Dogwood, also know as Cornus, encompasses many species of ornamental trees and shrubs, and there are numerous pests and pathogens that can affect their health. Fortunately, serious afflictions of this beloved plant are limited to particular species, and most diseases and insects can be treated, managed or controlled with fungicides.
Most but not all dogwood species are susceptible to a fungal disease known as canker. Canker's first symptoms are stunted, yellow and thinning leaf growth with an overall unhealthy appearance. Dogwoods affected by canker will show die-back of twig and stems in half of the tree, which then advances to the entire tree. At the base of the tree a slow growing canker will form; usually early on the canker oozes a dark fluid. Over time this canker creates a prime entrance for notorious dogwood borers.
Preventing injury to bark by mower blades, string trimmers and pruners is the best way to avoid canker. Maintaining optimal plant health by using fertilizers and planting in moist, acidic, well-drained soils will help stave off the disease.
If a dogwood tree becomes infected, an arborist can find the canker under the bark and excise affected tissue to eliminate the infection. If the tree is removed, do not replant with a species susceptible to the fungal disease.
Some cultivars are immune to the fungus. Try planting the cultivars "Cherokee Princess" or "Plena" for canker-free healthy plants.
Synanthedon scitula, also known as dogwood borer, is a clear wing moth that attacks stressed dogwoods. The insect will find a damaged area of the trunk and take up residence in a niche, feeding hungry larvae on wood sap. Many types of trees are susceptible to the dogwood borer, including pecan, oak, chestnut, hickory and apple trees.
The insect has a four-month life cycle and emerges as an adult yellow and black moth after eggs are laid near a tree wound then hatch into larvae that feeds on the leaves and then bores into the trunk of the tree to pupate. Symptoms of affected trees include peeling, fractured bark with burrowing tunnels on the inner cambium.
Since the borer likes to take up residence in stressed and injured trees, the best management for the disease is to avoid tree injury, such as nicks, scrapes or cuts to the tree flare, trunk or limbs.
Microsphaeria species, or powdery mildew, is a significant fungal disease present in many dogwood species. Powdery mildew looks just as it sounds, like a fine talc or powder covering leaves, twigs and stems. The fungus thrives in wet, shady areas with low air circulation. It overwinters on dead leaves and debris. Spring rain water droplets spread the fungus.
Fungicides can control the infection, but as a dogwood tree grows into a larger specimen, professional spraying may be necessary. The best prevention is to maintain clean planting beds and remove debris after fall leaf drop. Keep good air circulation by planting the tree with adequate room for air flow. Some varieties of dogwood are resistant to powdery mildew. Try the cultivar "Cherokee Brave"; it is highly resistant to mildew.