root rot or shoestring root rot is caused by Armillaria (Armillariella)
mellea, a common and damaging soilborne fungus worldwide. Armillaria
is used loosely to refer to a group of about 20 genetically distinct
fungal species that can be distinguished most readily using serological
techniques. Common names for this group include oak fungus, shoestring
root rot, honey mushroom, and honey agaric. The latter two refer
to the color of the mushroom fruiting structure of the fungus that
can sometimes be seen at the base of infected trees.
root rot is widespread in the relatively heavy soils of the
cooler parts of the temperate zones in the United States and
Canada, particularly in the Pacific Northwest. The fungi attack
about 700 species of mostly woody plants. Herbaceous plants
that are susceptible include blackberry, flowering bulbs,
potato, raspberry, and strawberry.
on image for larger version
Figure 1. Armillaria mellea Mushrooms from Base
of Infected Tree (photo courtesy of E. Dutkey)
shade and ornamental trees, oaks and maples are the ones most commonly
infected. Other woody plant hosts include azaleas, beeches, birches,
black locust, boxwoods, cedars, currants, dogwoods, Douglas-fir,
elms, firs, golden rain tree, hemlocks, hickories, hophornbeam,
Katsura tree, larches, lilacs, mountainashes, pines, planetrees,
poplars, privets, rhododendrons, roses, sassafras, spruces, sycamores,
tree of heaven, tuliptree, willows, yews, and many fruit and nut
trees. The Armillaria fungi may infect many other kinds of woody
plants if conditions are favorable for infection. Table 1 lists
ornamental, fruit, and nut trees and shrubs that are adapted to
Illinois and their relative resistance or susceptibility to Armillaria
plants that have previously been weakened by drought, flooding,
poor drainage, frost, repeated defoliation by insects or diseases,
other poor soil conditions, excessive shade, polluted air or other
chemical injury, or mechanical injury are most susceptible to attack.
The loss of fine feeder roots from this disease deprives affected
plants of sufficient nutrients and water, and often results in branch
dieback and staghead. The fungi can be of considerable importance
in the final death of weakened trees and shrubs. Serious radial
and terminal growth reduction of affected plants may occur. The
fungi are also responsible for butt rot in some species of trees.
In fact, Armillaria mellea and other species have been identified
as having a significant secondary role in disease complexes such
as oak decline, maple blight, and ash dieback.
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Figure 2. Armillaria mellea. Bark Peeled Back
is commonly found in most forest soils, so the disease may
occur in forested areas or areas that were previously forested.
Diseased trees may be found scattered throughout a forest
stand; or infection centers composed of one or several declining
trees may be scattered in the stand.
The aboveground symptoms cannot be differentiated easily from those
produced by other root or trunk injury. The most noticeable external
symptoms are premature autumn coloration and leaf drop, stunting
of growth, yellowing or browning of the foliage, a general decline
in the vigor of the plant, and twig, branch, and main stem dieback.
Such a decline usually occurs over several years but may appear
to progress very quickly as the tree shows advanced symptoms of
decline and death. As decline progresses, decay of the buttress
roots and the lower trunk is evident. Small plants die quickly after
the first symptoms appear with large trees surviving for a number
of years. Often a heavy crop of fruit (berries, cones) precedes
death. In addition, a severely infected tree exudes resin, gum,
or a fermenting watery liquid from the lower trunk.
signs are found at the trunk base or in the main roots near the
root collar. White or creamy white, paper-thick, fan-shaped sheets
of Armillaria mycelium can be seen growing over the water-soaked
sapwood when exposed. The Armillaria fungi have a strong mushroom
odor. By the time a tree or shrub wilts and dies, the trunk is usually
encircled by the fungus. With time, diseased wood becomes light
yellow to white, soft and spongy, often stringy in conifers and
marked on the surfaces by black zone lines. Decay in the butt and
major roots of birches, firs, and other trees results in vertical
cracks in the root collar. The cracks arise as trees weakened by
internal decay are stressed by wind or the weight of snow or ice.
death of only a few branches can result from the killing of one
or several main lateral roots. After the plant dies, rhizomorphs
(slender, rootlike, dark brown to black "shoestrings"
with a white interior) develop beneath the bark (Figure 2 and 3).
The rhizomorphs are 1 to 3 millimeters in diameter, round or flattened
and branched, and they consist of hyphal strands bundled together
and enclosed within suberized cells. The cordlike rhizomorphs grow
over infected roots and outward from a dead tree into the soil approximately
20 inches per month. Not all strains or species of Armillaria form
rhizomorphs in nature. Small or large clusters of yellowish brown
"honey mushrooms" appear in late autumn after a rainy
period across and are often speckled dark brown. The lower surface
is light brown to white with radiating gills which are attached
to and run a little way down the stem. The mushrooms have a persistent
whitish collar or ring around the upper part of the stem. The mushrooms
develop near severely diseased roots and emerge through the soil,
near the base of a trunk.
Armillaria mellea and most other species survive as rhizomorphs
and vegetative mycelium on and in the dead and dying wood
of tree stumps and roots. Sometimes the fungi can be found
several feet above the soil line on the trunk of dead trees
several years after being killed by Armillaria.
