Forest Service

North Central

Research Station

Research Note NC-388

2004

ABSTRACT.—Armillaria species were isolated from

coniferous and deciduous overstory species in 17 of 22

stands in the Chequamegon area of the ChequamegonNicolet National Forest. Armillaria calvescens and A.

sinapina were identified once each, and the remainder of

the isolates were A. ostoyae. These findings are related to

reports of Armillaria species from other areas of North

America, particularly the western Great Lakes States, and

to the potential role of Armillaria root disease in the

Chequamegon.

KEY WORDS: Armillaria, root disease, CheqamegonNicolet.

KATHRYN W. KROMROY is a Research Plant Pathologist

with the North Central Research Station, St. Paul, MN.

Phone: (651) 649-5105; e-mail: kkromroy@fs.fed.us.

Identification of Armillaria Species

in the Chequamegon-Nicolet

National Forest

Kathryn W. Kromroy

The Chequamegon area of the Chequamegon-Nicolet

National Forest in northwestern Wisconsin covers

348,000 hectares, nearly 90 percent of which are

classified as timberland (Haugen et al. 1998). In addition

to wood and wood products, the lands are managed for

wildlife habitat and numerous forms of recreation.

Beginning in the late 1980s, USDA Forest Service

personnel observed tree mortality and decline in some

areas of the Chequamegon. Armillaria root disease,

important in several areas of North America (Bruhn et al.

2000, Lundquist 2000, Mallett and Maynard 1998,

McLaughlin 2001b, Morrison et al. 2001, Rizzo and

Slaughter 2001), was suspected of contributing to these

observed declines, but no mycological investigations had

been done in this area.

The objectives of this study were to identify the species

of Armillaria present in the Chequamegon, especially in

declining stands, and to relate these findings to

previously collected information about Armillaria,

particularly in the western Great Lakes States, but also in

other areas of North America. This is the first step in

assessing the role of these fungi in the Chequamegon

forest.

MATERIALS AND METHODS

Stands with dead and dying trees were selected first

because the presence of Armillaria species was strongly

suspected, and other stands were selected to represent

the major overstory species of the forest. The root

collars, butts, and one meter of lateral roots of recently

killed or declining trees were examined for signs

(mycelial fans or rhizomorphs) of Armillaria. The soil

was carefully removed from around the base and lateral

roots of these trees, and small areas of bark were lifted

or peeled back. In stands where dying or declining trees

were not observed, stumps and snags were examined.

Samples of roots, root collars, and butts harboring

Armillaria-like mycelial fans, rhizomorphs, or white

stringy rot were collected for isolation, and the soil

surrounding the sampled substrate was examined for

rhizomorphs, which were collected if present. Host

species, substrate type, and diameter at 1.3 m or at

ground level, for trees and stumps, respectively, along

with the location and form (fan, rhizomorph, decayed

wood) of Armillaria observed were recorded for each

sample. Overall condition of the tree, and the presence

of decay, other fungi, and insect or disease problems

were also noted. One or two samples for isolation were

collected per stand. Sampled materials were placed in

paper bags and stored in a cooler while traveling (1 to 2

days), then placed in cold storage (4 C ̊) for 1 to 4 days

until isolations were completed.
2

Isolations were made following procedures described by

Rizzo and Harrington (1993). Small (< 5 mm) pieces of

mycelial fans and/or wood were taken from where the

bark was removed and plated both on water agar (WA,

15 g Difco agar per L) and on basidiomycete-select agar

(BSMA, 15 g malt extract and 15 g agar per L, amended

with 4 ug/ml benomyl 50WP and 100 ug/ml

streptomycin sulfate after autoclaving (Harrington et al.

1992)). Rhizomorphs were surface disinfected by

soaking in an aqueous solution of 20 percent ethanol

and 10 percent bleach for 2 minutes, then rinsed in

sterile distilled water. These were cut into 1-cm

segments and plated. Plates were incubated under

laboratory conditions (20-25 C ̊). To obtain pure

cultures, transfers onto 1 percent malt extract agar

(MEA) were made 1 to 2 weeks after isolation, and, if

necessary, a second transfer was made 4 weeks later.

