Pine Wilt: Death and the Pine Tree

My pine is dying! What do I do?? This is a question throughout Iowa and the Midwest

Several things can kill a tree, notably, age, weather stress, (rarely) herbicide damage, and several plant diseases, but most often it is a combination of these that do the deed. For a few species of ornamental pine trees, though, there is a specific disease called pine wilt that can take out a large tree in a matter of weeks or months. The primary species affected is Scot’s pine; unfortunately it is one of the most commonly planted ornamental pine species in Iowa.

Pine wilt is caused by nematodes (microscopic round worms) that feed and reproduce in the resin channels of the tree (where water is carried). They block the plant's ability to conduct sap, and that leads to rapid death of tree. The nematode cannot move far on its own but a common wood-boring insect called the pine sawyer beetle spreads the nematodes by carrying them from infested trees to healthy trees. Pine sawyer beetle larvae feed under the bark of pine trees, and as the grubs pupate and the ¾ to 1-¼ inches-long adults begin to emerge in May, the nematodes move into the breathing tubes of the beetles.   Shortly thereafter the sawyer beetle tunnels to the bark surface and flies to other pines (up to several miles) where it feeds lightly on young twigs before females lay eggs. The nematodes then leave the beetle and move into the tree through those feeding scars. If the pine is a resistant species (like white pine) the nematodes die. But if the tree is a susceptible species, especially Scots pine, the nematodes begin to rapidly multiply. Environmentally stressed trees are damaged more, because the tree is already weakened and also the beetles are attracted to stressed trees. Most Iowa Scots pines are under some environmental stress.

Pine wilt typically kills Scots pine within a few weeks to a few months. Almost all cases of pine wilt occur in established trees, those more than 10 years old – not Scots pine in Christmas tree plantations or other young trees. The disease begins with needles turning gray-green but rapidly becoming reddish- tan to brown. Dead needles remain attached to the branches. For some trees, the disease strikes all at once to the entire tree, but in other cases, the disease begins on a branch or two, but rapidly spreads across the tree.   Sadly, there is no effective cure once the disease begins. Because of pine wilt, Scots pine is no longer recommended for use here. 

If you have a pine tree that suddenly flags with dying branches, follow this strategy:

1.  Identify the tree (if you haven’t already). Over 90 percent of the trees killed have been Scots pine, but a few other species can be infected.
2. (Especially if it is Scots pine), you have a choice to make. One option is to take samples from the dying branches or tree to submit to a laboratory for positive ID of the nematodes. Cut a branch section at least 3 to 4 inches in diameter and an inch thick, or collect wood chips (not bark) with a brace and bit.  Send the samples to the ISU Plant and Insect Diagnostic Clinic along with $20 fee to 327 Bessey Hall, ISU, Ames IA 50011.
3. If the tree is rapidly declining or already dead, removal is the best option.
4. There is no effective control of pine wilt other than sanitation to slow further spread. Removed trees should be cut at ground level and burned, chipped, or buried to remove the overwintering beetles. Beetles emerge in mid May, so timeliness is important.
5. Do NOT plant Scots pine either initially or as a replacement tree.

For additional information, see ISU publication "Pine Wilt: A Fatal Disease of Exotic Pines in the Midwest."