New light on the dogwood blight.
Besides being the state flower of North Carolina and Virginia, the dogwood is planted by the millions by homeowners and landscapers, Spring brings visitors flocking to forests to see the brilliant white blossoms that seem to float just overhead like suspended snow. Sunday schools across the country teach students to revere the dogwood flower as a symbol of Christ -its prickly center like a crown of thorns, its four petals like the cross, their tips notched like wounds and each notch fringed by a brown stain recalling a trace of blood.
The facts the press publishes about dogwood blight have been selected for their drama. We are led to believe the disease is an alien invasion, having appeared near Pacific and Atlantic port cities in the late 1970s. We are told that in 10 years it has killed up to 95 percent of the trees in some northeastern forests. The News and Observer reported that foresters estimated the disease had affected a million acres in five southern states, "even though they found the first southern outbreaks only last fall." Early this year a report from the National Arboretum in Washington, DC, inspired stories that said no native dogwoods could resist the fungus. These are pieces of a story, but they are not the true and whole story of this new forest puzzle.
The infection is widespread, but it may not be an alien invasion by an exotic fungus. It is not infecting all dogwood stands, and it is not killing all the trees it does infect. Almost no one expects the dogwood to go the way of the chestnut or the American elm. As research progresses, however, we might well find a new dimension to the acid-rain story.
Dogwood blight begins on lower branches as brown withering between leaf veins or on leaf edges, or with purple-rimmed spots on the leaves. The cause is an anthracnose fungus in the Discula group. The disease can move into the twigs, down the branches, and finally into the bark, where it causes cankers before the tree dies. Infected trees often send out many water sprouts along the trunk, but these too die.
The idea that ships brought the fungus in from foreign shores grows from the fact that the disease was first noticed about 1977 in the states of Washington and New York. A disease that spread across the Northwest and up and down the entire East Coast in 12 years would be moving too fast for science to stop. Both the speed of the spread and its foreign origin, however, are far from proven fact.
Dr. Craig Hibben at the Brooklyn Botanical Garden's research center in Ossining, New York, has followed the blight since it first attracted widespread attention. He says we are not yet entirely sure the same fungus is active on both coasts, and we are not sure what species of the anthracnose fungus is doing the damage.
Margery Daughtrey, a plant pathologist at Cornell University's Long Island research station, says, "We don't really know if it was introduced or if it's a native disease whose conditions have changed-and we probably never will." The changing conditions that might have brought the disease to public attention include drought, light winter snow cover, gypsy-moth defoliation of forest canopy, excessive rainfall, and acid rain.
Wet leaves are the most direct link to the fungus, in Daughtrey's opinion. She says that the impact of the disease varies from year to year, depending on rainfall. In the forest, four to five days of gray skies and wet weather seem to be enough to unleash the fungus. New spring foliage is most susceptible, but a wet fall can allow the fungus to travel into the twigs and eventually get under the bark and cause cankers.
Ironically, drought may also contribute to the problem, Daughtrey says. The Northeast suffered a five-year drought period in the mid-1980s. The oak woods where many dogwoods grow were hard hit. The oak trees suffering from drought were attacked by the mushroom Armillaria mellea. Although Armillaria, the honey mushroom, is a fine edible, it also likes to eat, and its diet is tree roots. The spreading honey mushroom also attacks weakened dogwood.
Several other stresses that are not usually fatal could take a toll on weakened dogwoods. When gypsy moths defoliate the overhead canopy, forest dogwoods are suddenly exposed to additional light and heat. Light snow cover leaves many roots vulnerable to exceptional cold spells.
Whatever combination of events is killing dogwoods, most reports in the media suggest the epidemic is on a rampage. More careful scientists are not sure how much of this rampage is a product of the widening hunt for the disease that in turn brings more reports of damage. Ask more people to look harder for something, and they will find more of it.
Field data is also easy to misinterpret. An ongoing study by the U.S. Forest Service classifies a county as "infested" if a forester sends in just one leaf showing the fungus. This allows the creation of an accurate map of where the fungus is present, but the word "infested" too easily leads to alarming stories about epidemics.
Though foresters have confirmed dogwood blight across the entire state of Pennsylvania, state forest pathologist Glen Stanosz says, "I have no information about how long it's been here or how much damage it might have caused in the past. We really don't know. We feel that reports we're seeing from the western part of the state are merely a result of recognition of the problem."
Whatever the real rate of spread, the disease is hardly the botanical equivalent of Sherman's march to the sea. Dr. Hibben says that in the Northeast, "there are some woodland sites where perhaps 90 percent of the dogwood has been killed off, and we don't see much regeneration. But in other sites in the same region you find the dogwoods looking pretty good."
Last year Hibben went to the southern Appalachians to look at dogwoods there. "We identified the fungus in northwest Georgia at higher elevations, where it was quite bad, but at lower elevations we didn't find any indications." He found the same situation in North Carolina.
