deprived, even though sufficiently irrigated. Unlike fungi that thrive after rains, Microsphaera penicillata is common in high humidity, but also when days are warm and nights are cool. Fungicides that are effective against the mildew are myclobutanil, potassium bicarbonate, triadimefon and trifloxystrobin.
The first step in preventing disease is to purchase a healthy tree from a reputable nursery. In addition, choose a sunny location and plant your tree in well-drained, highly organic soil. Water from the roots and prune annually; do not over-mulch or overfertilize. Maintaining your dogwood with care is the most effective combatant against tree disease.