September 23, 2012

Cornell’s Trees In Danger

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Many years ago Cornell’s campus was home to a vast and picturesque collection of American elm trees, most of which were planted in the late 1800s as gifts.  But at the turn of the century, a tiny and unwitting hitchhiker from Europe would cross the Atlantic with a shipment of Dutch logs and, in the decades to come, prove devastating for the elm trees of North America.

This hitchhiker was none other than the smaller European elm bark beetle, the primary vector of Dutch Elm Disease (DED).  Caused by a fungus known as Ophiostoma ulmi, DED claimed its first American victims in the late 1920s, much to the alarm of Cornell’s agriculturists.  In 1934 the Daily Sun reported that an estimated “3,000 trees in [New York and New Jersey] [were] victims of the disease…”  By 1968, most of Cornell’s elm trees had, as one Sun reporter phrased it, “fallen to the tree-surgeon’s ax,” and by 1976, all of the elms on campus had been killed.

To this day, the loss of Cornell’s elms remains the most dramatic example of a tree disease outbreak this campus has known. But there is another fungal disease at large in the United States that, if it reached our campus, could wreak havoc on one of our current (and celebrated!) tree populations: the Northern red oak.  If you have ever crossed the Ag Quad, dined at Trillium in Kennedy Hall or simply walked along Tower Road, then you have seen these trees.

Dr. Wayne Sinclair, Emeritus Professor of Plant Pathology at Cornell, estimated the age of the largest Tower Road oaks to be around 110 years old, noting that they were the first generation of trees to be planted in their current location.  They line both sides of the road, though the ones on the south side, where the parking lot is, are stunted due to root confinement and damage from decades of cars parking beneath them.

According to the Cornell Urban Horticulture Institute, red oaks are plentiful on Cornell’s campus because they “are fairly easy to transplant, are fast growers among the oaks, and can tolerate salt and pollution along city streets.”  However, they are also the most vulnerable among the oaks to oak wilt, a fungal disease that has some of Cornell’s tree pathologists on the alert.

The oak trees along Tower Road are at an elevated risk should oak wilt strike because they are planted as a monoculture — that is, the same species of tree, one after another – which means that their roots are well connected, making it easy for the fungus to spread, perhaps before the disease is even detected.

The causative fungus is called Ceratocystis fagacearum, a mold related to the DED pathogen that is usually carried to a new site and onto an unwitting host tree by insects.  Once the fungus is introduced to the water conducting cells of an oak tree, subsequent spread to adjacent trees is most likely to be via grafted roots.  Inside the tree, at the site of introduction, the fungus plugs the large, water-conducting vessels while at the same time its spores are transported throughout the tree to new vessels where more plugging occurs. Eventually, trees are killed by dehydration, but not before there has been ample time for spread to the neighboring oaks.  Aside from wilting leaves, an infected tree will display a crack in its bark, beneath which the fungus grows, producing an odor reminiscent of rotting fruit to attract the nitidulid beetles that transport fungal spores.

While oak wilt has historically tended to occur in the Midwest and certain regions of Texas, several years ago it showed up in New York State.  Professor George Hudler of Cornell’s Plant Pathology Department responded to a call near Schenectady concerning the rapid wilting and death of oak trees in the aptly named Glen Oaks neighborhood.  “The…sobering thing for me was…when we realized that [oak wilt] had been brought to the neighborhood on infected firewood,” Hudler told the Sun.  “That’s when I learned that the movement of firewood around the country [has been] almost completely unregulated.”

Tree pathologists like Hudler do have containment strategies in place.  Unfortunately, they often involve the removal of apparently healthy trees that are near an infected one.  Schenectady’s Spotlight News reported in January last year that, after the removal of 75 infected trees, stump grinding, and planting of maple trees, the neighborhood had been declared free of oak wilt. Seventy-five trees may not seem like a lot, but for a neighborhood — or a campus — that has been growing for a century along with those trees, their absence will surely be felt.

Original Author: Sam Miller