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. Although the southern states have been hardest hit -- the disease has killed nearly all the native dogwoods in the Great Smoky Mountains National Park -- trees in New England also have been affected.
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 it's not just us humans being affected.
Everyone Loves Dogwoods
There's something about a dogwood in bloom that makes me want to take out a sketchbook and draw. The delicate flowers and graceful tiered branches would be beautiful any time, but this floral display is especially welcome in early spring. (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. Plus, dogwoods are widely planted as ornamentals, favored for their small stature, attractive branch structure, and spring blooms.
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 probably 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.
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. Right now it's in short supply; Greenwood Nursery (http://greenwoodnursery.com/page.cfm/5012) has a limited inventory. Growers are gearing up production of this variety, so you should be able to purchase it more readily in the next year or two.
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. 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 on top of this threat, too. They have released three mildew-resistant varieties, called Appalachian Snow, Appalachian Mist, and Appalachian Blush. Unfortunately, because of the time trees take to grow to a marketable size, these varieties won't be available to homeowners until 2005.
The dogwood research team continues to look for trees showing resistance to both anthracnose and powdery mildew - the "holy grail," in the words 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.
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