Root disease fungi affect all tree species in the Southwestern Region, with Douglas-fir, true firs, spruce, and aspen being the most susceptible to damage. Susceptibility also varies by tree age, genetics, type of root disease pathogen present and site history.

Root disease or decay fungi spread from roots of diseased trees to those of healthy ones. They start in a tree or stump and spread slowly outward in all directions, resulting in a slowly enlarging group of dying and dead trees. The oldest kills are located at the center of infection with a fringe of recently killed and dying trees around the outer edge. In the Southwest, these “centers” of disease are small in size, typically less than 1 acre.

Root disease is referred to as “a disease of the site” because the fungi colonize dead and dying trees and remain in dead roots and soils for many years. Many of these fungi have the ability to act as both pathogens in live trees and saprophytes in dead wood material. Not only are the current trees affected, but also trees that become established in the future. Some root disease fungi have been shown to remain alive and active in infested sites for more than 50 years.

The aboveground symptoms of trees affected by root disease include chlorosis, reduced needle length, progressive thinning of foliage, fading crown, reduced tree growth, and death. These symptoms are similar to those caused by drought, high water table, and bark beetle attack. However, the decline of trees affected by root disease usually extends over a period of a few to several years and not all trees succumb at the same time. The other causes of decline kill trees more rapidly, generally in 1 to 2 years, and a group of trees all die at the same time. Bark beetles commonly attack trees weakened by root disease infection.

The incidence of root disease often relates to the amount of human activity in forest sites. Harvesting timber from infected stands often increases root disease activity. In addition, changes in forest structure brought about by fire suppression and selective logging have led to increases in tree species more prone to root disease.