Most Nebraskans have heard of pine wilt, a disease of pine trees caused by the microscopic pinewood nematode. Each spring, it’s essential to remove trees killed by pine wilt to prevent or slow the disease’s spread. Trees that died suddenly last fall or have turned brown this spring are likely infected, and should be handled accordingly. Cut down infected trees by April 30th and burn, bury or chip them. Remove tree stumps down to the ground and don’t save firewood!
The pinewood nematode is unusual, compared to other plant-parasitic nematodes, because it lives entirely in the above ground parts of the tree and never enters the soil. It requires the assistance of another insect, called the pine sawyer beetle, acting as a vector to carrying it from tree to tree. The larvae of pine sawyer beetles are borers, that tunnel within the trunks of dead or dying trees. When the beetle larva pupates into an adult and begins to emerge from the tree it is infected with thousands of nematodes. As then beetles feed on the bark of small twigs at the tops of healthy pine trees, the nematodes enter the wound sites and infect the new tree. Adult pine sawyer beetles, which emerge from May though June, are strong fliers, and can move the nematodes several miles.
Infected trees die rapidly, usually within three months of infection. A typical early symptom of pine wilt infection is ‘fading’. Pine needles turn grayish-green, then tan and finally, brown. Often the entire tree will fade all at once, but sometimes the top of the tree may be affected first or some of the side branches. Needles can remain on a dead tree for a year or longer. Resin flow from the wood ceases and the wood is dry when cut, compared to wood from healthy pine trees which will quickly ooze resin and become sticky after cutting.
Scotch pine is the main host of pine wilt. Austrian pine is moderately susceptible, while native pines such as ponderosa and white pine are rarely affected. As pines age, their susceptibility to pine wilt increases. Most cases appear in trees more than 10 years old. When replacing dead trees consider species, such as spruces, firs, and red cedars or junipers, which are not susceptible.
Healthy trees can be protected from pine wilt with a trunk injection of Greyhound (abamectin). Injections protect approximately 70% of treated trees, and provide about 3 years of protection. Average cost for the treatment of a 10-inch diameter tree is $300.00 per tree. The cost makes treatment of large windbreak plantings prohibitive, but homeowners may consider treatment for high value landscape trees.
Sarah Browning is a Horticulture Extension Educator with University of Nebraska- Lincoln Extension in Dodge and Saunders Counties. She can be contacted by phone at 727-2775: by mail at 1206 W. 23rd Street, Fremont, NE 68025: or by e-mail at sbrowning2@unl.edu
