Oxidant air pollution injury to foliage of ponderosa and Jeffrey pines increased the susceptibility of roots to infection and colonization by Fomes annosus under field and fumigation chamber environments. Roots of ponderosa pine trees severely injured by oxidant air pollution became infected proximal to the inoculation point significantly more often than did roots of healthy trees. Proximal colonization rate of F. annosus in inoculated ponderosa pine also was much greater in trees severely affected by air pollution. Exposure of container-grown ponderosa and Jeffrey pine seedlings to ozone in fumigation chambers increased infection and colonization by the pathogen. (Copyright (c) 1980 The American Phytopathological Society.)