Diagnose and manage common diseases of dogwoods
The most common problems of flowering dogwoods in Maryland are described below. Refer to Diagnose Dogwood Problems for a chart of symptoms and possible causes.
Leaf scorch is a symptom of drought-stressed trees. Scorch symptoms include browning along leaf edges and a dropping or wilted appearance. This occurs when the amount of water leaving the plant (transpiration) exceeds the amount of water uptake by the roots. Drought-stressed dogwoods are particularly prone to disease problems.
- Avoid this problem by planting in a location where your tree will receive partial shade and supplemental moisture during hot, dry weather. Soil should be somewhat moist but not saturated.
- Leaf scorch alone rarely kills a tree but can make it more susceptible to pests and diseases.
- Dogwoods are shallow-rooted and frequently need irrigation during the drier months of July and August. If overhead irrigation is applied, water early to allow leaf drying before nightfall.
- Apply mulch at the base of the tree to conserve moisture in the root zone. Mulch should be no more than 3 inches deep. Keep it at least 3 inches away from the trunk.
Powdery mildew (Erysiphe and Phyllactinia guttata) has become a more severe and widespread problem in recent years. It appears as a powdery white coating on the leaves from early summer through fall. In some years the powdery appearance is less evident. Under severe conditions, the new leaves can exhibit leaf scorch, twisting, distortion, and smaller than normal growth. Older infected leaves often develop purple blotches that progress to dead areas. Infections cause the loss of water and photosynthetic leaf area which weakens trees and reduces growth.
- Select powdery mildew resistant cultivars of flowering dogwood.
- Place susceptible plants where there is adequate sunlight and good air circulation to reduce humidity levels. Allow proper plant spacing.
- Pruning for better air circulation may help. Pruning of dogwoods can be done in late fall/early winter (November-December) or after the trees are finished flowering in the spring.
- If disease symptoms are severe, especially on young trees, spraying a fungicide labeled for this disease on dogwood is an option. Spray at bud break in the spring, followed by additional sprays every two weeks as needed throughout the season.
This fungal disease can affect both flower bracts and leaves during wet spring weather. The symptoms are characterized by numerous small (1/16 -⅛ inch) spots with purple borders. When numerous, these spots can cause bracts and leaves to become wrinkled and distorted. This is a common disease and is not severe.
- Plant cultivars that are resistant to spot anthracnose.
- Treatment in landscape situations is rarely necessary. This is not a severe disease.
- Rake and discard symptomatic foliage.
- Fungicides will not cure visible symptoms. If spot anthracnose is severe on young trees, they can be protected with a registered fungicide applied in the spring at bud break followed by additional sprays every 7 - 14 days until leaves are fully expanded.
In the past, anthracnose was the most serious disease of dogwoods in the landscape and our forests but it is now less common. It causes dieback or even death of infected trees.
The early symptoms begin in mid to late May as leaf spots with tan or purple borders. In wet weather, these spots can rapidly enlarge and kill the entire leaf. These blighted, drooping leaves can remain hanging on the branches in wet weather before defoliation occurs.
The disease spreads from infected leaves into the twigs and branches and can cause dieback of the limbs. Young green stems and water sprouts are especially susceptible. Dark cankers will cause stem girdling and dieback. On older branches, the wood under the bark will appear dark brown in contrast to healthy light-colored wood. If the dieback reaches the main trunk the entire tree can be killed.
To distinguish this disease from other leaf spots, examine the underside of the leaves (with a hand lens or magnifying glass) for numerous small tan to brown dots, about the size of a printed period, scattered within the blighted tissue. These dots are the source of spores that will be washed away by rain or dew, or spread by insects to healthy leaves and neighboring trees. The disease overwinters in twig and stem cankers that initiate new infections in the spring.
- Avoid digging native trees from the woods and transplanting them into landscapes. This practice can introduce the disease into a neighborhood that was previously disease-free.
- Plant disease-resistant cultivars of flowering dogwoods. Tartarian dogwood (Cornus alba), redosier dogwood (C. sericea), and Cornelian cherry (C. mas) also are resistant to this disease.
- Avoid over-application of fertilizer which can result in succulent new growth with greater susceptibility to disease.
- Prune out all dead or dying twigs and limbs during dry weather. All water sprouts or suckers on trunks and branches should also be removed.
- In the fall, rake and remove fallen leaves. Remove any dead leaves still attached to the branches.
- Registered fungicides can be utilized on trees in landscapes in the spring at bud break, followed by additional sprays every 10-14 days until leaves are fully expanded. Trees should also be sprayed once in the fall after the leaves have changed color but before leaf drop.
Other leaf spots (Septoria cornicola and Phyllosticta)
Other leaf spots appear in late summer and fall on the upper leaf surfaces but rarely cause much damage. The dark spots are circular to angular in shape with purple borders and are about ¼ inch in diameter. Occasionally, very small black dots are visible inside these spots that indicate where the infectious spores are formed. Under severe conditions, the dogwood fruit may become discolored and shriveled.
- Generally, no chemical controls are needed for these late-season diseases.
- Rake and dispose of leaves in the fall to reduce disease incidence.
This fungal disease can be serious during wet spring weather. The symptoms range from a spotting of the flower bracts to a complete collapse of the bracts. Infected bracts eventually become covered with a gray fuzzy growth of fungus. This symptom gives this fungal organism the common name ‘gray mold’. Infected bracts that fall onto leaves can also cause leaf and twig infections.
Generally, no chemical controls are needed for this disease because drier weather will stop disease progression before serious damage occurs.
Twig and branch cankers (Botryosphaeria sp. and Diplodia sp.)
Although this disease is rarely serious it can cause limb dieback. Diseased sections on twigs and small branches begin as small discolored areas with sunken or cracked bark. The fungus Botryosphaeria appears to be more severe on pink flowering cultivars.
- Healthy trees are less susceptible to these diseases. Maintain healthy trees by irrigating during drought and avoiding over-fertilization to prevent excessive succulent growth.
- There are no effective chemical controls for this disease once diagnosed, so prune out infected branches promptly.
Crown canker (Phytophthora cactorum and P. cinnamomi)
This serious disease of dogwoods usually results from an injury to the bark near the base of the trunk that is invaded by this fungus-like organism called a water mold. Early symptoms include smaller than normal leaves, pale green leaves, and early red fall coloration, especially on individual branches along one side of the tree. Eventually, the entire tree may show these symptoms.
When a canker completely girdles the trunk, the tree will die shortly thereafter. In the early stages, the canker can be hard to find, but if cut into, will reveal discolored wood under the bark. As the canker enlarges it will form a sunken area on the trunk and the bark will dry and crack, exposing the wood underneath.
- Avoid buying trees with wounded bark. Bark injuries can occur when the plants are dug at the nursery, shipped or moved around within the retail sales yard.
- The best way to avoid this disease is to prevent wounds or damage to the trunk. Maintain a mulched, weed-free area around dogwoods to prevent injury from mowers and string trimmers.
- There are no chemical fungicide controls for this disease once symptoms are visible.
- Do not replant dogwood trees in the same spot that a diseased tree was growing.
By David L. Clement, Principal Agent, University of Maryland Extension, Home and Garden Information Center, revised and edited 11/2019