Armillaria

The Microbial World:
Armillaria mellea and other wood-decay fungi

Produced by Jim Deacon
Institute of Cell and Molecular Biology, The University of Edinburgh

Wood decay fungi

Wood is one of the most abundant energy sources on earth, but it is difficult to degrade. Only a few specialised fungi of the groups basidiomycota and ascomycota have the ability to do this. Some of them are important pathogens of trees, including Armillaria mellea which is considered here and Heterobasidion annosum which is considered in a separate Profile. Some cause structural damage to the timbers of buildings - for example, the dry-rot fungus, Serpula lacrymans (Figure A). Most, however, are saprotrophs in natural environments, with major roles in nutrient cycling.

Wood-decay fungi are of increasing biotechnological interest, because their enzyme systems can detoxify pollutants and can delignify agricultural wastes, leaving cellulose as a potentially cheap commercial substrate for industrial fermentations.

Here we consider:

the special features required for growth on wood
the types of wood-decay fungi
Armillaria mellea, a major tree pathogen
other decay fungi, including generalists and host-specialists
the enzymology of wood decay.

Figure A. Serpula lacrymans (dry rot). The image shows part of an internal door post with extensive dry rot. The rotten wood shows brick-like cracking (white arrowheads) typical of this fungus. At bottom left we see the sulphur-yellow spreading fans of hyphae of the fungus on the wood surface - the advancing margin is indicated by the three black arrowheads.

Special features of wood-decay fungi

Wood-rotting fungi need to be specially adapted to overcome three major constraints.

Complexity of the organic substrates. Wood contains only small amounts of easily usable substrates such as simple sugars and starch. These are present initially in the ray parenchyma cells, but are utilised rapidly by sapstain fungi (Figure B) which invade freshly cut logs. The principal components of wood are cellulose (40-50% dry weight, composed of long, straight chains of beta-1,4-linked glucose), hemicelluloses (25-40%, consisting of mixed polymers of glucose, mannose, xylose, arabinose, etc.) and lignin (20-35%; a complex 3-dimensional polymer consisting of three types of phenyl-propane units). Lignin is highly resistant to enzymic attack, and it encrusts the other wall materials or forms chemical complexes with them, preventing them from being degraded easily.
Low nitrogen content. Wood typically has a very low content of nitrogen (usually less than 0.1% by weight) and phosphorus - the two mineral elements needed in largest amounts for microbial growth.
Presence of potentially fungitoxic compounds. These are particularly common in the non-living heartwood, and include tannins in broadleaved trees and various phenolic compounds (terpenes, stilbenes, flavonoids and tropolones) in coniferous trees. The most toxic tropolones are the thujaplicins, which uncouple oxidative phosphorylation. The natural decay-resistance of cedar wood is due to these compounds, and the specificity of several fungi for particular types of wood might be related to their tolerance of particular compounds.

Figure B. Sapstain fungi seen in thin sections of wood stained with safranin. The low-power image (left) shows a section through two parts of the ray parenchyma traversing the xylem. The right-hand image shows darkly pigmented fungal hyphae growing in the ray parenchyma.

Types of wood decay fungi

Wood-decay fungi can be grouped in various ways:

by their method of degrading wood, which reflects fundamental differences in enzymic activities - the white-rot, brown-rot and soft-rot fungi
by their general biology - whether they are pathogens which attack and destroy the living sapwood, "parasites" which attack and kill moribund or stressed trees, or saprotrophs which colonise dead wood
by whether they are primary colonisers or secondary colonisers, cause heartrots of standing or fallen trees, are host-restricted or "generalists", etc.

White-rot fungi

These fungi degrade all the major wood components (cellulose, hemicelluloses and lignin) more or less simultaneously, so that the wood becomes progressively more fragile but remains white as it decays. White rots are caused by the two major root-rot pathogens of trees, Armillaria mellea (Figures C, D) and Heterobasidion annosum, and also by many saprotrophic fungi, including the common coloniser of stumps, Coriolus versicolor (Figure N), and the common wood-rot ascomycota Xylaria hypoxylon (Figure Q) and Xylaria polymorpha (Figure R).

