ROOT ROT, SHOESTRING ROOT ROT, HONEY MUSHROOM
all coniferous species.
Highly susceptible: True firs, Douglas-fir, western hemlock
Moderately susceptible: spruces, western redcedar, lodgepole
pine, western white pine
Least susceptible: ponderosa pine (except in south central
Washington), western larch
- Resin flow
from tree base
- Crown thinning
or changing color to yellow or red
crop of cones
- White mycelial
fan under bark
- Black rhizomorphs
penetrating root surfaces
mushrooms near base of tree in fall
trees often in groups or patches on the east side of the Cascades;
usually killed singly on the west side.
Stand: The crowns of affected trees will begin to thin and change
color, turning red, brown or yellowish. Resin blisters may be found
on the stems and branches of young Douglas-firs.
Dead tissue may be found on the outside of the roots as decay progresses
inwards; root centers often remain sound. Decayed wood may be stained
grey to brown in the early stages, often
with a water-soaked appearance. Later, the wood becomes soft, yellowish,
spongy and stringy, marked by frequent black zone lines. Occasionally
there will be heavy resin flow from the tree
base. A distress crop of cones may be produced in trees of conebearing
age. Large trees may be easily windthrown, with roots of down trees
often broken crosswise near the base of the tree.
On the east
side of the Cascades, Armillaria will commonly develop root rot
centers, where the disease has spread out over time from an initial
infection. In the center will be either an old stump or an empty
area, or occasionally new regeneration. Trees within several yards
to hundreds of feet (depending on the size and age of the infection)
will be dead or dying, trees further out will show early crown symptoms,
and trees beyond those will not yet show symptoms though they may
be infected. These centers may be many acres in size. On the west
side of the Cascades, trees are more commonly killed singly, and
are mostly trees younger than 20 years of age that are under stress.
In autumn, light brown to honey-colored gill fungi (mushrooms) with
stalks 3 to 10 inches in length and caps 2 to 5 inches across may
be found in clusters at the base of dead or dying trees and stumps.
It is easier to diagnose this disease by stripping away a portion
of bark at the root crown. A creamy white "fan" of leathery
mycelia (fungal fibers) develops between the bark and wood of the
infected host, and may be found beneath the bark of infected roots,
root crowns and lower stems.
black shoestring-like structures, also provide another reliable
method of diagnosis. These may be found between the bark and the
wood, on bark surfaces below the soil line, and in the litter and
soil around the roots and root crown. They may be distinguished
from rootlets in that they adhere to and penetrate roots as well
as branching in a different manner.
May be confused
with: Mottled rot (Pholiota limonella), which has similar mushrooms,
but only if mycelial fans not present.
cycle: Armillaria is the most common root rot in the Pacific
Northwest. Armillaria is highly pathogenic, able to kill apparently
healthy vigorous trees. Airborne spores don't seem to be very important
in disease spread. Most commonly, trees are infected by rhizomorphs
or by root contact with a diseased individual. Rhizomorphs grow
out through the soil from an established infection and penetrate
directly into the root surface of uninfected trees to spread the
disease. When roots from a healthy tree touch an infected root or
stump a new infection results.
young trees are quickly killed, while older trees may be able to
block the progress of the fungus to the root collar and thus survive
for many years, albeit in a weakened state (predisposed to windthrow,
other diseases and bark beetle attack). Over time, rotted stumps
accumulate in an area; these may retain viable inoculum for decades
which may infect regeneration.
agents: Stress generally predisposes trees to attack by assorted
agents; a tree that is not receiving enough water, light, or soil
nutrients, or one that has been exposed to temperature extremes,
pollution, insect attack, disturbances from partial cutting, or
other fungal diseases may have a reduced resistance to Armillaria
attack. Armillaria occurs more frequently in dry areas, on less
productive sites, and on sites disturbed by human activities (including
fire suppression). Armillaria fre quently occurs with other root
diseases such as laminated root rot and Annosus root disease. Armillaria
itself is a predisposing factor for bark beetle attack. Young trees
seem to be more susceptible than older trees of the same species.