Diploid-haploid pairings, per Korhonen (1978), were

used to identify the unknown Armillaria isolates to

species. Each unknown presumably diploid isolate was

paired with 8-12 haploid testers (table 1), which were

obtained from David Rizzo (Rizzo 1993). One 3-mm

plug of the tester strain was placed on 1 percent MEA, 5

mm away from a plug of the unknown. Plates were

incubated under laboratory conditions for 5-7 weeks, at

which time all pairings were subcultured. As described

by Harrington et al. (1992), three 3-mm plugs were

taken from the tester (haploid) side of the pairing—one

from the margin of the tester advancing toward the

outside of the plate (away from the plug), one from the

middle of the tester growth area, and one from between

these two—and transferred to new plates of 1 percent

MEA. These were incubated for 10-14 days and then

visually evaluated. Rizzo and Harrington (1992)

Table 1.—Tester isolates used in haploid-diploid pairings for

identification of Armillaria species

Species Isolates

A. ostoyae A371

, A100

A. gallica A74, A102, A103

A. gemina A80, A781

A. sinapina A48, A521

A. calvescens A771, A71

A. mellea A721

1

Listed in: Harrington, T.C.; Rizzo, D.M. 1993.

demonstrated that nuclear migration occurs from the

diploid to the haploid in conspecific pairings and that,

in most cases, this method should be considered a

reliable means of identifying Armillaria to species.

Mycelial growth of single-spore haploid isolates (the

testers) is usually whitish and fluffy, with much aerial

mycelia. Growth of diploid isolates (unknowns) tends to

be flat, brown to dark brown, and becomes crustose in

many cases. If the growth of the subcultured plug from

the middle of the tester colony and at least one other

subculture differed in morphology from the original

tester strain, i.e., were flat and/or crustose rather than

fluffy, the unknown was considered to be the same

species as the tester strain.

RESULTS AND DISCUSSION

Twenty-two stands in the four ranger districts of the

Chequamegon (Hayward, Glidden, Washburn, and Park

Falls) were inspected for Armillaria between June and

August 1992 (fig. 1). Stands varied in age and some

were recently clearcut. While most stands were

predominantly conifer overstory, stands of pure and

mixed overstory were included. No fruiting bodies of

Armillaria were observed during this survey, because

their production occurs in the fall. However, Armillaria

fans and/or rhizomorphs were observed in 86 percent of

the stands and Armillaria species were isolated from

material collected in 90 percent (17 of 19) of those

stands (table 2, fig. 1). Although the distributions of

Armillaria species are probably irregular throughout the

Chequamegon, the genus is considered to be ubiquitous

in forested areas, and a more thorough examination of

potential substrates would be required to claim that

Armillaria species were “absent” from a stand. Armillaria

fans and/or rhizomorphs were observed on 78 percent of

37 samples collected and Armillaria species were

isolated from 76 percent (23 of 29) of those samples.

Whereas Armillaria species were isolated from both live

and dead conifers, the fungi were isolated only from

dead deciduous trees (table 3).

Of the 23 Armillaria isolates identified from the

Chequamegon, 21 were A. ostoyae, 1 was A. sinapina,

and 1 was A. calvescens. Since the late 1980s, each of

these species has been identified in the western Great

Lakes region, and root disease associated with A. ostoyae

has been reported on several conifer and deciduous

hosts (table 4). Species of Armillaria are not confirmed
3

Figure 1.—Stands in the Chequamegon-Nicolet National Forest that were examined for the presence and identification of

Armillaria species. Each stand is designated by a letter and number (e.g., H5 = Hayward Ranger District, stand number 5).

Washburn

Ranger

District

Four Districts in the ChequamegonNicolet National Forest

μ

!(

!( ^_

GF

!(

^_

!(

!(

!(

!( ")

!( !(

!(

!( !( ^_

!(

!(

^_

!(

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BAYFIELD

ASHLAND

IRON

SAWYER

PRICE

V

HWY C

FR 251

US HWY 2

FR 236

CTY HWY M

US HWY 63

ST HWY 77

FR 164

CTY 

HWY B

ST HWY 77 FR 163

ST HWY 70

FR 161

ST HWY 70

ST HWY 182

FR 149

Hwy C

US

HWY

63

Hayward

Ranger

District

Glidden

Ranger

District

Park Falls 

Ranger District

W1

W2

G3

G1

H5 G2

H2

H3 H4

H6

H7

G6 G5

G4A G4B

PF2

PF3

PF4 PF5A

PF5B

PF1

H1

G

!

