Armillaria Root Disease
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Armillaria root disease is found
throughout temperate and tropical regions of the world. In the continental
United States, the disease has been reported in nearly every State. Hosts
include hundreds of species of trees, shrubs, vines, and forbs growing in
forests, along roadsides, and in cultivated areas.
The disease is caused by fungi, which
live as parasites on living host tissue or as saprophytes on dead woody
material. The fungus most often identified as causing the disease is
Armillaria mellea (Vahl: Fr.) Kummer. Recent research, however, indicates
that several different but closely related species are involved. Therefore, the
generic term Armillaria is used to refer to this group.
These fungi are natural components of
forests, where they live on the coarse roots and lower stems of conifers and
As parasites, the fungi cause
mortality, wood decay, and growth reduction. They infect and kill trees that
have been already weakened by competition, other pests, or climatic factors.
This type of activity occurs throughout the United States--especially in
deciduous forests of the East. The fungi also infect healthy trees, either
killing them outright or predisposing them to attacks by other fungi or insects.
Such behavior typically occurs in the relatively dry, inland coniferous forests
of the Western United States.
Armillaria and the disease it causes have
several common names. Shoestring root rot refers to the rootlike fungal
structures, called rhizomorphs, that spread the fungi. The names honey mushroom,
honey agaric, mushroom root rot, or toadstool disease refer to the mushrooms
produced. Conifers often respond to infection by producing a copious flow of
resin, hence, the names resin glut or resin flow. When oaks are the common host,
Armillaria is often called the oak fungus.
Detection and Diagnosis
Characteristics of Infected Trees. Because
these fungi commonly inhabit roots, their detection is difficult unless
characteristic mushrooms are produced around the base of the tree or symptoms
become obvious in the crown or on the lower stem.
Crown symptoms on conifers and
broad-leaved trees vary somewhat. Generally, however, the foliage thins and
discolors, turning yellow, then brown; branches die back; and shoot and foliar
growth are reduced.
On large, lightly infected or vigorous
trees, crown symptoms develop over a number of years (fig. 1), until the trees
die. Conifers, particularly Douglas-fir and western larch, frequently produce a
larger-than-normal crop of cones, known as stress cones, shortly before they
|Figure 1. - The
Douglas-fir on the right with sparse foliage and poor shoot growth has been
infected for a number of years.
||Figure 2. - This
subalpine fir was killed rapidly and retained its full complement of
|Figure 3. - Crown symptoms
alone, such as those on this ponderosa pine, are not sufficient to identify
the presence ofArmillaria
On small, extensively infected or
low-vigor trees, crown symptoms develop rapidly: the foliage quickly discolors,
and the tree often dies within a year (fig. 2). On such trees, premature foliage
loss and reduced shoot and foliar growth may not be apparent.
Trees affected by prolonged drought or
attacked by rodents, bark beetles, or other fungi, particularly other root
pathogens, can produce crown symptoms similar to those caused by Armillaria.
Thus, additional evidence, often found on the roots and on the lower stem, is
needed to diagnose the disease (figs. 3 and 4).
conifers, the infected portions of the lower stems are somewhat enlarged and
exude large amounts of resin (fig. 5). Infected portions of the roots frequently
become heavily encrusted with resin, soil, and sometimes fungal tissue.
||Figure 4. - Removing the bark
at the base of the symptomatic ponderosa pine in figure 3 reveals the
characteristic mycelial fans.
||Figure 5. - Resin on a
Douglas-fir. The first resin produced in response to the fungi turns dark
brown but later is often covered with resin that retains a white sheen.
In contrast, infected portions of
broad-leaved trees sometimes develop sunken cankers covered with loose bark or
bark infiltrated with gum and other exudates. But most often these cankers are
inconspicuous or absent.
If Armillaria is present,
removing the bark covering infections will expose the characteristic, white
mycelial mats or the rhizomorphs that grow between the wood and the bark.
The white mycelial mats are marked by
irregular, fanlike striations; hence, they are often referred to as mycelial
"fans." The thick mats decompose, leaving impressions on the resin-impregnated
|Figure 6. - Shoestringlike
rhizomorphs be-tween the bark and the wood of a grand fir.
Rhizomorphs growing beneath the bark
are flat, black to reddish brown, and up to 0.20 inch (5 mm) wide (fig. 6). They
have a compact outer layer of dark mycelium and an inner core of white mycelium.
Rhizomorphs also grow through the soil. Except for being cylindrical and about
half as wide, subterranean rhizomorphs are similar to those produced beneath the
Mushrooms, the reproductive stage of
these fungi, confirm the presence of Armillaria. The short-lived mushrooms may
be found growing in clusters around the bases of infected trees or stumps (figs.
7 and 8). They are produced sporadically in late summer or autumn, and are most
abundant during moist periods.
|Figure 7 - A
cluster of mushrooms at the base of a western white pine. The spores seem to
be of limited importance in spreading the fungi.
||Figure 8 - A
cluster of mushrooms on the root of a red oak.
The mushrooms of the different species
vary somewhat but generally have yellow or brown stalks about 2 inches (5 cm)
long, and a ring is sometimes found around the stalk just below the gills. The
stalks have honey-yellow caps, 2 to 5 inches (5 to 12.5 cm) across. The upper
side of the cap may be slightly sticky and dotted with dark brown scales;
underneath, the cap has light-colored gills, which produce millions of light
yellow to white spores.
