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ANNOSUS
ROOT DISEASE
(Heterobasidion annosum, Fomes annosus)
Hosts:
All coniferous species.
Highly
susceptible: Grand fir, mountain hemlock, western hemlock
Moderately
susceptible: Subalpine fir, western white pine, ponderosa pine,
lodgepole pine
Less
susceptible: Douglas-fir, larch, spruce, western redcedar, hardwoods
Quick ID:
- Crown yellowing
and thinning
- Distress
crops of cones
- Trees killed
in disease centers
- Windthrown
trees
- Conks on
declining live trees, dead trees, and stumps
- Annual rings
separate with pits on one side of sheets
 Field
Identification
Tree &
Stand: Symptoms vary by species. In hemlock, crown symptoms
are uncommon and trees often look healthy until they are windthrown.
In other species such as pine, affected trees
show marked reduction in height and diameter growth. The crown thins
and yellows, and may frequently produce a distress crop of cones.
Resinous hosts, particularly pines, may have
pitch soaking of root wood, and crown symptoms are more apparent
than in nonresinous species such as true firs. Decay appears initially
as a reddish-brown stain with an irregular margin
in the interior of butts and roots. In the roots, decay has several
symptoms: the bark separates easily from the wood, the wood is streaked
with darker brown lines and the surface of the inner
bark has numerous small silver to white flecks. Later, the roots
may have small, shallow pockets of white fibers with black flecks.
In the trunk, wood will often separate into annual rings with small
oval pits on one side (as opposed to the pits of laminated root
rot, which are on both sides). Advanced decay of the trunk is stringy
with large white streaks, and black flecks are usually found in
decayed wood. Disease centers where the disease has spread out over
time from an initial infection are common. In the center, an old
stump or empty area may be found. Trees within several yards to
hundreds of feet (depending on the size and age of the infection)
will be dead or dying, and trees further out will show early crown
symptoms. Trees within 50 feet of the apparent edge of a disease
center are very probably infected as well. Trees in the disease
center may be windthrown, either living or dead.
Fungus:
The most obvious sign of Annosus root rot is the fruiting body (conk),
found inside old stumps or in the duff layer on the exterior of
roots and root collars of infected or dead trees. The conk may take
several forms. Small, button-like conks may be found on the wood
beneath the bark of pine stumps and on the surface of small pine
roots in the soil. On the root collar of infected trees, conks ranging
from 1/2 to 2" in size may be found under the litter layer.
Inside rotted stumps or on the underside of exposed roots of windthrown
trees, conks tend to be large (up to 10" wide) and irregularly
shaped. The conks, which are perennial, have a dark upper surface
and a
creamy white lower surface covered with tiny pores.
May be confused
with: Laminated root rot, Armillaria root rot.
Disease
cycle: Annosus root rot is considered third in severity in the
Pacific Northwest, after laminated root rot and Armillaria root
rot. Researchers have identified at least two host-specific forms
of the disease: an s-type that infects mostly spruce, firs, Douglas-fir,
western redcedar, and hemlock, and a p-type that occurs mainly on
pines but will infect the same species as the s-type, as well as
brush and hardwood species. Unlike laminated or Armillaria root
rot, Annosus spreads both by root contact and by spore infection.
When the roots of a healthy individual touch the roots of a diseased
individual, a new infection results. As well, windborne spores that
are deposited on freshly-cut wood surfaces such as stumps or basal
wounds germinate and infect the surface.
Annosus can live for 50 years in large stumps of resinous trees;
in smaller stumps and nonresinous species survival time is shorter.
Species susceptibility to attack varies. In some species such as
hemlock, the host is rarely killed because of its ability to compartmentalize
the decay in the butt log. In other species, notably pines and grand
fir, the disease causes tree death. Trees are often killed standing
rather than being windthrown.
Predisposing
agents: Certain management practices such as harvesting old
growth can create heavy inoculum levels. If the old stumps become
infected subsequent regeneration will alsobecome infected. This
creates a potential for future disease spread that is very serious,
as windborne spores could be carried for some distance. Stumps less
than 18" in diameter rarely become established infections.
Partial or selective harvesting increases the likelihood of infection
by creating new stumps and causing basal wounds from mechanical
injury. Annosus is itself a predisposing factor for bark beetle
(particularly fir engraver) attack, and is frequently found on
the same tree with Armillaria root rot or laminated root rot.
Impact:
Total losses in board feet are not known, but Annosus causes damage
by tree killing, butt rot, windthrow, and decrease in growth of
affected trees. Losses due to Annosus are known to be increasing,
probably as a result of intensified logging. Bark beetle epidemics
mask the incidence of root disease by being the obvious killer,
but many if not most trees killed by bark beetles are predisposed
by root diseases. Additionally, bark beetle epidemics become established
in diseased trees and spread out to healthy ones.
Management:
It is important to search carefully for signs of other root diseases
once Annosus has been diagnosed, as they often occur together. The
presence of Armillaria or laminated root rot will affect choice
of species for reforestation and silvicultural methods to follow.
Unlike the other two major root diseases, there is a chemical control
for Annosus. Borax powder spread 1/8" thick on a cut stump
within two days of harvest will prevent colonization by Annosus
spores. Stumps smaller than 18" in diameter need not be treated
with borax as they rarely become sources of inoculum. Pine stumps
should only be treated if they are within one mile of an infected
pine stand.
Control of
Annosus disease centers is best attempted at final harvest. A systematic
survey of the stand can be used to identify the borders of the disease
center; every infected tree should be marked low on the trunk so
the mark remains after logging. A buffer strip 50 feet wide beyond
the border of the disease center should be established. It is suggested
that disease centers should be mapped as well as marked to enable
them to be tracked over time. Global Positioning Systems (GPS) may
make mapping centers and tracking them over time easier, and that
technology is rapidly becoming affordable. Within the disease center
and the buffer strip, every tree should be cut and stumps treated
with borax unless they will be excavated. During the harvest, trees
outside
the designated boundary should be checked for symptoms and the boundaries
adjusted accordingly. Following harvest, stumps may be excavated
if it is economical and practical considering species desired, terrain,
and soil type. Otherwise, another, more resistant species may be
planted. This is where detecting the presence of other root diseases
is critical: Douglas-fir is not seriously damaged by Annosus, but
is highly susceptible to Armillaria and laminated root rot. Ponderosa
pine is moderately susceptible to Annosus, but in south central
Washington is
highly susceptible to Armillaria. The best bet is blister rust resistant
western white pine in stands with high mortality. If the previous
stand was true fir, ponderosa pine or lodgepole pine may be planted
as the strain of Annosus affecting true firs will not infect pines.
Short rotations of 40 to 120 years in stands of grand fir and white
fir will also minimize problems with Annosus, especially if the
number of intermediate stand entries that may cause wounds is reduced.
Thinning to increase stand vigor is recommended early in the rotation
before the potential for wounding becomes significant; small stumps
from thins do not need to be treated with borax. In mixed-species
stands, favor species oher than true fir. Salvage logging should
be minimized in true-fir stands as
it increases wounding.
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