How to Solve Some of the Problems With Established Dogwood Trees
Proper Care is Vital to Dogwood Trees
Introduction to Dogwoods
The flowering dogwood is an extremely popular ornamental tree around the world because of its year-round appeal and its moderate size that can fit on almost any size yard. They easily become a landscape specimen, blooming with gorgeous flowers in white, pink or red with small yellow flowers at the center. The leaves and fruit become a stunning bright red/purple color in the fall, and in the winter there are large, flattened buds with dark bark that resembles alligator skin.
Unfortunately, for some people, their dogwood trees become problematic over the years causing them to go scrambling for solutions to the many different problems that can arise. Growing in the wild, dogwoods are understory trees that flourish in organically-rich soil and the shade. The environment you provide for them needs to be similar.
Hopefully, this article will provide at least some of those solutions. It is not intended to provide instructions on planting but instead is directed toward those with already-established dogwood trees.
Dogwood trees are considered hardy in U.S. Department of Agriculture zones 3 through 8. The redtwig dogwood (Cornus alba) and the pagoda dogwood (Cornus alternifolia), are hardy to USDA zone 2; with the pagoda dogwood being heat hardy only through zone 7. The trees planted in partial shade generally perform better. Planting dogwoods in full sun can cause them to be stressed, making them more susceptible to disease, heat stress, and borers.
Anthracnose: The Most Common Disease
There are several diseases that can affect your dogwood trees. The following will list the symptoms of each, along with a possible solution (if there is one):
Spot Anthracnose - This fungal disease usually occurs in the spring due to a fungus called Elsinoe corni (primarily on Cornus florida). The flowers, fruit, leaves and young shoots are all affected by small, purple lesions that are no larger than the head of a pin. The spots often have a yellow center, which can drop out, and often, the spots will merge together to cause larger areas to become infected. These numerous lesions normally will occur on dry leaves, which will die. Areas with lower humidity see less of this disease, which normally kills the tree within two to three years.
What You Can Do
- Prune your tree regularly and destroy any infected shoots.
- Spray the foliage in the early spring with organic copper-sulfate fungicide.
- Then spray with synthetic fungicide registered for dogwoods (active ingredients mancozeb, myclobutanil or maneb). Please always follow the directions on the label.
- Apply a 5-10-10 fertilizer and water it well, especially during dry weather.
- Improve water retention with mulch.
Septoria Leaf Spots
Larger Spots in Late Summer
Septoria leaf spots - The lesions caused by septoria leaf spots are larger than those caused by spot anthracnose, although still small (1/8-1/4 inch), and are angular to irregularly shaped, usually bordered by leaf veins. They occur usually in late summer and are more common in wet conditions or areas of high humidity.
There are several different species of septoria (purple spots with gray centers) that can infect dogwoods but already-stressed trees are the most vulnerable to the disease, which usually does not become a severe problem until mid to late summer depending on weather conditions. The disease has a dramatic appearance but the overall impact on the health of the tree is minimal.
The infected leaves will remain on the tree so, despite the necrotic tissue, there is still enough green tissue producing carbohydrate reserves to sustain the tree for the season. The health impact of these spots is limited to one season; the leaves will drop in the fall and the tree will produce more leaves the following year.
What You Can Do
- Always avoid having your trees wet for extended periods of time by staying away from overhead irrigation methods.
- Rake and destroy the fallen leaves in the fall.
Dogwood Borer Moth and Larvae
Knotty, Calloused Bark
Dogwood borers (Synanthedon scitula) - Dogwood trees attacked by borers (clear-winged moths which begin emerging in late May to September) will display swollen, knotty, calloused or gall-like areas on the trunk, at or immediately below the ground surface (possibly between the ground surface and the branches above). Injuries can also be seen at the union of the trunk and branches or smaller twigs and branches. Young dogwoods are usually attacked at the crown.
Dogwood trees are damaged by the feeding activity of the dogwood borer larva under the bark of the trunk and limbs. They are found throughout the area where flowering dogwoods are grown.
What You Can Do
- Keep a layer of mulch around the plant to keep the soil cool and moist, which in turn will help to keep the tree healthy.
- Avoid wounding the trunks and branches with lawnmowers, trimmers, etc. Those wounds provide excellent entry points for dogwood borers. The borers cannot chew through uninjured bark but borers in various stages of development can be found at the edge of bark wounds throughout the year. Borers are also able to enter bark damaged by woodpeckers or squirrels.
- Prune at the proper time, taking care not to leave stubs. No pruning compound or sealer is needed.
- Spray the bark with a synthetic insecticide registered for dogwoods (active ingredient chlorpyrifos) in mid-May and mid-June.
Note: To protect people and the environment, pesticides should be used safely and responsibly. Read and follow label directions carefully before you buy, mix, apply, store or dispose of a pesticide. According to laws regulating pesticides, they must be used only as directed by the label. Persons who do not obey the law can be subject to penalties.
Yellow/Brown Leaves and Twig Dieback
Dogwood anthracnose - This disease is common in the 30-60 species of dogwoods in the Cornus genus. It causes yellow leaves, twig dieback, and sunken discolored areas of tissue; however, these symptoms are also common with branch canker and crown canker as well.
Symptoms on leaves - The most common symptoms are large, brown, irregularly-shaped blotches with dark brown to purple margins. The infection begins at the tip, spreading down the midvein, producing a wedge-shaped appearance. The infected leaves will usually drop before the fall season, defoliating the tree.
Symptoms on twigs - Infected twigs will have sunken spots that are tan to brown with purple borders. They will eventually get larger and girdle the twig, resulting in twig dieback.
What You Can Do
This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.