Invasive pest spreads disease, threatens black walnut trees
In the pasture below my childhood home stands an old black walnut tree. My friends and I loved hurling the green-husked fruit at one another; consequently we would come home with green-tinged fingers.
My mother would chastise me since she usually discovered my stained hands moments before we were headed out for a family gathering or other event.
Whether you played with the messy fruit or not, many of us have some association with walnut trees.
Black, or American, walnut (Juglans nigra) are native to the Eastern half of the United States and are extremely valuable hardwood trees. Black walnut has been used for centuries for food, medicine and building. Native Americans relied on it for medicine, using the inner bark for digestive woes and toothaches.
The fine-grained wood is much sought after for furniture, cabinetry and firearms and has become scarce over time, reducing its use to veneers.
Home gardeners are often leery of planting black walnut trees because many plants are sensitive to a substance secreted by their roots called juglone. But some plants, including most grasses, are immune to the toxic effects and do quite well near these large trees. Many homeowners simply don’t care for the large, messy fruits dropped each fall.
Walnuts contain high levels of manganese and heart healthy fats. Many of us enjoy them baked into cakes or enveloped in chocolate. The shells may seem like a by-product but are actually used in myriad ways from cleaning jet engines to making explosives, or in cosmetics and textured paint.
In the past several years, an emerging threat to these important trees has come to the attention of the Clemson University Department of Plant Industries.
Invasive Species Program coordinator Sherry Aultman says the threat lies in the walnut twig beetle (Pityophthorus juglandis), a small bark-boring insect native to a small region of the Southwest, including Arizona and New Mexico.
The walnut twig beetle typically only attacks the Arizona walnut, a small scrubby western walnut species, but for some reason, it began to expand its native range, attacking the multitude of planted black walnuts from California to Colorado.
Researchers believe the insect may have hitched a ride to the East Coast on lumber or dead wood and is now feeding on the Eastern species in Central and Eastern states.
The problem isn’t the insect itself but in the fungus spread by the beetle that causes “thousand cankers disease.” Symptoms include crown dieback, yellowing of the upper leaves, wilting of leaves and dark amber stains along branches and stems.
Recently, the disease has been detected as close as Tennessee and now in North Carolina. Once infected, the trees cannot be saved.
To help stem the spread of this disease, the Clemson University Department of Plant Industry staff is conducting a “Thousand Cankers Survey” in South Carolina this summer. The survey is a cooperative project between Clemson and U.S. Department of Agriculture in an effort to protect this important natural resource.
While the Lowcountry is beyond the natural range of the black walnut, which extends only as far as the Sandhills, many landowners plant them for their many uses.
Aultman says that even though we don’t have large native populations of the trees in Lowcountry forests, “We have to look at the big picture and help protect our statewide forests and neighboring states from this threat.”
Field staff members conducting the survey are combing the state looking for declining walnut trees, those that appear to be dying or in poor health. Once they inspect the trees, they will place pheromone traps called Lindgren funnel traps about 10 feet away from the tree. Each trap will be left in place for six weeks and will be monitored for the presence of the beetle.
Aultman notes that this disease has not been found to affect hickory or pecan trees, but may affect other native trees related to the walnut tree, such as butternut (Juglans cinerea L.).
If you don’t have a walnut tree on your property, you can still help to protect them.
Sarah Morrison, a program assistant for the Clemson Invasive Species Program, notes that moving firewood, or any wood with its bark intact, greatly contributes to the problem of spreading invasive species. She says the rule of thumb is to avoid moving firewood more than 50-100 miles from home. Read more at the website www.dontmove firewood.org/.
Since there is no available treatment for the disease, Clemson staff will work with tree owners to isolate and remove affected trees in an effort to curb the spread of this potentially devastating disease.
To contact the Clemson Invasive Species Program and find out more go to www.clemson.edu/invasives or call 864-646-2140.
Applications are now being accepted for the 2013-2014 Clemson Extension Tri County Master Gardener Training Course.
The course will be begin Sept. 19 and will be held 9 a.m.-4 p.m. Thursdays (except holidays).
Candidates must apply by Aug. 2. Candidate interviews will be Aug. 12-16.
For more details and to apply online go to http://bit.ly/XOONhg.
Amy L. Dabbs is the urban horticulture extension agent and tri-county Master Gardener coordinator for the Clemson University Cooperative Extension. Send questions to firstname.lastname@example.org.