A bacterium is attacking our red oaks. Will they survive this scourge?
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Most leisure activities at Cadwalader Park are pursued at eye level.
That is why, if you had found yourself in the Trenton park one recent dank, grey morning, you might not have noticed Mark Chisholm swinging from the branches of a massive red oak, 90 feet off the ground.
Chisholm, 38, is a third-generation arborist, tree fanatic, and champion climber. Up in the red oak, he had spotted something. “Lot of dead wood on this tree,” he called down.
For a centenarian, the tree looked pretty good. Its buttress roots resembled the talons of some great mythical bird, and while the lowest of its outstretched boughs bent toward the sodden ground, the tallest reached above the morning fog. One would never have guessed it was dying.
Nearly half the red oaks and related species that have been tested in the park, which was laid out in the late nineteenth century by the famous landscape architect Frederick Law Olmsted, are afflicted with a fatal disease known as bacterial leaf scorch. There is no known cure. An affected tree shows signs of infection only in late summer and early fall, and even then, it’s not easy to distinguish its thinning yellow crown from the symptoms of other opportunistic diseases.
Scorch is but one indicator of Xylella fastidiosa, an elusive pathogen with a checkered history that dates to 1880, when it was discovered on grapevines in California. Called Pierce’s disease when referring to infected grapevines, it continues to plague vineyards today. In shade trees, this microscopic organism colonizes and gums up the xylem, the vascular tissue that conducts water and nutrients from the roots to the leaves. The agents of infection are leafhoppers (or sharpshooters), treehoppers, and spittle bugs, insects that reside in the canopy and suck xylem fluid from the stems and leaves, spreading the disease from infected trees to healthy ones. More than 40 species are susceptible to this strain of the pathogen, but in New Jersey it mostly affects that leafy emblem of the suburban landscape, the northern red oak, Quercus rubra—New Jersey’s state tree.
The sad plight of the red oak is not unusual. There is a consensus among foresters and plant pathologists that environmental stress—a broad term embracing drought, rising temperatures, and changing weather patterns, possibly as a result of global warming—is inhibiting the ability of our native plant life to fight off pests and disease.
That’s a heartbreaker for Chisholm. “When you climb a tree, it becomes part of your history,” says the Howell resident. He first encountered the Cadwalader oak in 2003 at the New Jersey State Tree Climbing Championship—an event he’s won every year for the last sixteen years. Through his work in the family business, the Aspen Tree Expert Company in Jackson, he has watched big pieces of that arboreal history disappear.
There’s hardly a county in New Jersey where the red oak is not in decline. According to one estimate based on research by the New Jersey Forest Service, in the southern counties of Camden, Burlington, Cumberland, and Salem, up to 50 percent of red oaks that have been tested and, to a lesser extent, pin oaks, are dying. Riverton in Burlington County loses ten to fifteen big specimens each year.
Until recently, bacterial leaf scorch was considered a street tree disease, but with the discovery that red oaks are dying in the forests and woodlands, that perception has changed. Based on aerial surveys, ground observation, and sampling done at Parvin State Park in Salem County, where 90 percent of sampled trees tested positive, and Belleplain State Forest in Cape May and Cumberland counties, where 17 of 32 trees tested positive, Jeremy Webber of the New Jersey Forest Service says the blight could be “catastrophic.” Alan Iskra, a U.S. Forest Service plant pathologist studying the problem in the Northeast, concurs that the early findings are alarming. To track the disease’s progress on a map, you can draw an almost unbroken line from New Jersey to Washington, D.C.
One can’t consider the demise of the red oak and not recall the specter of Dutch elm disease, an aggressive fungus that arrived in the United States from Europe in 1930, courtesy of a global traveler called the elm-bark beetle. By the time it had finished its tour of the country, it had wiped out 70 million elms.
When the outbreak subsided, streets everywhere were haunted by empty space. In the wake of that plague, many shade-tree commissions and homeowners planted red oaks, the epitome of the obliging tree: adaptable, easy to transplant, and tolerant of poor soil. Arborists like them, because, as Chisholm says, “They’re so big and strong, there’s not much that’ll go after ‘em.”
As farms and woodlands were replaced by suburban developments, the red oak prospered, and as a consequence many New Jersey residential communities are almost Quercus rubra monocultures. Lack of diversity means trouble; because harmful insects are specialist feeders, a uniform environment means they need not fly far in search of their next meal.
Nick Polanin, the agent for Somerset County at the Rutgers Agricultural Experiment, warns that bacterial leaf scorch could eventually degrade the landscape as severely as Dutch elm disease, a view seconded by other foresters. But unlike the Dutch elm fungus, X. fastidiosa is slow-moving and progressive—an infected tree might hang on for eight years. Treatment with antibiotic sprays can mitigate symptoms at great expense but will not cure the disease. Diagnosis must be confirmed through laboratory tests, because to the naked eye, the bacterium is a secret agent, a shape shifter that mimics gypsy-moth defoliation one day, old age the next. Blurring the picture further is the fact that mortalities are often ascribed to other causes and go unrecorded.
And then there is the malady’s image problem. The name bacterial leaf scorch just does not sound as diabolically threatening as other tree slayers, such as the Asian long-horned beetle or the hemlock wooly adelgid. Because of the speed with which these aliens infest and kill their victims, they are hunted down by the U.S. Department of Agriculture. Quarantine areas are set up, and the hosts are destroyed. Since trees with bacterial leaf scorch are not subject to such protocol, the disease has largely escaped public notice. As Polanin puts it, “A tree dies and you take it down.”
