Sudden oak death case causes worries in Pennsylvania

University Park, Pa. -- Forgive Pennsylvanians if they are paranoid about fungi that can kill trees. In the last century the state saw the most important tree in its forests, the American chestnut, wiped out by an insidious, seemingly unstoppable blight caused by a fungus. And most everyone has heard about Dutch elm disease, a fungus that has killed the majority of American elms in cities across the Keystone State.

So it is understandable that the announcement last month by Pennsylvania Agriculture Secretary Dennis Wolff that the first case of sudden oak death caused by the fungus Phytophthora ramorum had been confirmed in a bonsai camellia shipped this year to a southeastern Pennsylvania homeowner sent shock waves through the forestry community.

"It is not paranoia that people are feeling," says Donald Davis, professor of plant pathology in Penn State's College of Agricultural Sciences. "It's just that we are wary because we know how serious this disease could be for our forests. We saw what happened to the American chestnut trees."

It is an ironic quirk of fate that sudden oak death most threatens the very tree that grew into the forest niche previously filled by the chestnut. Oak trees provide acorns and high-quality timber so important to wildlife and Pennsylvania's economy. And foresters already were concerned about the oak's future in Pennsylvania because some combination of deer overbrowsing, forest fire control and soil acidification from acid rain seems to be preventing oak trees from regenerating at historic levels.

But despite the name, it is not just oaks that are threatened by sudden oak death, says Seong-Hwan Kim, a Penn State adjunct professor of plant pathology who runs the Plant Disease Diagnostic Laboratory for the Pennsylvania Department of Agriculture in Harrisburg. Kim, who has studied the disease since about 1995, points out that sudden oak death can infect a wide variety of hosts. "It can infect both overstory trees and understory plants such as rhododendron and mountain laurel," he says. "It doesn't kill every plant. That's what makes it so dangerous and easily spread. Some plants, such as camellia, are simply carriers of the fungus."

Plant fungi have had major impacts around the world. A fungus in the same genus (Phytophthora) as the one that causes sudden oak death was responsible for causing the great potato famine in Ireland in the mid-1800s, according to Davis. This same fungus that caused the Irish potato famine is still around after 150 years, reducing Pennsylvania's potato and tomato harvest in wet years. However, scientists are not even sure that this organism is a fungus. Most scientists agree that Phytophthora is a "fungus-like" organism, but actually may be in a class of organisms separate from fungi.

"We're not even sure where this fungus comes from," Davis says. "It may be native to the United States, or it may be introduced. And we aren't sure about its potential for destruction here in the Northeast. I'm a forest pathologist who has worked with oaks quite a bit, but I honestly don't know what to expect. We do know that in certain areas of California it has killed oak trees in forests. Of course, it is a big problem on ornamental plants such as rhododendrons and similar plants in nurseries. Pin oak and red oak seedlings that were sent to California and inoculated with the sudden oak death fungus were capable of being infected with it."

Just how much of a threat is sudden oak death in Pennsylvania? "That's the million-dollar question," Davis says. "In Pennsylvania, the disease could range from an epidemic like chestnut blight to some weak disease -- such as oak wilt or oak decline -- that kills some oak trees, but many trees survive with these diseases for many years."

The infected camellia with the confirmed sudden oak death has been kept indoors since arriving in Pennsylvania. Agricultural Department officials are confident that there was virtually no opportunity for the disease to escape to the outdoors. However, Kim said that samples are being taken from the surrounding area to verify it is contained. So this time Pennsylvania apparently dodged the bullet. "But we may not be so fortunate next time," says Kim, who points out sudden oak death could be transported into the forest on an unsuspecting hiker's boot and then splashed up onto the leaves of mountain laurel or other low-growing plants. "Once it is introduced, under the right conditions, the fungus can spread very quickly."

Kim believes, however, that Pennsylvania's weather may offer its trees some protection from sudden oak death. "The fungus doesn't survive well in weather extremes," he says. "It grows well at about 68 degrees Fahrenheit. But when it gets warmer than 80 degrees, it slows down. When the temperatures drop below about 59 degrees, it also slows down."

So in Kim's opinion, sudden oak death poses the greatest threat in the southern portions of the state. "However, that is really hard to determine," he says. "It all depends on the micro-climate conditions, such as average temperature, relative humidity and rainfall. The fungus needs free water for disease development. The threat is enormous if sudden oak death starts. Although the window is narrow, if it gets into, say, rhododendron in a wetland, it will be a serious situation."

Last Updated March 19, 2009