Pine wilt is a serious disease of pines, especially Scots pine, in the Midwest, including Iowa. Infected trees wilt and die within a matter of weeks to a few months. Although a nematode is the primary disease agent that blocks water flow in affected trees, pine sawyer beetles are the vectors that carry the nematodes from infected to helathy trees. The major recommendation to help control spread of the disease is to remove infected pines promptly and burn, chip, or bury them.
A question that often arises is: can chips from infected pines be used as organic mulch without risk of transmitting pine wilt to helathy pines? A recent research article in the Journal of Nematology demonstrated that the nematode which causes pine wilt can survive for brief periods in wood chips, and can even enter healthy trees if fresh chips are in direct contact with wounded roots or stems. A scenario in which this could possibly occur might be when fresh chips from an infected tree are incorporated into the soil around a newly transplanted Scots pine, because digging and transplanting typically creates root and even stem wounds. The risk of transmission from infected chips is probably very small even under this scenario. In view of the new findings, however, it may be advisable to stockpile chips from infected trees for a few months before using them as mulch, especially around recently transplanted Scots pine. In addition, always place chips on top of the soil, never mix them into the soil. Finally, avoid piling chips around the trunk of any tree; instead separate the mulch zone from the base of the trunk by at least 6 inches.
Once again, the risk of transmission of pine wilt by infested wood chips is probably very slight. Nevertheless, by stockpiling fresh chips before use and not incorporating them in the soil around trees, you can probably cut the risk factor pretty close to zero.
This article originally appeared in the June 9, 1993 issue, pp. 1993 issue, pp. 88-89.