"

^

Fungal species

F Armillaria calvescens

( Armillaria ostoyae

) Armillaria sinapina

No Armillaria found

Washburn

Ranger

District

↑N
4

Table 2.—Numbers of stands and samples examined for Armillaria species by ranger district in the Chequamegon area of the

Chequamegon-Nicolet National Forest

 Ranger district

Stands Hayward Glidden Park Falls Washburn Total

Total no. examined 7 7 6 2 22

No. with signs1 of Armillaria 5 6 6 2 19

No. with symptoms2 only 0 1 0 0 1

No. from which Armillaria was isolated 5 6 4 2 17

Samples

Total no. examined 12 10 13 2 37

No. with signs of Armillaria 9 8 10 2 29

No. with symptoms only 1 1 2 0 4

No. from which Armillaria was isolated 8 8 5 2 23

1Only assessed belowground, signs are mycelial fans, rhizomorphs.

2Only assessed belowground, the main symptom is white stringy rot of the wood.

Table 3.—Numbers and condition of trees from which Armillaria species were isolated by ranger district in the Chequamegon area of

the Chequamegon-Nicolet National Forest

 Ranger district

Tree type Condition Hayward Glidden Park Falls Washburn Total

Conifer Living 1 0 1 0 2

Dead 3 4 1 1 9

Stump, snag 4 1 2 0 7

Total 8 5 4 1 18

Deciduous Living 0 0 0 0 0

Dead 0 3 1 1 5

Stump, snag 0 0 0 0 0

Total 0 3 1 1 5

for the earliest publications from this region, either

because isolations were not made or because the work

occurred before several intersterile groups were

identified within the original A. mellea complex

(Anderson and Ullrich 1979, Korhonen 1978, Shaw and

Kile 1991, Volk and Burdsall 1995).

The finding of A. calvescens on Acer saccharum Marsh. is

consistent with other reports from the western Great Lakes

States, and from New York and Ontario (Banik et al. 1995,

Blodgett and Worrall 1992, McLaughlin 2001a). Although

most often causing a butt rot, A. calvescens has been

identified as a factor in a decline of sugar maple in New

York (Bauce and Allen 1992).

McLaughlin (2001a) reported A. saccharum as a host of A.

sinapina in Ontario, but the identification from the

Chequamegon may be the first report of A. sinapina on this

host from the western Great Lakes States. Banik et al.

(1995) identified A. sinapina on several conifer and

deciduous species in Wisconsin and Michigan, but not on

A. saccharum. In both Ontario and New York, A. sinapina

has been associated with cambial infection as well as butt

rot (Blodgett and Worrall 1992, McLaughlin 2001a). More

intensive sampling is required to suggest roles for either of

these two Armillaria species in the Chequamegon forest.

The predominance of A. ostoyae in the Chequamegon is

consistent with results of other collections of Armillaria

from sites in the western Great Lakes States that are, or

once were, dominated by conifers (Banik et al. 1995,

Rizzo et al. 1995, Smith et al. 1994). In the

Chequamegon, A. ostoyae was identified on five
5

Table 4.—Previously published accounts of Armillaria species in the western Great Lakes States

Reference Armillaria spp. Associated host or Vegetation on the site

 substrate at the time / past (if known) Location

Kromroy A. ostoyae Abies balsamea 8-yr-old conifer plantation / Rhinelander, WI

1999 mixed hardwoods

A. ostoyae Quercus rubra, Populus 30-40 yr-old Pinus resinosa Black River Falls, WI

deltoides plantation / Quercus spp.

A. ostoyae Corylus cornuta, Vaccinium P. resinosa plantations / Cloquet, MN

angustifolium, Populus spp., upland forest of P. resinosa,

Amelanchier sanguinea P. strobus, & P. banksiana,

Picea-Abies & Populus-Betula

A. sinapina Debris in the soil Same as above Cloquet, MN

A. gallica Rhizomorphs in the soil 20-60-yr-old P. resinosa

plantation / Quercus spp. Black River Falls, WI

Banik et al. A. Acer saccharum, Betula Not described Upper Peninsula (UP),

1995 calvescens papyrifera, Lonicera spp., MI; central MN;

Populus tremuloides, Quercus southwestern WI

rubra, Q. velutina, Tilia

americana, Ulmus spp.

