Saving the trees that kill each other.
BILL PETERS went to a meeting of his neighborhood association and learned to his horror that his yard is one of the centers of a virulent disease attacking oak trees. "I had seen dead trees, but I didn't know there was a real problem until people focused on my yard," recalls Peters, who has been a resident of Austin, Texas, for 25 years. "They had made drawings of the neighborhood from aerial photographs, and I recognized my corner."
It was too late to save his trees, but Peters found himself called on to help save his neighbors'.
The epidemic attacking trees in Austin is called "oak wilt." The disease is caused by a fungus that clogs the water-conducting vessels of infected trees. The trees become dehydrated as water is prevented from moving from the root system to the crown. The leaves wilt, and, almost invariably, a quick death follows. During the past 20 years, oak wilt has killed more than 10,000 trees in Austin. It is considered one of the most virulent tree diseases in the United States.
Disease or natural disaster can strike any community and can cause a devastating loss of tree canopy. Should that happen, the effect is significant: Your neighborhood could be stripped of its trees, your house could lose its shade, and, as your air conditioner works harder to cool your home, you could find your utility bills rising. The best way to protect your trees? Plant a diversity of species. That way, you will not lose your entire tree canopy (see "Whose Trees Are at Risk?" on page 38).
Bill Peters, 61, whose business is wholesale industrial supply, has lost six trees so far; in the end, all his oaks will probably die. A few elms, ashes, and magnolias will remain, leaving less than half the original tree canopy to protect his home from the harsh Texas sun. The challenge for Peters and many who have faced such a loss anywhere in the U.S. is to find ways to reestablish a healthy tree canopy.
Oak wilt is caused by a fungus called Ceratocystis fagacearum. Many oaks are resistant, but live oaks (Quercus fusiformis and Q. virginiana) and the red-oak family, particularly Spanish (Q. texana) and blackjack oaks (Q marilandica), are quite susceptible. Mortality is 100 percent in the red-oak family, only slightly less in live oaks.
The fungus is transmitted in two ways--by airborne spread and underground through roots.
In late fall and early spring, diseased red oaks produce "fungal mats"--masses of oak-wilt spores--beneath their bark. These sweet-smelling mats attract free-flying, sap-feeding insects called nitidulid beetles. The fungal spores adhere to the beetles' bodies as they feed. If air currents blow the insects to uninfected oak trees, the beetles deposit the spores as they feed on sap oozing from any fresh wounds caused by pruning cuts or broken limbs. Scientists believe the beetles can spread the spores from fungal mats to uninfected trees a mile away. Only red oaks--not live oaks--produce the fungal mats that initiate this process of establishing new centers of disease. For this epidemic, the red oak is the Typhoid Mary.
Live oaks generally grow in large stands with an interconnected root network, so they are particularly susceptible to the second, localized form of transmittal. A characteristic trait of live oaks is prolific root sprouting, and when the sprouts become mature trees, the root connections remain. The disease travels from an infected tree along this communal root structure to adjacent healthy oaks. Oak wilt can expand in this manner by as much as 100 feet a year.
In live oaks, the symptoms are a distinctive yellowing and browning of the mid-vein and lateral veins of the leaf while the remainder stays green. The incubation period--from the time the fungus is introduced to the appearance of symptoms--is brief. Once live oaks are infected, most die within three months to a year. Symptoms in red oaks are less distinctive. The sign to watch for is leaves that turn pale green to brown very quickly. Red oaks die within two weeks to several months after symptoms appear.
At this time there is no cure for oak wilt, so the only treatment is preventative measures and suppression. Precautions include removing diseased red oaks before they can form fungal mats. Bill Peters cut a couple of his dead trees even before he knew what had killed them.
