Use Common Sense to Prevent Spread of Tree Diseases from Firewood

When laying in a supply of firewood for the winter, the responsible citizen should consider the possibility that, in some cases, the firewood pile may pose a risk to the health of nearby trees in forests and landscapes. Recently cut logs with bark still tightly attached may harbor insects that can overwinter in the pile and emerge the following spring to infest live trees, perhaps spreading diseases in the process. In Missouri, the diseases with the highest potential for spreading to live trees from firewood through insect activity are oak wilt, Dutch elm disease (DED) and pine wilt.

Oak wilt and DED are fungal diseases that invade the vascular systems of oak and elm trees, respectively with devastating effects. The causal agents for the two diseases are closely related and affect their host plants in very similar ways. They are spread from diseased to healthy trees either by root grafting or by the activity of beetles. Once infected, a susceptible tree species (such as red or pin oak and American and Siberian elm) often dies within one or two seasons as the fungus spreads throughout the tree. In the case of oak wilt, the disease organism creates a fungal mat under the bark of a recently killed tree. Sap-feeding nitidulid beetles live and feed in these mats, then fly to fresh wounds on living oak trees and carry the fungus to healthy trees. In the case of DED, bark beetles feed under the bark of dead and dying elm wood and the DED fungus produces spores in the beetle galleries, then beetles emerge with fungus spores on their bodies and feed in twig crotches of healthy trees, introducing the fungus to healthy trees.

In the case of pine wilt disease, the causal agent is a nematode, which proliferates in the branches and trunks of pine trees (particularly Scotch pine) and kills the trees by clogging resin ducts. The nematode is spread from dead or dying trees through the activity of a wood boring beetle. Adult beetles emerge from the trees in spring, carrying nematodes in their breathing tubes, which infest new trees through feeding wounds on the branch tips.

While oak, elm and Scotch pine firewood should be considered a potential source of insects that can spread diseases, a few common sense measures can reduce the risk associated with firewood storage. For oak or Scotch pine, if one knows the source of the wood, it may be possible to confirm that the source trees were free of visible symptoms. If so, there is little likelihood that the wood will be infected with oak wilt or infested with pine wood nematodes. Also, seasoned firewood, with loose bark and obvious cracks on the log ends will be too dry to harbor viable fungal mats or insects.

If, however, the wood still has tight bark, and it is not possible to confirm that the source trees were symptom free, some precautions are in order. Also, the bark beetles that spread DED can infest non-DED-infected trees and the fungus can sporulate in beetle galleries under the bark of dead elm logs. Thus elm logs in general constitute some risk for the spread of DED. Keep firewood as dry as possible and, whenever possible, burn it all before April 1. If the wood comes from trees known to be infected with one of the diseases discussed above or you are storing elm logs, consider covering the wood pile with a tarp or heavy plastic sheeting. Cover the edges of the plastic with soil to make it impossible for insects to escape in spring. Leave the pile covered until late July and then open it up to expose it to hot, dry conditions in August. Do not treat firewood with insecticides. This has not been shown to be effective in reducing insect populations and may cause toxic vapors when logs are burned in the fireplace.

Another important pest that can be spread from firewood is the emerald ash borer (EAB). This exotic insect has killed millions of ash trees in Michigan. While it has not yet been found in Missouri, it could easily be introduced here from firewood brought in from an EAB-infested area. For more information about this pest, visit the Michigan Department of Agriculture Web site: http://michigan.gov/mda.

The USDA Forest Service has some good "How-To’s" related to the organisms discussed in this article. They can be found on the Web at:
http://www.na.fs.fed.us/spfo/pubs/howtos/ht_ded/ht_ded.htm
http://www.na.fs.fed.us/spfo/pubs/howtos/ht_oakwilt/toc.htm
http://www.na.fs.fed.us/spfo/pubs/howtos/ht_pinewilt/pinewilt.htm
Thanks to Linda Haugen, Plant Pathologist, USDA Forest Service, for technical assistance.

Chris Starbuck, Wood Ornamental Horticulture, UMC (573) 882-9630