A. gallica A. balsamea, P. resinosa, P. Not described except for UP MI; eastern MN; WI

strobus, Acer rubrum, A. some collected in P.

saccharinum, A. saccharum, tremuloides stands

B. papyrifera, Carya ovata,

Fraxinus pennsylvanica, P.

tremuloides, Prunus serotina,

Quercus alba, Q. ellipsoidalis,

Q. macrocarpa, Q. rubra, Q.

velutina, Ulmus spp., duff

A. mellea A. saccharum, Quercus spp. Not described UP MI; SE MN; westcentral & southern WI

A. ostoyae A. balsamea, Picea glauca, Not described except for UP, MI; central MN; NE

P. banksiana, P. resinosa, P. some collected in P. & central WI

strobus, Thuja occidentalis, tremuloides stands

Tsuga canadensis, A. rubrum,

A. saccharum, Betula

alleghaniensis, B. papyrifera,

Fraxinus nigra, P. tremuloides,

Q. rubra, Q. velutina, Ulmus

spp.

A. sinapina A. balsamea, T. occidentalis, Not described except for UP, MI; northern WI

T. canadensis, A. rubrum, B. some collected in P.

alleghaniensis, F. nigra, P. tremuloides stands

tremuloides, Q. rubra

(Table continued on next page)
6

Reference Armillaria spp. Associated host or Vegetation on the site

 substrate at the time / past (if known) Location

Rizzo et al. A. ostoyae P. banksiana; hardwood Clearcut; P. banksiana Cloquet, MN

1995 regeneration; conifer & planted / P. resinosa - P.

hardwood stumps banksiana

A. ostoyae P. resinosa, P. banksiana, P. resinosa - P. banksiana Cloquet, MN

A. balsamea, A. rubrum, B. with mixed deciduous /

papyrifera estab. after 1894 fire

Smith et al. A. ostoyae P. resinosa, A. rubrum, P. resinosa seedling Northern MI

1994 Populus sp., B. papyrifera plantation / northern

stumps hardwood forest

A. sinapina Populus sp. stump Same as above Northern MI

Smith et al. A. bulbosa = Fruiting bodies & Northern hardwood Northern MI

1992 A. gallica rhizomorphs in soil forest

Smith et al. A. ostoyae & Fruiting bodies P. resinosa seedling Northern MI

1990 A. bulbosa = plantation / northern

A. gallica hardwood forest

Proffer et al. A. ostoyae Prunus cerasus, P. avium, Prunus orchards Lower Peninsula, MI

1987 P. persica, Malus pumila

NABS1 III = P. cerasus Prunus orchards Lower Peninsula, MI

A.

calvescens

A. mellea P. cerasus , Quercus sp. Prunus orchards Lower Peninsula, MI

stumps

Stanosz & A. mellea Populus spp; suckers Short rotation Populus plots / Grand Rapids, MN

Patton sensu lato2 & stump mature Populus stand

1987 (no isolation)

Livingston A. mellea A. balsamea A. balsamea plantation / Grand Rapids, MN

et al. 1982 sensu lato 50-60-yr-old hardwood

stand

Pronos & A. mellea Q. alba, Q. rubra Natural Quercus stands, Black River State

Patton sensu lato sprout origin, herbicide Forest, WI

1978 treated and untreated

Pronos & A. mellea P. resinosa P. resinosa plantation, Northwest, northeast,

Patton sensu lato Quercus overstory, & west-central WI

1977 (no isolation) herbicide treated / natural

Quercus stands, sprout origin

1

 North American Biological Species, an intersterile population identified within the original A. mellea complex. 2

 Refers to the original A. mellea complex before its separation into several species.

(Table 4 continued)
7

conifers—Abies balsamea (L.) Mill., Picea glauca

(Moench) Voss, Pinus banksiana Lamb., P. resinosa Ait.,

and P. strobus L. and three deciduous species—A.

saccharum, Populus tremuloides Michx., and Betula

papyrifera Marsh (table 5). All have been previously

reported as hosts of A. ostoyae in this region.

Although its pathogenicity toward deciduous species is

less clear (Gregory et al. 1991), A. ostoyae is a serious

pathogen of several conifer species (Blenis 2000,

Cochran 1998, Rosso and Hansen 1998, Whitney

1995). It invades the roots of trees already stressed, but

may also penetrate the bark and kill the cambium in the

roots and root collar of healthy trees, resulting in their

death (Gregory et al. 1991, Kile et al. 1991). Root

disease from Armillaria species is a contributing factor in

several declines. “Decline” is a disease syndrome caused

by interactions among predisposing (e.g., site quality,

tree age), inciting (e.g., drought, insect defoliation) and

contributing (e.g., two-lined chestnut borer) factors

(Manion 1991). Some species in this genus, such as A.

ostoyae, may also act as a predisposing factor by

requiring a small but constant drain of host resources to

defend against chronic low-level infections that occur on

otherwise healthy roots (J. McLaughlin, personal

communication). By also deriving nourishment

saprophytically from stumps and debris, A. ostoyae can

survive on a site for many years.

