Forest for the trees?
How pathogens are threatening the health of American trees and forests
The pathogen came from overseas. It swept through the vulnerable native population, leaving behind a few disfigured survivors, and many more mortal remains. This wasn’t the 1918 flu, or the plague. Instead the pathogen was Dutch elm disease, and its victim was the American elm tree.
Globalization means that human diseases like H1N1, AIDS, and others can travel swiftly around the globe, invading new human populations. But the quick passage of people and goods around the globe affects many more species than just our own. Trees in particular are vulnerable. Unable to move to escape introduced pathogens, and with limited reproductive capabilities, they can’t adapt quickly to the lethal organisms that hunt them down. As the speed at which we exchange goods has accelerated over the past century, more and more pathogens have been afforded the opportunity to travel, placing tree populations around the globe at increased risk.
Dutch elm disease: the first pandemic
Nearly every city or town in the United States has an Elm Street, but very few of them have elms growing there today. The trees are long since dead, their remains cut down, and probably burned in a fiery purification ritual that attempted to stem the tide of Dutch elm disease. Now only a few elms exist in urban areas, protected by armies of arborists, while other trees live in pockets of forest so isolated that the scourge of Dutch elm disease hasn’t reached them yet.
Dutch elm disease can be caused by three different species of fungus, Ophiostoma ulmi, Ophiostoma himal-ulmi, and Ophiostoma novo-ulmi. It is a type of vascular wilt disease, which means it attacks the plant’s internal system that transports water and nutrients. The tree responds by closing off that branch or area, in a valiant but ultimately hopeless effort to protect itself. The tree’s own immune system clogs the vascular intake system of the plant, preventing water from reaching the branches above the infection site, usually killing the tree within a single year. For American elms, which regularly live for 80 to 120 years if uninfected, this is a swift killer.
The deadly elm disease got its name by an accident of geography. “The Dutch have nothing to do with it,” quipped Martin Hubbes, a professor emeritus of forest pathology at the University of Toronto. Hubbes explained that the disease was first noticed in Holland in the aftermath of World War One, and spread through Europe, eventually making its way across the Atlantic, probably in a log, or another infected wood product. Here in the United States, sometime after the product arrived in New York’s harbor, a tiny insect came to examine it. The insect took a bite, and then headed out to find its next meal. That small act would set in motion a devastating epidemic.
The tiny insect was a bark beetle, which drills into the space between the wood and bark of a tree to reproduce. It bored into the infected elm wood that arrived from Europe, and then returned to its preferred host, the American elm, carrying spores of the fungus. The elm trees were the perfect host for this pathogen, and the bark beetle the perfect superhighway between the trees.
American landscapers loved the elm, and lined long stretches of road with the tall, magnificent trees. Elms soared 80 feet above the ground, and their thick canopies provided shade on roads, town squares, and college campuses around the United States. Planted on either side of mid-western roads, they formed a green tunnel that stretched through farmland and towns, easing the harsh sunlight for the travelers below.
Because they were planted so close together, a single infected tree could have devastating results. “The infected tree can infect trees for miles around,” said George Ellmore, a biology professor at Tufts University who has worked on Dutch elm disease for years. Bark beetles could easily travel from one tree to the other, and with the trees planted so close together, the roots would entwine and connect, transferring the fungus directly from tree to tree. The only way to prevent the infection from spreading during the 20th century was to cut ravaged trees down and thoroughly burn the remains, but even this did little to temper the devastation.
Dutch elm disease, though devastating, is far from the only disease affecting the trees and forests of the United States. Other vascular wilt fungi that kill oaks and pines have emerged. Other types of fungi function as parasites. Parasitic fungi can resemble shelves on tree trunks. They rot away the wood inside a tree using the tree’s nutrients and water as their own, and build their ‘bodies’ on the nearly-dead trunk. Chestnut blight, a fungus brought into the country from Asia in the early 1900s, has reduced the American chestnut to the ‘living extinct,’ with a dying population of elderly, infected trees, suffused with cankers that will eventually kill them. There are attempts to breed the few surviving uninfected American chestnuts with more resistant chestnut species from Asia, but the process is long and slow.
