Is Our Native Dogwood Toast?

I love driving around Atlanta this time of year and seeing all the flowering dogwoods (Cornus florida) blooming. Not only are these dogwoods famous for their spring bloom (called bracts), they also add fall color that is a crimson red, a beautiful addition to many a residential property.

However, all is not well in Dogwood Land. The trees are suffering from a fungal disease called anthracnose, which is incurable and is spreading down the Eastern Seaboard. The disease first appeared in the New York Botanical Gardens in the late seventies and has since spread rapidly, especially during wet seasons, when the fungus has the moisture it needs to proliferate.

Dogwoods have been dying from this disease from the northern-most states down to and including Georgia. It is also spreading westward past Pennsylvania, Ohio and Alabama. Disease spores spread during spring rains and thunderstorms and so far have infected over two million acres of forest lands and wood lots, wiping out 90% of the dogwood trees in some locations.

Tree experts hope this is not going to be the equivalent of Dutch elm disease or chestnut blight, which virtually eliminated the affected species of trees from the American landscape. Hopefully, in the case of the dogwood, there will be trees that are resistant to anthracnose, and they will pass this immunity onto their offspring. This is the process of natural selection.

However, so far, samples from American forests all proved susceptible to anthracnose spores. As far as the urban landscape goes, simple rules for placement of dogwoods will help their survival. Dogwoods are an understory tree, and should be placed along the periphery of a woods facing east or north or in the canopy of large trees. Never plant a dogwood in full sun. Chinese dogwoods (Cornus kousa) are a resistant variety; however, they develop their leaves first in the spring, then they bloom, and are not as pretty as Cornus florida in my opinion.

Anthracnose, which is also called spot anthracnose, starts out with, well, spots on the leaves. This causes the leaves to turn brown and then yellow, leading to leaf abscission and flight (they eventually fall off). Cankers then develop around the trunk, resulting in girdling of the trunk and stems and, ultimately, death.

The following recommendations from the Horticulture Research Institute should minimize problems from spot anthracnose:

1. Practice drought-stress prevention by mulching around the base of the tree; keep mulch material approximately 3 to 4 inches away from the trunk. Irrigation is urged, but limit watering to the base of the tree to prevent wetting the foliage, when possible. Improving air circulation around the tree is also recommended: prune or thin dogwoods and surrounding trees.
2. Fertilization with a balanced N-P-K (nitrogen, phosphorus, and potassium) formulation, based on a soil test recommendation, is suggested for improving tree vigor and natural resistance. But it is also important not to over fertilize.
3. Pruning out dead twigs (where feasible) and removing healthy epicormic branches (water sprouts at the base of the tree) may be helpful.
4. Particularly during the period of leaf expansion, periodic use of accepted fungicides may be used. Although the fungicide Benlate had shown a statistical advantage in control of anthracnose, it has the disadvantage of giving rise to resistant strains of fungi when used alone. Control can probably be best realized by alternating between Benlate and Daconil. (Arborists who have treated landscape specimen dogwoods with water and fertilizer have reported improved appearance of diseased trees without the use of fungicide sprays.)
5. When selecting new trees, purchase only nursery grown trees. DO NOT TRANSPLANT TREES FROM THE FOREST, AS ANTHRACNOSE IS PRIMARILY A FOREST DISEASE. Transplanting can bring the disease into your landscape. Select the proper site: Trees need well drained soil, good air movement, and at least 6 hours of indirect sun on an average day. Do not plant trees too deep!
6. In the fall, dispose of dogwood leaves rather than using them as mulch.
7. Avoid mechanical injury from mowers or stringtrimmers.

So keep on the lookout for this terrible disease and let’s keep Atlanta’s dogwoods blooming. If you spot the disease and would like a recommendation of a tree service, please call us at 770-447-6037 ext. 2.