Brown-rot fungi

Brown-rot fungi degrade the cellulose and hemicellulose but leave the lignin more or less intact as a brown framework. Only about 6% of wood-decay fungi cause brown rots, and all these fungi are members of the basidiomycota. They include Serpula lacrymans (dry-rot fungus, Figure A) and the common birch polypore, Piptoporus betulinus (Figure H).

Soft-rot fungi

Soft-rot fungi degrade only the cellulose and hemicelluloses, and typically occur in wood of high water content and high nitrogen content. They are most commonly found in rotting window frames, wet floor boards and fence posts, etc., where nitrogen is "recruited" from soil or from atmospheric contamination. Some of these fungi are common decomposers of cellulose in soil (e.g. Chaetomium species) and they are the least specialised of the wood-rot fungi.

Armillaria mellea: a destructive pathogen of trees

The genus Armillaria contains about 40 species of important wood-rot fungi which are widely distributed across the world.. Their basic behaviour is similar, because all the species invade the roots and cause a progressive white rot (see later). For this reason, all these fungi were at one stage grouped as a single species, Armillaria mellea. But they are now separated on the basis of morphology, physiology, pathogenicity and geographical distribution.

Some of these species are destructive root-rot pathogens of trees - they can invade through the bark of the major roots, progressively destroying the living root tissues and leading to serious decline and ultimate death of their hosts. In economic terms, these pathogenic species are almost as important as Heterobasidion annosum in commercial forestry. The major examples include Armillaria mellea (in the new, restricted sense) and Armillaria ostoyae in Europe and North America, and Armillaria luteobubalina in Australasia. Other species, such as A. gallica in Europe and North America, are usually weakly pathogenic but can cause disease of stressed or weakened trees. Further distinctions can be made in terms of the characteristic host ranges of the different species. For example, A. mellea is mainly a pathogen of broadleaved trees in ornamental parklands, natural woodlands, fruit orchards, etc, but it can kill young coniferous trees (pines, spruce, etc.) planted in sites where the broadleaved species were felled. In contrast, A. ostoyae seems to be a more important pathogen of coniferous trees, causing major damage in even second- and third-rotation stands of conifers. Thus the genus Armillaria seems to have diversified into a number of forms with different degrees of pathogenic virulence and host preference. Here we treat them all as "Armillaria" because their basic behaviour is similar.

Armillaria is commonly seen in British woodlands, where it produces large numbers of honey-coloured fruitbodies on or around the bases of dead trees (Figure C). These fruitbodies give rise to the common name of A. mellea - the "honey fungus". Armillaria is also known as the "bootlace fungus" because many of the species can spread through soil or under the bark by producing rhizomorphs that resemble bootlaces (Figure D). These rhizomorphs are aggregates of thousands of hyphae, with a black "melanised" outer rind, and they have an important role in infection, discussed below.

Armillaria can establish in new sites when airborne basidiospores land on exposed stump surfaces, in a similar manner to Heterobasidion annosum. It then colonises the stump tissues which serve as a food base for spread to the roots of other trees. In some species of Armillaria this spread from tree to tree is achieved by mycelial growth during root-to-root contact. But in many cases it is achieved by the rhizomorphs which can grow along the root surfaces, under the bark of dead trees, or can spread several metres through soil from an established food base. Thus, once established in a site Armillaria often forms an extensive network of rhizomorphs which can either aggressively invade the roots of trees or, in the case of the less pathogenic species, can invade when the trees are stressed by environmental factors.

The rhizomorphs are remarkable structures because they function as units, with an apical meristem which superficially resembles a root tip - quite different from the normal growth of individual fungal hyphae. They grow much faster than the normal hyphae of Armillaria - up to 19 mm per day in laboratory conditions - and they translocate nutrients to the tips from a food base (a colonised stump. etc.) that can be several metres away. The rhizomorph tip, supported by nutrients from a food base, has sufficient "inoculum potential" to penetrate the bark of a tree root and progressively destroy the underlying root tissues - an essential feature in the biology of these fungi. Little is known about the factors that regulate the growth of rhizomorphs, but the initiation of rhizomorphs in culture is strongly enhanced by low-molecular-weight alcohols such as ethanol and propanol.