Eastern Washington, with its dry forests, is among the areas where
Armillaria occurs most frequently and severely. The problem has
probably increased substantially in this century due to incorrect
forest management practices such as selective logging of the ponderosa
pine/western larch overstory and fire suppression, both of which
favor regeneration of Douglas-fir and true fir which are more susceptible
to Armillaria on most sites. In south central Washington, ponderosa
pine is considered to be most susceptible and Douglas-fir appears
tolerant, but in other areas Douglas-fir and true fir are the most
susceptible. In Douglas-fir/true fir forests mortality begins shortly
after regeneration and continues through the life of the stand,
but in other species such as western redcedar, mountain hemlock,
western larch, western white pine, ponderosa pine, and lodgepole
pine, damage tends to diminish with stand age beyond 20-30 years.
probably more true on the west side rather than the east side of
Armillaria often occurs with other root diseases and is a predisposing
factor for bark beetle attack. For that reason, trees killed by
bark beetles should be examined for signs of Armillaria or other
root diseases, as that may affect choices about which species to
replant and silvicultural methods to use. Armillaria can remain
viable in stumps for 50 years. Chemical treatments have not been
shown to be cost-effective. Nor is it possible to eradicate the
fungus entirely. The most frequent and effective approach to managing
root disease problems is to attempt to control them at final harvest
by replanting site-suited tree species that are disease tolerant.
In eastern Washington that typically means replacing Douglas-fir
or true fir stands with ponderosa pine, western larch, western white
pine, lodgepole pine, western redcedar, or spruce. Species susceptibility
varies somewhat from location to location, so check what seems to
be least affected on the site.
Prior to harvest,
root disease centers should be marked by examining outlying trees
for symptoms (e.g. mycelial fans, rhizomorphs). Infected trees should
be marked low on the bole so the mark remains after harvest. 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. All trees in
the disease center as well as uninfected trees within 50 feet should
be cut. During the harvest, trees outside the designated boundary
should be checked for symptoms and the boundaries adjusted accordingly.
No tree from a highly susceptible species should be planted within
100 feet of a disease center.
expensive alternative to changing species is to remove diseased
stumps and trees from the site by pushing them out with a bulldozer.
It is not necessary to burn the stumps as air drying will kill the
fungus, and any small roots left underground will decay before they
can reinfect the new seedlings. After stump removal, any species
may be planted.
the most important control measure is to manage for reduced tree
stress. Use silvicultural practices to regulate species composition,
maintain biological diversity, reduce chances for insect pest buildup
and increase host vigor. Thinning early in the rotation to promote
species diversity is encouraged. Mixed-species forests are more
resistant to insect defoliation, and also slow the spread of species-specific
pests such as dwarf mistletoe, which are both predisposing agents
should be checked for root disease symptoms at 15-20 years of age.
Stands with four or more disease centers per acre are severely diseased
and should not be precommercially thinned; instead, they should
be either left untreated until rotation (at a younger age than normal),
replanted with a more resistant species, or destroyed and replanted
with Douglas-fir or other desirable species after stump removal.
Stands with fewer than four disease centers per acre may be thinned
by cutting infected trees as well as susceptible trees within 60
feet. However, any precommercial or commercial thinning program
that removes existing ponderosa pine while leaving Douglas-fir and/or
true fir is not recommended.
of root-disease infested grand fir stands makes the problem worse
by creating a fresh supply of stumps for colonization; for this
reason, uneven-age management is not recommended (also because it
intrinsically favors Douglas-fir and true fir stands). Salvage operations
of grand fir and Douglas-fir in eastern Washington have apparently
increased the amount of root disease losses. One major salvage every
ten years seems to cause fewer problems than salvages every two
to three years.
Armillaria Root Disease, USDA Forest Service Agricultural Handbook