Table 5.—Armillaria ostoyae1 and host associations from the Chequamegon area of the Chequamegon-Nicolet National Forest

Host species Host condition D.b.h.2 (cm) Stand3

Acer saccharum Dead 10 G4A

Betula papyrifera Dead, in clump with live 15 W2

Populus tremuloides Dead <1 G3

Abies balsamea Dead <1 G3

31 H2

32 H5

Stump, broken 30 PF2

Picea glauca Dead 38 G5

NR4 H1

Stump, cut NR H1

26 H3

15 H3

Pinus banksiana Live, declining 20 PF1

Dead 13 PF5A

28 W1

Snag 20 PF5A

Pinus resinosa Live, suppressed 15 H7

Dead 10 G3

Stump, cut 60 G2

38 H7

Pinus strobus Dead 44 G1

1 A single isolate of A. calvescens was collected from a dead stem of A. saccharum, 4 cm d.b.h., growing in a clump of

live and dead, site PF4. A single isolate of A. sinapina was collected from a single dead A. saccharum, 13 cm d.b.h.,

site G4B.

2 D.b.h.: Diameter at 1.4 m above ground for stems, at ground level for stumps.

3 Refers to the ranger district and stand number as shown in figure 1.

4 Not recorded.
The most notable mortality observed in this study was of

the jack pine in stand 5A in the Park Falls district and

the white spruce in stand 1 in the Hayward district.

Although the exact role of Armillaria root disease in

these stands was not clear, it probably contributed to the

situation. Other factors in the jack pine mortality

appeared to be the maturity of the trees, the exposure of

the site, and the presence of bark beetles and other root

and butt rot fungi. Factors in addition to Armillaria root

disease that may have been contributing to the white

spruce mortality were heart rot associated with Phellinus

pini, premature needle loss and branch mortality

associated with Rhizosphaera kalkhoffii (Juzwik 1993),

drought stress, and poor soils (USDA 1990).

Between the 1983 and 1996 cyclic inventories, the

USDA Forest Service, Forest Inventory and Analysis

(FIA) unit reported that the total area of deciduous

forest types in the Chequamegon increased by 23

percent while the total area of conifer forest types

decreased by 18 percent, due largely to reductions in

jack pine and white spruce forest types and reduced

growing-stock volume for both these species (Haugen et

al. 1998). Changes in harvest practices, natural forest

succession, and weather patterns contributed to these

changes, but most likely so did insect outbreaks and

diseases, including Armillaria root disease. Evidence of

the significance of A. ostoyae in forest succession and

composition has been reported by McLaughlin (2001b)

in areas of southern Ontario. Hardwood species are

colonizing openings where A. ostoyae has killed the red

pine. Although some mortality is occurring among the

hardwoods, in general they appear less susceptible than

the red pine and are expected to eventually dominate

these sites.

CONCLUSIONS

The occurrence of A. ostoyae on eight overstory species,

coniferous and deciduous, and its presence in most of

the stands that were examined are consistent with other

reports documenting the broad host range and

widespread distribution of A. ostoyae, both in the

western Great Lakes States and in other areas of North

America. Although additional data are required to draw

more definitive conclusions, based on the findings of

this study, A. ostoyae should be viewed as an integral

component of the Chequamegon forest landscape,

capable of causing root disease and thus a likely

contributor to decline and mortality of overstory species,

especially conifers.

ACKNOWLEDGEMENTS

Partial funding for this study came from the USDA

Forest Service, Forest Health Protection Unit. I thank

Joseph O’Brien for his assistance in identifying sites and

collecting sample material, and Paul Castillo for his help

in preparing the manuscript. I am grateful for the help

and insight provided by my three manuscript

reviewers—Joseph O’Brien, Plant Pathologist, USDA

Forest Service, State and Private Forestry; Mike Ostry,

Research Plant Pathologist, USDA Forest Service, North

Central Research Station; and John McLaughlin, Forest

Research Pathologist, Ontario Forest Research Institute.

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8
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9
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