Leaves as a clue: Surveying for laurel wilt disease

by | Jan 29, 2014

Along with the frigid temperatures we’ve seen lately, the bare branches of trees around us serve as a reminder that we are still deep in the throes of winter. But not all branches have shed their leaves. Some trees are evergreen while others, such as the American beech, retain their light brown leaves until late winter. However there is one tree that fits both of these categories: the redbay tree. Usually an evergreen tree with aromatic leaves, the state’s redbay trees are under attack by an invasive pest – laurel wilt disease. A small beetle attacks the tree, spreading a deadly fungus to it. When the tree dies, it retains dead leaves for several years.

Left: N.C. Forest Service survey teams keep an eye out for the reddish-brown leaves of dead redbay trees when looking for laurel wilt. Right: When a dead redbay tree is found, a closer inspection may reveal dark streaking under the bark, a symptom of laurel wilt disease. Images: K. Oten, N.C. Forest Service.

Despite the unpleasant appearance, the retention of dead leaves following tree death is a helpful survey tool. Each winter, the N.C. Forest Service canvases areas near the known range of laurel wilt in the state to determine if the disease has spread to new areas. They scan forests for small- to medium-sized trees that are holding onto their reddish-brown leaves. Upon spotting one, the survey team will pull over for closer inspection. Winter is the ideal time for this because the leaves of other trees do not get in the way and surveyors can view much farther into the forest.

Laurel wilt is a fungal disease spread by a small beetle called the redbay ambrosia beetle. As a group, ambrosia beetles bore into trees, inoculate the galleries within the wood with a fungus, and feed upon the fungus. Most species of ambrosia beetles attack weakened or dying trees and typically the fungus does not kill the tree. However, in the case of the redbay ambrosia beetle, it not only attacks seemingly healthy redbay trees, but the fungus is essentially toxic to the tree.  A single attack by a beetle can inoculate a tree with the fungus and result in tree death in a few short weeks.

To date, close to half a billion trees in the Southeastern U.S. have been killed by laurel wilt since it was initially detected in 2002 near Savannah, Ga. Laurel wilt was first found in North Carolina in 2011 and is now known to occur in six counties: Bladen, Brunswick, Columbus, New Hanover, Pender and Sampson. While the disease has only affected redbay trees in North Carolina, sassafras, avocado, spicebush, pondberry (a federally endangered species), pondspice (a species of concern in North Carolina), and other plants in the laurel family are also susceptible (mountain-laurel is not affected).

So, enjoy your Florida-grown guacamole while you can… it may have a limited lifespan. In fact, the Florida Department of Agriculture & Consumer Services launched a ‘Save the Guac’ campaign to raise awareness of the disease and the harmful impact that moving firewood may have in spreading this disease. To learn more about laurel wilt, refer to the N.C. Forest Service laurel wilt page.