While all of these diseases are awful, some are worse than others. “One of the bad ones out there right now is sudden oak death,” said Karen Snover-Clift, the director of the Plant Disease Diagnostic Center at Cornell University. Sudden oak death (caused by the fungus-like pathogen Phytophthora ramorum) is a vicious disease that can infect over 120 species of tree or shrub. “When it moves through the forest it looks as though the forest almost completely dies,” said Snover-Clift.
Sudden oak death is one of the most recent entries into the United States, first noticed in 1995 when large number of oak trees began dying in California. Researchers were able to identify the fungus-like pathogen, P. ramorum in 2000, but no consensus has been reached as to where it originated. The disease was also noticed in Europe at around the same time, and it has since become a scourge to forests across the continent, particularly in the United Kingdom. In the United States, sudden oak death seems to have been confined exclusively to the West Coast. But there’s no telling how fast the disease might spread if one infected plant slipped past the controls along state lines. Nurseries in infected states must be inspected before they send any of their merchandise over state lines, and hikers exiting a sudden oak death area of forest must wash their boots at way stations along the trail before they walk into another area. Despite these precautions, East Coast plant pathologists like Snover-Clift keep an eye peeled for sudden oak death, among the many other plant diseases that they look for every day.
“Our laboratory basically serves as a place where people can send in sick plants and we try to determine the causes,” said Snover-Clift. People send in pieces of dying plants to her lab, or entire plants and trees if at all possible. Then Snover-Clift and her team identify the pathogen and alert the owner as to what measures might need to be taken. She works with everyone from curious homeowners to golf courses, to the New York State Department of Agriculture and the United States Forest Service, diagnosing plants around the entire northeast. Sometimes it’s just a matter of environmental factors leading to the plant’s poor health, and sometimes it’s insects, but “about fifty percent of the time it’s a pathogen,” said Snover-Clift.
Pathogens do enter the scene, like uninvited, irritating, deadly neighbors. Luckily, there are preventative measures already in place to combat the global spread of disease. The front line? United States Customs and Border Protection. In addition to being charged with looking for terrorists, drugs and illegal weapons, customs officers inspect plants before they are allowed in. “The rules and regulations that we have for the entire United States work quite well” said Snover-Clift.
“Quite well” wasn’t good enough for the United States government after September 11. Concerned that pathogens would be used in a terrorist attack against our agricultural crops, they decided to set up the National Plant Diagnostic Network, which linked diagnostic labs like Snover-Clift’s with similar ones all over the country. Now, in addition to their normal duties, these labs are charged with ensuring that dangerous plant pathogens entering the United States are identified and quarantined with all possible speed.
Tree pathogens don’t inspire the same terrorist threat level as pathogens that infect agricultural crops but the rapid movement of people and goods affects them all the same. Fungi are certainly the most common pathogens out there, but there are other kinds of infections too. The plum pox virus, potentially devastating to fruit orchards, affects trees that bear fruit with stones, including apricots, peaches and plums. It was identified in Pennsylvania in 1999, and appears to be a strain that was imported from Europe. Snover-Clift’s lab found the virus in New York in 2006, after its annual survey of 15,000 trees. Because of its dramatic effect on New York’s agricultural industry, her lab and orchard owners around the state are watching the path of the virus carefully, ready to stamp it out vigorously wherever it might emerge.
Plants themselves can be plant pathogens, as evidenced in the western part of the country where dwarf mistletoe reigns. Jim Worrall, a forest pathologist with the United States Forest Service, says that the dwarf mistletoe is one of the more problematic pathogens plaguing western forests today. “Mistletoe has a big impact on their form,” said Worrall, explaining that a tree parasitized by mistletoe will grow misshapen and will eventually die as the parasitic plant sucks away its water and nutrients, leaving dark, twisted forests behind.
Fungi, parasites, and viruses … oh my. It seems pretty bleak for trees in America these days, with pathogens traveling from near and far to drive them into extinction. But there is hope. Plant and forest pathologists around the world are studying treatments and methods of prevention every day. “Tree pathology as a science is a bit behind agricultural sciences because [trees] have not been prioritized,” said Ellmore. Despite a lagging start, pathologists are learning more and more about tree pathogens, accumulating a large body of knowledge that could help manage disease outbreaks.
