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.
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.
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.