In the Garden:
Is there anything prettier than a dogwood in bloom?
New Dogwood Varieties
Our beloved flowering dogwood (Cornus florida) has been having a tough time of it lately. Over the last 20 years a disease called dogwood anthracnose has taken its toll on dogwood trees throughout the country. The southern states have been hardest hit; the disease has killed nearly all the native dogwoods in the Great Smoky Mountains National Park.
The native range of flowering dogwood covers much of the eastern half of the country, from New England to Minnesota and south to Texas and Florida. Like many native plants, dogwoods provide food for wildlife. Dozens of bird and mammals rely on the fruit for sustenance, including songbirds, wild turkeys, deer, and black bears. So the decline of the dogwood affects not only our home landscapes but may also have implications for wildlife populations.
Everyone Loves Dogwoods
The dogwood's delicate flowers would be beautiful any time, but this floral display is especially welcome in early spring. Dogwoods are widely planted as ornamentals, favored for their small stature, graceful, tiered branch structure, and spring blooms. (Technically speaking, the white "petals" are bracts; the true flowers are the small yellow structures in the center.)
How popular is the dogwood? It's the state flower of North Carolina, the state tree of Missouri, and the state flower and state tree of Virginia! New Jersey chose the dogwood as its state memorial tree, and a related tree -- the Pacific dogwood -- is the official flower of British Columbia.
Dogwood anthracnose threatens to send dogwoods the way of the American chestnut and elm, both of which were wiped out by a fungal disease. And like chestnut blight and Dutch elm disease, the fungus that causes dogwood anthracnose is likely foreign in origin and was inadvertently introduced into the U.S. Symptoms of infection include leaf spots and stem cankers; infected mature trees may live up to three years, but young trees often die soon after infection.
Anthracnose is most destructive to dogwoods in native woodlands where they grow in the dappled shade of larger trees. Individual dogwood trees in the home landscape appear to be less bothered, at least right now, possibly because as specimen trees they receive more sun and better air circulation than trees in a wooded setting.
A Bright Spot on the Horizon
A team of University of Tennessee researchers has been hard at work trying to find anthracnose-resistant varieties, and it's not just altruism on their part. Growers in Tennessee supply about 80 percent of the dogwoods sold in the nursery trade, valued at up to $50 million annually. The team's first variety release, 'Appalachian Spring', was cloned from a dogwood tree found in Catoctin Mountain Park in northern Maryland -- the only tree left standing in an area otherwise devastated by the disease. While 'Appalachian Spring' may not be immune to anthracnose, it has shown strong resistance to infection.
Crosses of flowering dogwood with Kousa dogwood (Cornus kousa) also have shown strong resistance to anthracnose, and several varieties are commonly available, including the Stellar series released by Rutgers University. The hybrids begin flowering just as the flowers of the native dogwood begin to fade. Some references indicate that the branching structure on the hybrids is weaker than on the species, and the trees are prone to splitting.
As if one disease weren't enough, the incidence of powdery mildew on dogwoods also is on the rise. Although rarely fatal on its own, powdery mildew weakens trees, and the strain that affects dogwoods seems to be especially virulent. Unlike anthracnose, powdery mildew readily attacks trees in the home landscape. The one-two punch of anthracnose and powdery mildew is of great concern. As you might guess, the research team at the University of Tennessee is addressing this threat, too. They have released three mildew-resistant varieties, called 'Appalachian Snow', 'Appalachian Mist', and 'Appalachian Blush'.
The dogwood research team continues to look for trees showing resistance to both anthracnose and powdery mildew -- the "holy grail," in the words of one of the team leaders. Whether they'll find a naturally occurring resistant variety or create one with traditional crossbreeding techniques, the outlook seems hopeful.
In the meantime, growers are gearing up their production of the new disease-resistant dogwoods. Since the trees take several years to reach a marketable size, supplies are limited. Greenwood Nursery (http://greenwoodnursery.com) will have 'Appalachian Spring' for sale this fall.
The Stellar series of hybrid dogwoods are widely available; mail-order sources include Carroll Gardens (http://www.carrollgardens.com) and Greer Gardens (http://www.greergardens.com/).
You can help the University of Tennessee Dogwood Research group locate disease-resistant trees to use in their breeding program. For more information, go to http://eppserver.ag.utk.edu/Dogwood/June2002/helpus.htm.
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