To map out the course of the disease, pathologists come up with disease cycles and pinpoint areas of weakness in them. “A disease cycle is like a life cycle but it takes into account other factors, including the vectors that carry it,” said Linda Haugen, a plant pathologist with the United States Forest Service.
Different diseases can have very different vectors, or ways that the disease is transferred from tree to tree. Dutch elm disease is transmitted by insect, or by direct connections between trees, while dwarf mistletoe shoots its seeds 30 to 40 feet into the air, hoping that one will land on a viable host. Dwarf mistletoe, a pathogen native to the United States, has taken advantage of dense forests enabled by fire controls in the west to spread rapidly through the trees. “If there’s not fire to clean things out, you get into a chronic disease condition in the forest,” said Worrall. While fire wouldn’t help the American elm population, many of the species of trees that are now burdened by dwarf mistletoe have evolved to be fire resistant, and a good wildfire could help restore the balance of some mistletoe-infested forests in the west.
Hoping for a wildfire is a pretty indirect way to treat disease. Just as human diseases are sometimes treated with injected antibiotics, it is possible to give trees injections of fungicide.
In order to give a shot to an 80-foot-tall elm tree, “you drill little holes in the base of the tree every May,” said Ellmore. Then eighty gallons of pressurized fungicides are pumped into the tree, a process that only takes two to three hours. The fungicide destroys the chitin within the fungal cells, weakening them and preventing them from reproducing, potentially saving the tree from its own deadly immune response. Ellmore developed the method, called “shallow pit injection” in the 1980s, and its use has become widespread among very important trees in very important places, like the Washington Mall. “It’s something you would only do if it was really valuable, a historically important tree,” said Ellmore.
“In forest pathology we generally can’t think about curing or treating a single tree,” said Worrall. The cost is prohibitive, with each injection costing hundreds of dollars. “In the forests we’re more interested in saving an entire population,” said Haugen, adding that usually the most effective way of dealing with an infected tree in the forest is to remove it from the population before the disease can spread. “If you’re saving an entire population you may be cutting down large numbers of trees,” said Haugen.
City tree or country tree, one thing remains the same. Early detection of disease can keep it from spreading, and avoid potential disaster. At Cornell, where Snover-Clift works, the campus was dominated by American elm trees, all of which died of Dutch elm disease. The bare campus taught a harsh but valuable lesson: “One of the things we’ve learned is that it’s not good to have huge monocultures of one thing,” said Snover-Clift. “That way if a pathogen comes in, it’s not going to kill every tree.” Landscapers are increasingly encouraged to use a variety of trees when designing a landscape.
Globalization means that “species are being challenged with species in combinations that have never before existed,” said Ellmore. Unfortunately, in the species versus species battle the pathogens are likely to beat the trees every time.
“The arms race is very much in favor of the fungus,” said Ellmore. Fungi grow swiftly, with multiple generations within weeks. Trees, by contrast, take years to reach maturity, let alone reproduce, severely curtailing their ability to adapt to new challenges like pathogens from other parts of the globe. Ellmore put it bluntly: “A tree cannot outrun a fungus, and the fungi are coming.”
Global warming plays a role in the fight as well, often coming down on the side of the fungi and other pathogens. “It seems like things are always changing, and with changing climate we expect things to change even more,” says Worrall. Accelerating climate change puts considerable stress on tree populations that can’t move to escape the new conditions, and stressed trees are more susceptible to pathogen attacks.
Changing climate and fungi with advantages mean that many American forests will endure dramatic shifts in species. Larger, older hardwoods are likely to get pushed out by shrubs and more generalist, less vulnerable species of trees. For now, the only advantage that the trees have is the dedicated team of researchers and forest pathologists committed to understanding the diseases affecting forests. It is an uphill battle. “We can’t test every single plant for every single pathogen,” said Snover-Clift, but that doesn’t stop those involved from staying vigilant. With goods being exchanged globally at a rapid pace, “there certainly are more of those [pathogens] waiting to be transferred by humans,” said Worrall. But as long as there are people waiting and ready for the pathogens, the trees might just have a chance.