In general, the existence of roads seems to have little effect on forest tree diseases, but there are some examples where building or using roads has caused significant local effects. These problems, where they exist, appear to be specific to the pathogen, host, and site. Nearly always, the negative effects can be ameliorated through simple modifications in how roads are built and used.
Building and maintaining roads may exacerbate root diseases. Wounded trees and conifer stumps created and not removed during road building provide infection courts for annosus root disease, which may then spread through root contacts to kill a patch of trees (Otrosina and Scharpf 1989). Trees can be damaged or stressed by road building through wounding of stems and roots, covering of roots with side castings, or compacting of soil over roots. These trees become susceptible to various tree diseases. Armillaria root disease is an example. In deciduous stands only injured trees are attacked but in coniferous stands an injury can initiate a pocket of disease involving several to many trees (Shaw and Kile 1991). Oak decline is associated with poor sites, older stands, and road building or other disturbance (Wargo and others 1983). Black stain root disease (Leptographium wagneri) attacks conifers stressed by disturbances, especially compaction caused by road building; in pinyon pine (Pinus monophylla), it is associated with roads and campsites (Hansen 1978, Hansen and others 1988, Hessburg and others 1995). Droopy aspen disease is associated with road building and compaction (Jacobi and others 1990, Livingston and others 1979). Sap streak disease in sugar maple is associated with compaction from roads and from direct injury to trees (Houston 1993).
Roads indirectly contribute to disease spread by giving people ways to transport diseased material long distances. New pockets of both oak wilt and beech bark disease (Houston and OBrien 1983) may have resulted from moving firewood from the forest to a homesite (Appel and others 1995, Rexrode and Brown 1983). Pitch canker (Fusarium subglutinans) was recently reported on Monterey pine (Pinus radiata) in California; previously, it had known to cause littleleaf disease on slash pines in the South. A single introduction is thought to have been responsible for the disease occurence in California (Correll and others 1992, Storer and others 1995). Campers who use roads to get to remote sites in Colorado and other states have caused significant mortality by carving on aspen and birch trees. The wounds provide pathways for various fungi that cause cankers and quickly kill the trees. Many trees are unintentionally damaged, for example, when campers hang a gas lantern on a branch too close to the trunk of a tree, thereby causing heat damage.
Road building also may set the stage for an insect attack that further stresses the trees, creating conditions for a disease outbreak (Boyce 1961).
One benefit of roads is to provide access for silvicultural activities that limit the damage from tree diseases (Bull and others 1997). Road building can be planned to help reduce the spread of some forest tree diseases: mistletoe is spread by the forcible ejection of the mistletoe seeds. In young plantations or pole-sized stands, roads can subdivide an area to prevent mistletoe seeds from reaching a healthy stand (Hawksworth and Wiens 1996). In Texas, roads could be planned to separate a portion of a stand with oak wilt from healthy trees. The act of building the road (if extensive enough) severs root connections and prevents tree-to-tree movement of the pathogen (Appel and others 1995, Rexrode and Brown 1983). In other areas, new or established roads may have the unintended effect of breaking the continuity of host roots and thus halting the spread of laminated root rot (Phellinus weirii) and other root diseases (Hadfield 1986, Thies and Sturrock 1995).