Another preventative measure is to avoid pruning during fall and spring when the fungal mats are forming and the nitidulid beetle is most active. In addition, an extremely important precaution is to seal wounds immediately by covering them with an asphalt-based paint. Normally foresters no longer recommend this treatment for pruning cuts or other wounds, but in the case of oak wilt, research has shown that sealing prevents sap from attracting the beetles. The paint must be applied within 48 hours. After that, the wound is no longer attractive to the insects, and thus it is unnecessary to paint old wounds.
Firewood from infected red oaks can contain fungal mats and nitidulid beetles. Every autumn, Jim Rooni and Jay Culver of Austin's Parks and Recreation Department mount an educational campaign advising homeowners to avoid buying logs that come from red oaks. But if a homeowner is unsure of the type of wood being sold, the best thing is to store it under plastic and bury the edges so the beetles cannot escape. Clear plastic should be used rather than black plastic, which allows light to penetrate through rips and puncture holes, showing the insects potential escape hatches.
A final preventative measure for individual high-value trees that have not yet begun showing symptoms is to inject them with a fungicide called propiconazole (trade name Alamo). The rate of success drops dramatically if this treatment is used as a therapeutic measure after a tree has already begun showing symptoms, Culver says. In that case, injection is considered a last resort because the disease simply goes into remission instead of being cured. Also, the fungicide may prolong the life of an individual tree, but it does not kill fungus in the roots and thus does not prevent underground transmittal of the disease.
In 1992 the cost of a fungicide treatment, including labor, for a single live oak 30 inches in diameter ranged from $360 to $450. A 10-inch tree would run about $100. Bill Peters had six trees injected.
The main line of attack is suppression--checking the spread of the disease by isolating affected areas. Trenching machines are used to sever the communal root systems. The disease can still be transmitted by the beetles, but the vast majority of spread is through roots.
"The contractors came in with a big wheel, like they'd use to dig a utility line," recalls Peters. "They went down about three feet by eight inches wide to cut the roots" to separate the intertwining root systems. "They trenched the backyards and sideyards of the house behind me and my neighbor on one side."
The trench was refilled the same day. Roots take eight to 10 years to grow back together, by which time the disease will have run its course. Grass grows back over the trenching scar quickly, but shrubs and flowers have to be replanted.
Trenches for containing oak wilt average 2,000 feet in length--nearly half a mile. "Austin has shallow soils, so a rock saw has to be used in many cases, and the cost can run as high as $8.80 a linear foot. When the path reaches underground utilities, water or sewer lines, fences, or sprinkler systems, the trench has to be dug by hand. When it crosses streets or driveways, it has to be refilled with concrete.
The red tape can be daunting (involving some 31 steps for planning the project and obtaining funds), and the effort and cost are beyond the reach of individual home-owners. To stop this disease, people have to cooperate with their neighbors--whether they like them or not. Austin's parks department works with neighborhood associations to walk them through the bureaucracy.
The meeting at which Peters found out about his trees was held by the Castlewood-Oak Valley Neighborhood Association. Some 15 or 20 people attended the association's early meetings on oak wilt, but later gatherings attracted 50 to 70 homeowners. Castlewood is a quiet, upper-middle-class neighborhood with three- and four-bedroom brick houses. The Castlewood association formed an oak-wilt steering committee, which raised more than $8,000 during a two-month, door-to-door campaign. The Texas Forest Service provided a matching grant from U.S. Forest Service funds.
Even though Bill Peters' oaks are mostly gone, he made a contribution to the trenching project.
Bemoaning the loss of his trees, Peters says, "When you've lost something that you've manicured over the years, you know it doesn't just grow up to 15 or 20 feet in height overnight."
With the help of a program run by the city, he's already picked five trees to replant along the curb. Needless to say, none of those will be oaks.
NORAH DAVIS is former managing editor of AMERICAN FORESTS, writes on natural-resources issues aboard her sailboat, Richmond Studio, in Washington, DC.
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|Title Annotation:||oak wilt|
|Author:||Davis, Norah Deakin|
|Date:||Jan 1, 1995|
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