I'm worried about this disease of pine trees I have been hearing about. What is it and can it be prevented?
Scots pine, a non-native pine that has been widely used in Nebraska windbreak plantings, is facing a serious, new threat called Pine Wilt, which is caused by the pine wood nematode, Bursaphelenchus xylophilus. This 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.
The pine wood nematode is a microscopic, worm-like animal that feeds on the living plant cells surrounding the water-conducting tissues of pine trees. Once inside a susceptible pine tree the nematodes reproduce rapidly, and move throughout the tree.
As they destroy the water-conducting tissues the tree's water-moving system becomes clogged and resin flow slows, then stops. The tree then begins to display wilt symptoms and soon dies. This wilt typically kills Scots pine within a few weeks to a few months.
Symptoms
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. 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. Needles can remain on a dead tree for a year or longer.
Once the pine sawyer beetle introduces the nematode, the infected tree typically dies within a few weeks or months. Some trees will fade during the summer, and more will begin to fade in August and September, continuing through the fall and into spring.
Transmission
Pinewood nematodes are transmitted from tree to tree by an insect, the pine sawyer beetle, Monochamus spp. Also known as longhorn beetles, pine sawyers are attracted to the dying trees and the females lay their eggs under the bark. The grub-like larva hatch out and tunnel through the wood of the dying tree. When the beetle larva pupates into an adult and begins to emerge from the tree it is infected with thousands of the nematodes that enter its breathing holes. As the adult beetle flies to a new tree it carries the nematodes with it. Nematodes enter the new tree through feeding site wounds made by the adult beetle.
Hosts
Scotch pine is the main host of pine wilt, but the disease also occurs in Austrian, Jack, Mugo and Red pine. In the Midwest more than 90 percent of the trees killed by this condition have been Scots pine. The disease does not affect other conifers, such as spruces, firs, and red cedars or junipers.
Tree age also influences the risk of pine wilt. Although trees at any age can be infected, the disease is generally seen in non-vigorous trees at least 10 years old. Nebraska's hot, dry summer conditions may place pine trees under stress, making them more prone to attack by the pine wood nematode and provide the warm conditions needed by the nematode for rapid reproduction.
Monitoring
In fall homeowners should check their yards and windbreaks for trees showing signs of pine wilt. Nematodes are not visible to the eye, but can easily be spread by the pine sawyer beetle to entire windbreaks or plantings in a few years. To determine if dead or dying trees are infected with the pine wilt nematode, cut a piece of wood from the lower trunk or the base of the lower limbs. A disk of wood, 1 inch in thickness and 3-4 inches across, makes an adequate sample. Place the sample in an air-tight, plastic bag and take it to your local Extension Office. They will send it to a diagnostic lab for positive identification. (There is usually a small charge for each sample submitted to the diagnostic lab.)
Control
Sanitation can prevent or slow the spread of pine wilt. Cut down infected trees and burn, bury or chip them. The stump should be removed down to the ground, if possible. This should be done as soon as the infection is discovered to prevent pine sawyer beetles from emerging from the tree the following spring and carrying the disease to other healthy trees. Do not hold the wood for firewood. Dead trees must be removed and destroyed by May 1.
A new protective treatment for pine wilt became available in 2006. Greyhound (abamectin) an insecticide/ nematicide product can be trunk-injected into uninfected trees, and when injected into healthy trees will give about 95% protection from the disease for about three years. Because of the large amount of product needed, the treatment is expensive and will probably be worth considering only for high value trees. As an example, a tree with a trunk diameter of 10 inches would cost the owner about $200 to treat. Contact a local arborist for treatment prices.
For more information, view:
Pine Wilt: A Fatal Disease of Exotic Pines In The Midwest