dogwood (Cornus florida) is widely planted in home and commercial
Dogwood anthracnose, caused by the fungus Discula destructiva, was first reported in New York and Pennsylvania in the late 1970's. Since that time it slowly moved south along the Appalachian Mountains through Maryland, West Virginia and Virginia, and reached western North Carolina in 1987. Currently the disease is found in all counties to the west of, and including, Rockingham, Wilkes, Alexander, Burke and Cleveland.
Initial symptoms are small purple-bordered leaf spots or larger tan blotches, especially on leaf margins. These initial symptoms can be confused with leaf spots caused by Elsinoe corni and Septoria cornicola, or other foliage diseases. Blighted leaves do not abscise in the fall and frequently remain on the twigs until spring. The fungus can spread to the twigs and, in some cases, to the trunk, causing brown, elliptical, annual cankers. Epicormic branches may develop on the trunk and larger branches, and also may become infected. Multiple cankers can girdle branches or the trunk, and may result in the death of the tree. The disease is most severe in cool moist forested areas above 1800 feet elevation. It is less of a problem in the landscape than in the forest.
The fungus produces masses of spores (conidia) on infected leaves or bark and presumably spreads by splashing rain and possibly by birds. Disease spread and development are favored by cool, moist weather.
Control is centered around cultural practices and fungicidal sprays. Maintenance of healthy, vigorous dogwoods is recommended (see section on planting and cultural practices at end of note). Pruning and disposal of diseased twigs and branches, removing epicormic branches that develop on the trunk, and raking and disposal of leaves may be of some value. Pruning low branches on taller trees and thinning other understory plants to improve air movement may also help.
should be used as a supplement to the cultural control program at elevations
above 3000 feet; or below 3000 feet elevation if the plants occur in full
shade on north-facing, moist slopes. Chlorothalonil (Daconil 2787), mancozeb
+ thiophanate methyl (Cleary's 3336 or Domain) or propiconazole (Banner)
will protect against leaf infections. Apply 3 or 4 sprays during leaf
expansion in the spring at 10-14 day intervals. Additional fungicide applications
may be necessary later in the growing season if weather conditions are
favorable for infection.
Leaf and Flower Diseases
Spot anthracnose, caused by Elsinoe corni, affects the flower bracts (petals), leaves and young shoots. The most conspicuous symptoms are small (1/25 to 1/16 inch in diameter), circular to elongate reddish-purple spots on the bracts in early spring. The spots may become numerous and merge, losing the distinctive characteristics of individual spots. Severely infected bracts may be stunted and disfigured and fall prematurely.
leaves are very small and dark purple in color, but the centers may turn
pale yellow-gray and drop out. Heavily infected leaves are smaller than
normal, distorted and often killed. Infected young shoots and berries
develop elongated, scabby lesions with a purplish margin. The causal fungus
produces spores on the lesions, and overwinters on infected twigs and
Septoria leaf spot, caused by Septoria cornicola, usually does not become severe until mid to late summer, but this may vary depending on spring weather conditions. The spots are angular and are bordered by veins. At first, they are a uniform, purplish color, but later in the season the centers become grayish, while keeping the dark purple border. The centers rarely drop out. The spots are much larger than those caused by spot anthracnose.
An effective spray schedule to control of leaf and flower diseases must start before disease appears, usually when the flower buds are beginning to open. A second spray should be made when the leaves are unfolding, a third spray about the last week in July, and a fourth before leaf drop in the fall. Full coverage of the plant is necessary. Fungicides containing azoxystrobin (Heritage), propiconazole (Banner), and myclobutanil (Systhane, Eagle, Immunox) are effective in controlling all the leaf and flower diseases. Fungicides containing mancozeb (Dithane, Fore, Mancozeb DG and Protect) will control spot anthracnose and septoria leaf spot. Spraying may not be necessary unless leaf or flower diseases were a problem the previous year. However, newly planted trees should be protected by fungicides until they are well established in the landscape.
Crown canker, caused by Phytophthora cactorum, is characterized by a slowly developing canker on the main trunk near the soil line. The disease usually occurs on newly transplanted trees or those that have injuries to the roots, and is more common on poorly drained areas. Cankers appear as constricted or sunken areas that may eventually girdle the stem and kill the top of the tree. Leaves on diseased trees may be smaller than normal, become chlorotic and be shed prematurely. Trees with cankers often produce numerous sprouts at or below the soil line. Treatment with metalaxyl (Subdue 2E, Subdue 2G) at 2-3-month intervals will prevent the disease.
This disease, of unknown cause, results in the production of cankers on the main trunk and larger branches. Cankers may be of two types; a) sunken areas in the bark that may girdle the trunk and result in the death of the tree above the canker, or b) swollen areas with roughened bark on the trunk or main branches. These roughened areas are often invaded by insects. Trees with cankers often produce sprouts from the base of the tree. No control measures are known. Only canker-free plants should be purchased.
Dogwoods are susceptible to several root and crown rot fungi. These fungi may be present in the soil and attack the roots when the vigor of the tree is reduced by unfavorable soil conditions or by some type of injury. Often the first symptom observed is the drying of the leaf margins followed by death of the plant during the summer months. This is the final stage of a disease that began with an infection of one or more of the lateral roots. After infecting a part of the root system, the fungus spreads along the roots to the basal portion of the tree, which is often girdled. As the fungus progresses, the tree may show symptoms of decline, such as yellowing of the foliage, dying of the leaf margins, branch dieback, and a general unthriftiness. One of the most common root rots on dogwood is caused by Armillaria sp. This disease is characterized by a relatively thick, white, fan-shaped mass of fungal tissue (mycelium) beneath the bark at the base of the tree or on large roots, and by black, shoestring-like structures (rhizomorphs) that can be found on or under the bark and in the adjacent soil. Another root rot, caused by Phytophthora cinnamomi, usually affects the smaller roots and occurs most commonly in poorly drained soils. This disease can be diagnosed only by laboratory tests.
To prevent root rot, plant only in well-drained soils where root rot has not been known to occur, preferably away from areas where large trees have been removed. Fertilize and water during dry periods and control foliage diseases to maintain the vigor of the tree. By the time a general decline of the tree is observed, it is too late to save it. Remove the tree and all roots possible from the soil, since the fungi can persist in dead roots and infect nearby shrubs or other plants set in the same site.
Other Dogwood Problems
The poor appearance of dogwood trees is often due to factors or conditions other than diseases, with symptoms the same as might be expected with a damaged or inefficient root system. These include stunting or slow growth, light green to yellow foliage, dead leaf margins, dieback of branches or even death of the plant. Marginal leaf burn and premature reddening of the leaves are common symptoms on dogwoods during the summer months when rainfall is below normal. Any situation in which the leaves are losing water faster than the plant can take it up can result in these types of symptoms.
Proper planting and maintenance are the best way to avoid these problems. Some of the most important factors to consider are presented below:
Recommendations of specific chemicals are based upon information on the manufacturer's label and performance in a limited number of trials. Because environmental conditions and methods of application by growers may vary widely, performance of the chemical will not always conform to the safety and pest control standards indicated by experimental data. All recommendations for pesticide use were legal at the time of publication, but the status of registration and use patterns are subject to change by actions of state and federal regulatory agencies. Last printed 04/91
by North Carolina Cooperative Extension Service
Revised March 2001 by A.V. Lemay