The Rutgers University ento-mology laboratory overlooks a lane of dying red oaks. In the palm of his hand, Jim Lashomb holds what looks like a tiny bit of chaff. Examined under a microscope, the speck becomes a bark-colored, onyx-spotted insect with a fin-shaped protuberance called the pronotum. A species of treehopper, Ophiderma flavicephala is one of the culprits identified as transmitting fastidiosa to shade trees; previously, scientists had believed only leafhoppers and sapsuckers spread the bacterium.
But Lashomb’s unpublished research confirms that members of the treehopper family (Membracidae) play a significant role, too. This is an important discovery, because of the approximately 3,200 known species of treehopper throughout the world, few are considered serious pests. Their lives are brief (just two to three months) and their diet is limited to xylem fluid, which they drink by piercing leaf stems with their needle-like mouth parts. Unlike the leafhopper, which can cover several miles in a day, the treehopper’s entire world is typically limited to a single tree.
“These insects are so much more complex than I ever dreamed,” says Lashomb, who is still grappling with how the insects transmit the pathogen from tree to tree. The entomologist, a white-haired, bespectacled man of square build and rumpled demeanor, wears a baseball cap, baggy trousers, and a wine-colored jersey that is stained with a cretaceous substance. Caterpillar frass? Old breakfast? The sartorial message is that he’d rather be mucking about in the woods.
Earlier, Lashomb and his research partner, the plant pathologist Ann Gould, talked hosts and vectors with the excitement others reserve for discussing Chekhov. Gould explained that bacterial leaf scorch was discovered in New Jersey in the mid-1980s when a beautiful old grove of red oaks in Moorestown went into decline. Researcher John M. Wells of the U.S. Department of Agriculture was stationed at Rutgers when he isolated samples from the leaves and identified the lozenge-shaped cells as a strain of fastidiosa, the same microbial menace that withers grapevines in California, stunts peaches in Florida, blights coffee bushes in Brazil, and makes a general nuisance of itself throughout the Western hemisphere. (In Brazil, fastidiosa has caused so much crop damage that the investigators who completed the first genome sequence of the bacterium in 2000 were hailed as national heroes.)
Even though there is no cure in sight, Lashomb is in high spirits over his treehopper breakthrough, the culmination of five years of collecting, sorting, identifying, and cataloging insects. With great pride he shows off his freezer, which he’s converted into a treehopper morgue. It’s crammed with bugs—thousands of them. Bugs in plastic ziplocks. Bugs in lidded paper cups. Bugs wrapped in butcher paper. Not one of these specimens is bigger than a newborn’s eyelash, and many are a great deal smaller. In the canopy of a mature oak exists an invisible city of such insects, some of them benign, some deadly. It fell to Lashomb and his sole assistant, a PhD student, to identify the creatures, but the real challenge was locating the one or two specialists in the world capable of confirming those identifications.
Once he had labeled each suspect, he had to behead and eviscerate it, extract the bacterium, and figure out how to amplify it for study so that Gould could compare it to the bacterium found in the sick tree. Most laymen would assume that, if the bug is found at the scene of the crime, holding the goods, then it’s guilty as charged. But as the novelist Marilynne Robinson has written, “In environmental issues, a standard of proof is demanded that makes the claims of the Flat Earth Society look easy.”
“We’ve just barely clarified what we’re dealing with,” says Lashomb. “As the data came in, I had a running joke about the ‘hypothesis of the week.’ The next week we’d get more data and the whole story would change. And it’s changing still.”
We want science to provide us with tidy answers and certainty, but as with many scientific puzzles, the research on bacterial leaf scorch raises more questions than it answers. While the U.S. Forest Service is midway through a two-year survey to quantify the disease’s occurrence in the Northeast and Mid-Atlantic states, researchers say it will be years before they can project the number of infected trees. A systemic cure, were one to be developed, could save most ornamentals, but not the approximately 70,000 forest oaks that grow on state land.
The only recourse at present, says Jon Kliesches of the New Jersey Forest Service, is to thin the affected areas, though there is no guarantee such a program will inhibit the disease’s spread. The agency might broaden its forest survey to include central and northern New Jersey, but monitoring is expensive.
For Emile DeVito of the New Jersey Conservation Foundation, an environmental group, the restoration of the state’s forests, which are plagued by overabundant deer and invasive species, must be part of the solution. “If the red oaks are evicted from the forest, they’ll be replaced by weeds. When the natives aren’t regenerating, you have a gigantic problem.”
Meanwhile, in Jersey towns, it’s more important than ever to avoid an over-reliance on the red oak. Some communities might follow the lead of Riverton, which, in response to the crisis, transformed its urban landscape into a model of tree diversity—winning the 2008 New Jersey Arborist Gold Leaf Award in the bargain. Some years ago, Riverton made the decision to ease off on planting red oaks; today the town is home to 150 different kinds of shade trees, including hackberries, seedless sweetgums, liberty elms, and hedge maples.
“In the long run, we’ll be better off,” says Barry Emens, chairman of the Riverton Shade Tree Commission. “This epidemic has obliged us to search out new species. Really, it’s quite amazing. Now every street in Riverton is green.”
Laurel Berger is a freelance writer whose work has appeared in Men’s Journal, National Geographic Adventure, and other publications.
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