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Figure 3. Rhizomorphs, or "Shoestrings",
Means by Which Armillaria Fungi Spread (IL Nat. History
the late autumn, mushrooms may arise from the rhizomorphs. Millions
of microscopic whitish spores (basidiospores) produced in the mushroom
caps are carried by the wind to dead stumps or injured bark at the
base of living plants. Under favorable conditions of moisture and
temperature, a few basidiospores germinate and produce a mycelium
that infects the bark and later the sapwood and cambial regions.
It is doubtful that the basidiospores play an important role in
the occurrence of this disease. White "fans" of the mycelium
develop on the sapwood, followed by the formation of rhizomorphs.
The rhizomorphs advance through the soil at the rate of one or more
meters (3 to 8 feet) per year in Illinois. The spread of the disease
is not so much a matter of the fungus growing toward the roots of
a healthy tree or shrub as it is of a healthy plant's roots growing
through the soil to wood already infected with Armillaria. Some
species of the fungus or perhaps strains within species, are virulent
parasites while others are opportunistic and act selectively on
small or weak individual plants. Armillaria also colonizes the declining
root systems of plants felled or killed by other agents.
occurs when Armillaria mycelium comes in contact with and adheres
to young roots of a susceptible plant by means of a gelatinous secretion.
The mycelium penetrates a root by the action of secreted enzymes
that partially digest the cell walls of the young root. The fungus
then grows into the root tissue between the cells. Once a plant
has been invaded, the Armillaria fungus continues to ramify through
the root and trunk tissues, even after the host plant has been dead
for several years. A large stump can support the growth of rhizomorphs
for decades. Trees killed by other diseases, such as Dutch elm disease,
annosus root rot (Heterobasidion annosum), or Phytophthora root
rot can be colonized by Armillaria and thus lead to severe local
outbreaks of the disease.
or shrub may die in one to several years after initial infection,
depending on the vitality of the plant and environmental conditions.
Armillaria can pass from tree to tree via root grafts. Roots of
trees under stress are most easily infected. Armillaria is generally
inhibited at soil temperatures above 79°F (26°C).
- Armillaria can be excluded if care is taken to insure that all
planting material brought into an area is disease free. Plant
only well adapted trees and shrubs in sites suited for vigorous
- Fruit trees, pine plantations, or ornamental trees and shrubs
should NOT be planted in recently cleared areas where Armillaria
has been a problem. These areas should be planted with nonsusceptible
crops such as corn, small grains, and grasses for a few years
to help eliminate the fungus. Another possibility: use the infested
area for lawn, vegetable garden, rockery, or for annual and biennial
- Clean cultivation of an orchard can help distribute Armillaria
infected wood to other areas not infested with the fungus. A groundcover
crop should be used to replace the procedure of clean cultivation.
- In orchards and other areas where Armillaria is established,
diseased trees and shrubs should be carefully dug up, including
the stump, all large roots, stakes, or other wood harboring the
fungus, and burned on the site instead of transported to a dump.
All pruning wastes should be burned rather than incorporated into
the soil to prevent the formation of new disease centers. Deprived
of their food supply, any rhizomorphs left in the soil will soon
die. Eradicating Armillaria from a site requires a thorough removal
of all diseased and dead wood.
- Plants found to be infected in only a few roots or a small
part of the root collar can be saved for a time by carefully removing
the soil to expose the root collar and buttress roots to aeration
and drying from mid-spring to late autumn. Infected bark and wood
on large roots, buttress or trunk should be excised back to healthy
tissue. Replace the soil with Armillaria free soil before the
first heavy frost.
- Maintain tree and shrub vigor by good cultural management practices:
(1) regular fertilization, based on a soil test; (2) thorough
watering during extended droughts; and (3) insect and disease
control. Where possible, provide for adequate soil drainage in
heavy, poorly drained sites. Avoid all root damage to established
woody plants in areas where construction is to occur. This is
particularly relevant to oak groves. Avoid soil fill and soil
removal around valuable trees and shrubs.
- If the precise source of infection is known and cannot be removed,
it should be possible in some cases to prevent the rhizomorphs
from reaching the trees and shrubs to be protected by sinking
a sheet of heavy polyethylene vertically into the soil between
diseased and healthy plant(s), provided it extends far enough
laterally (several feet beyond the outer dripline) and at least
a meter (3 feet) into the soil. A suitable deep ditch would have
the same effect.
- Fungicides applied to infected trees are not recommended.
1. Resistance or Susceptibility to Armillaria Root Rot of
Certain Woody Plants Adapted to Illinoisa
or Highly Resistant
or flowering maple
mahonia, Oregon grape
or sweet elder
maple, Oregon maple
pine, Scotch pine
or white fir
Johnswort (shrub form)
maple, Amur maple
abelia, white abelia
(except Japanese privet)
paulownia, Empress tree
A partial listing taken largely from Resistance or Susceptibility
of Certain Plants to Armillaria Root Rot, by Dr. Robert D. Raabe,
Division of Agricultural Sciences, University of California,
further information on diseases of trees and ornamentals contact
Nancy R. Pataky, Director of the Plant Disease Clinic and Extension
Specialist, Department of Crop Sciences, University of Illinois