Armillaria causes a white
rot of infected wood. When wood first begins to decay, it looks faintly water
soaked; then it turns light brown. In the advanced stages of decay, wood becomes
light yellow or white (fig. 9) and may be marked by numerous black lines.
Advanced decay is spongy in hardwoods but often stringy in conifers.
||Figure 9 -
Wood of Douglas-fir in advanced stages of
light yellow and stringy.
In live trees, stem decay, referred to
as butt rot, is confined largely to the inner woody tissues. Butt rot seldom
extends more than a few feet above the ground.
Patterns of Infection.
Trees of different species and sizes may be killed individually throughout
stands. This pattern often occurs in managed stands reforested with species
unsuited to the site but may also occur in unmanaged stands.
Armillaria also kills
trees-primarily conifers-in a pattern of progressively expanding disease centers
(fig. 10). These centers develop in managed or unmanaged stands and vary from
small areas affecting several trees to areas of up to 1 ,000 acres (400 ha).
Within disease centers and on their expanding margins, trees in varying stages
of decline are normally present. One or all species and sizes of conifers may be
||Figure 10 - Three disease
centers in a virgin, mixed conifer forest in western Montana. The lowermost
center covers nearly 20 acres (8 ha).
Infection and Spread
Armillaria may live for decades in
coarse woody material. From this food source, the fungi spread to liv-ing hosts.
Spread occurs when rhizomorphs, growing through the soil, contact uninfected
roots or when uninfected roots contact infected ones.
Rhizomorphs can grow for distances of
up to 10 feet (3 m) through the upper soil layers, and they penetrate the roots
by a combination of mechanical pressure and enzyme action. The rhizomorphs'
growth and ability to penetrate roots depend upon the specific fungus, the type
and amount of the food source, the soil environment, and the host species.
When uninfected roots contact infected
ones, the fungal mycelium invades uninfected roots without forming rhizomorphs.
Such spread is common in dense stands where root contact is frequent.
Vigorously growing trees often confine
the fungi to localized lesions and limit their spread up the roots by secreting
resin and rapidly forming callus tissues. But when infected trees are in a
weakened condition, Armillaria spreads rapidly through the roots. If the
growth of the tree improves, fungal growth is checked. Such interaction occurs
throughout the life of an infected host until (1) it outgrows the fungi or (2)
the fungi reach the root collar, girdle the stem, and kill the tree.
When infected live trees are cut,
Armillaria rapidly spreads into the uncolonized parts of roots and stump. As
a result, the food source increases and may be responsible for initiating new
Outright mortality is the most frequently observed result of infection;
it can be a problem in timber stands, recreation areas, or orchards. On the
other hand, mortality can improve resource values - particularly in dense, young
coniferous forests of the Western United States.
Infection also results in growth
reduction and wood decay. Growth reduction often goes undetected or is ascribed
to other agents and thus is probably underestimated. Likewise, decay extends
only a few feet into the lower stem and will often go unnoticed until the tree
fails or is cut. Tree failures are significant hazards in recreation and urban
Because these fungi are indigenous to many areas and live on a wide
variety of plants and woody material, their eradication or complete exclusion is
not feasible; management should be directed toward limiting disease buildup or
reducing its impact.
Where individual trees are of high
value, chemical fumigants, including chloropicrin, methyl bromide, and carbon
disulfide, can reduce the infection level. These fumigants are applied in and
around the base of infected stems or in holes left after trees have been
Cultural management shows promise for
dealing with Armillaria in commercial forests. Management considerations
include (1) reforesting stands with a mixture of species ecologically suited to
the site and not obviously infected by Armillaria; (2) maintaining
vigorous tree growth without causing undue damage to soils; (3) minimizing
stress to and wounding of crop trees; and (4) reducing the food source by
uprooting infected or susceptible root systems and stumps.
Where infection is limited,
integrating the first three considerations into management prescriptions may be
Where infection levels are high, such
as in root disease centers, all four considerations may be used. Stumps and
roots should be removed in a zone extending at least 33 feet (10 m) beyond the
visible margin of the disease center because root systems in this area are
pests or stand conditions may be more significant than Armillaria. A
thorough evaluation of existing or potential pest activity, site and stand
characteristics, and the feasibility of various options should always be made
before selecting a management alternative.
Assistance in recognizing and dealing with Armillaria is
available from Extension offices; municipal or State forestry offices; or Forest
Pest Management and Research staffs, U.S Department of Agriculture, Forest
R.E. Williams,1 C.G. Shaw,
III,2 P.M. Wargo,3and W.H. Sites4
1Plant Pathologist, U.S.
Department of Agriculture, Forest Service, Intermountain Region, State and
Private Forestry, Boise, ID.
2Research plant pathologist, U.S. Department of Agriculture, Forest
Service, Pacific Northwest Forest and Range Experiment Station, Juneau, AK.
3Research plant pathologist, U.S. Department of Agriculture, Forest
Service, Northeastern Forest and Range Experiment Station, Hamden, CT.
4Plant pathologist, U.S. Department of Agriculture, Forest Service,
Southern Region, State and Private Forestry, Asheville, NC.