Joe’s Journal, Spring Edition: When Microbes Go Wrong – Dogwood Anthracnose
The dogwood (Cornus florida) is a popular native tree, which displays dramatic white or pink flowers. The branches grow horizontally out from the main stem, making the tree distinctive in the landscape. Branching is dichotomous (that is, buds grow opposite the other).
Unfortunately, an introduced fungal disease, dogwood anthracnose (Discula destructiva) is contributing to the trees decline throughout its range. Dogwood anthracnose infects Cornus florida on the east coast, as well as western dogwood (C. nuttallii) in the Pacific Northwest.
Foliage is susceptible in cool, wet springs. Lesions form as a result of infection. These spots appear reddish-purple with a tan center. Left untreated, infections can spread into twigs and stem, forming cankers.
Stressed trees will produce epicormic (adventitious) sprouts to compensate for lost foliage. However, this growth is very susceptible to infection, and ought to be removed.
Infected trees display symptoms of dieback and later, decline and death.
Managing Dogwood Anthracnose
If you are planning to plant dogwood, seek cultivars that are resistant to the disease. Resistant crosses of Korean dogwood (Cornus kousa) and C. florida are available, among them ‘Stellar’, ‘Constellation’ and ‘Aurora’. A native remnant tree found in Maryland was developed called ‘Appalachian Spring’, which displays resistance to the disease.
Although it is a native understory tree, plant trees in full sun in well-drained soil. Foliage of trees planted in open locations will dry more quickly, which will help to reduce the incidence of infection. Apply a 2-4” layer of an organic compost as a top dressing, and mulch well. Keep woodchips well away from the base of the stem (or trunk flare). Water in well after planting.
During bud break in early spring, apply a protectant fungicide such as propiconazole or a copper product. In areas of protracted wet springs, maintain a spray program. Always follow label recommendations when applying a pesticide.
Remove epicormic sprouts when they form, and rake and discard leaf-fall from under the trees to reduce inoculum and potential for future infection.
Avoid overfertilization to avoid succulent (susceptible) growth. When applying a nitrogen fertilizer, use a slow-release product such as ArborPlex (14-4-5, 50% slow-release nitrogen). Slow-release fertilizers support moderate growth in plants, when applied according to label directions.
It is worth the extra effort to help keep this increasingly rare native tree healthy. It will return your efforts one-thousandfold with blooms in the years to come!