Both Daughtrey and Bill Jackson, a U.S. Forest Service researcher in West Virginia, have seen dogwoods throw off the fungus. Daughtrey describes a "shot-holing" effect in which the leaves seem to surround the fungus spots with a barrier and drop it. jackson's studies in Maryland's 5,770-acre Mount Catoctin Park, however, show a much more alarming situation. When the Catoctin forests, next to the presidential retreat at Camp David, were first cruised in 1984, 33 percent of the dogwoods were dead and only 3 percent of the stems were free of symptoms. When jackson repeated the cruise in 1988, "We found that 79 percent of our dogwoods were dead. And we found none that were healthy." What's more, Jackson saw no new dogwoods growing. Because a large deer herd browses Catoctin, Jackson is not yet willing to blame the fungus for preventing seedlings from sprouting. Except for the dogwoods, the forests of Mt. Catoctin are healthy.
An even darker picture emerges from surveys reported by the U.S. Forest Service's pathologist in Asheville, North Carolina. Bob Anderson believes the blight could become as total as chestnut or elm blight. He already has seen areas as large as 500 acres where more than 60 percent of the trees are dead. "And normally these areas of severe mortality," he says, "are surrounded by hundreds or thousands of acres with a large percentage of the dogwood infected."
"If you come over here and stand in the middle of some of these stands," he says, "it's scary."
The University of Georgia, Anderson says, has developed a very effective fungicide against the blight, but it is not practical for forest protection. "We have no control over the forest situation at this time."
For Anderson, as for other experts, why some dogwoods are blighted and others are not, and why some survive while others die are the central questions in understanding the blight. When the Catoctin epidemic was discovered, Frank Santamour, a geneticist at the National Arboretum, decided to test 174 dogwoods from 20 eastern and midwestern states. He took the potted seedlings to Catoctin and set them out under infected trees. The fungus quickly infected almost all the seedlings. After three years, only the Chinese dogwood, Cornus kousa, had no leaf loss.
"The question is wide open at this point," says Dr. Craig Hibben at the Brooklyn Botanical Garden. The answer could lie in climate, soils, or air pollution. The dogwood blight may in fact yield some interesting dimensions to the acid-rain picture. Hibben doubts that acid rain is setting up the trees for a coup de grace administered by the fungus.
"I have a feeling, from what we've done with the fungus," he says, "that the stress angle is the reverse of what is usually proposed. The fungus that causes this disease is an anthracnose type fungus. It's fairly aggressive. In our experience, it doesn't need weakened trees. We think the disease gets its start and stresses the trees." Infected dogwoods, for instance, are more easily killed by severe winter weather than healthy trees are.
Daughtrey, who works closely with Hibben, believes that if the fungus has been around for some time, its suddenly more lethal effects could be explained by acid precipitation. Chemicals could change the interaction between the leaf and the fungus, rupturing or weakening the cuticle, thus removing physical barriers the tree uses to protect itself.
At the U.S. Forest Service's Bent Creek Experimental Forest near Asheville, Dr. Anderson has brought acid rain onto center stage. In the greenhouse he has simulated several levels of acid precipitation. The more acidic the mist applied, the more intense the fungal infection. Anderson is hoping Forest Service requests for field-test funds will soon be granted.
At Cornell's Long Island research center, Margery Daughtrey is testing Anderson's hypothesis. At Penn State, Dr. Bruce Nash said his mind was completely open about the acid-precipitation question as he prepared to test Anderson's idea.
Whatever the present research turns up, few experts expect that the dogwood will follow the chestnut and elm toward extinction. Pennsylvania's forest pathologist, Glen Stanosz, says "We don't consider this a forest problem of major concern to us."
For many homeowners, however, media coverage of the dogwood blight has made it a major concern. Although research has not produced a simple defense against the fungus, Daughtrey says that a knowledge of how the disease works in the natural setting can help homeowners.
First, homeowners should know whether they live in an area where the dogwood blight has appeared (see map on page 48). So far it has been confined to the Northeast and Northwest and cool mountain areas of the Southeast. Daughtrey feels that in warm climates where trees grow more rapidly, they may have a greater chance of defending themselves. Anderson, however, says he now has reports of dogwood blight from the southern piedmont area around Spartanburg, South Carolina.
Even in areas where the blight flourishes, trees planted in the open are seldom hit as hard as forest trees. The better air circulation and greater amount of drying sunlight seem to discourage the growth of the moisture-dependent fungus. However, even homeowners growing it in open sunlight need to take precautions.
For the present, neither homeowners nor forest lovers need fear the extinction of the familiar dogwood. Despite the media alarm, the people who know most about the disease still don't know how it started, whether it is related to acid precipitation, or how it will run its course. For those who have the patience to wait for science, the dogwood blight is just beginning to open another window on our understanding of forest ecology.
Defending Your Dogwoods
Most ornamental dogwoods in open sunlight are not seriously endangered by dogwood blight. However, homeowners can and should take precautions:
* Because dogwoods are shallow rooted, heavily mulch the base of each tree to retain water and keep the tree healthy.
*Water the tree in dry weather, but avoid wetting the leaves.
* Fertilize trees in early spring or late fall with a balanced fertilizer, not a high-nitrogen lawn formula.
*Prune out and burn diseased limbs.
*Clip off water shoots (epicormic branches) to prevent infection and cankers.
* Fungicides may provide some protection, though they are probably of little use in wet weather, when the fungus takes hold. Limited protection has been obtained from Daconil 2787.
* If dogwood borers are a problem in your area, spray the trunks. Borers put trees under stress and may make them more susceptible to blight.
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|Title Annotation:||includes related information|
|Date:||Nov 1, 1989|
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