Armillaria is notable in two other respects. First, the pathogenic species have astonishingly wide host ranges which extend far beyond forest trees. For example, Armillaria can cause serious damage to orchard crops (peaches, almonds, citrus, avocado, cocoa, coffee, kiwifruit, etc.) whenever these are planted in sites where indigenous woodland or shrubland was cleared. The fungus infects these crops from the stumps or major root tissues that were left after clearing, and it can only be eradicated by removing all the major root fragments that provide a food base - an expensive operation. Second, some Armillaria species can act as mycorrhizal fungi to support the growth of orchids and other non-photosynthetic plants, similar to the more common role of Rhizoctonia species (see Mycorrhizas). For example, Armillaria species have been identified as the fungal symbionts of the orchids Gastrodia elata, G. cunninghamii and Galeola septentrionalis, and of Monotropa uniforma in the family Pyrolaceae. In some cases the fungus produces coils in the cells of the orchid tubers, and these coils are digested by the orchid as a source of nutrients. In some cases, also, the Armillaria rhizomorphs that infect the orchid are also attached to tree roots. So the fungus essentially acts as a bridge, supplying nutrients to the orchid from a tree host - an indirect form of parasitism.

Further reading:

Many aspects of the biology, infection behaviour and ecology of Armillaria are covered in depth in the chapters of Armillaria Root Disease (CG Shaw & GA Kile, eds) 1991. Agriculture Handbook No. 691. Forest Service, United States Department of Agriculture, Washington D.C.

Ganoderma adspersum and other heartrot fungi A few fungi cause extensive heartrots of standing trees by growing in the central, non-living woody tissues. They can be specially adapted to tolerate the potentially fungitoxic compounds in this type of wood. A classic example is Ganoderma adspersum, a white-rot fungus which causes heartrot of beech and other broadleaved trees (Figures E, F). It is not a pathogen because it grows in the non-living tissues, but it can weaken the tree, making it susceptible to wind damage. Ganoderma establishes from airborne spores that enter wounds caused by shedding of branches, etc. Its characteristic "woody" fruiting bodies release large numbers of basidiospores from extremely narrow pores - it is one of the 'polypore' fungi, with pores instead of gills. Its fruitbodies are perennial, and the white margin (Figure F) represents the expanding edge of the current season's growth.
Figure E. Base of a living beech tree (Fagus sylvaticus) showing large, bracket-shaped fruitbodies of the heartrot fungus Ganoderma adspersum. Figure F. Close-up of a fruitbody. These are perennial and produce a fresh zone of growth each year. Brown-coloured basidiospores are released from basidia that line the pores of the fruitbody. Some spores have fallen onto the tree base; others are seen on top of the fruitbody, owing to electrostatic attraction.
Figure G. Polyporus squamosus fruiting from the trunk of an elm tree. Figure H, Piptoporus betulinus fruiting on a fallen birch log.
The white-rot fungus Polyporus squamosus (known as Dryad's saddle) (Figure G) causes a heartrot of standing elm, beech and sycamore trees. In contrast, Piptoporus betulinus (Figure H) is a brown-rot fungus, seen commonly on standing birch trees, but it is fruiting on a fallen birch log in Figure H. It is known commonly as the razor-strop fungus because the rubbery, fine-textured fruitbody can be used as a strop. It is a host-restricted species, found only on birch, and often is regarded as a parasite because it rapidly colonises stressed or moribund trees.
Figures I, J. Fruitbodies of Daedalea quercina, fruiting on a fallen oak trunk. The fruitbodies are seen in their natural orientation in Figure I. The underside of a fruitbody is shown in Figure J.
Daedalea quercina is another host-restricted species, found almost exclusively on fallen oak wood in Britain (Figures I, J). It is a brown-rot fungus that colonises the heartwood. Its fruitbodies have large, irregular, maze-like pores, giving rise to the common name, maze-gill. In laboratory culture, this fungus tolerates high concentrations of phenolic compounds that would inhibit most other species. It is also highly tolerant of acetic acid in culture, consistent with its growth in oak heartwood which can contain as much as 0.45% (dry weight) of acetic acid and have an acidity as low as pH 3.
Figure K. Old tamarisk trees (Tamarix gallica, the "salt cedar") on a slope just above the seafront at Biarritz, southern France.These trees are noted for their drought-tolerance and ability to withstand salt spray.
An extreme example of the effects of heartrots is shown in Figure K, where the tamarisk trees have almost completely degraded heartwood. Yet the trees are still alive because the fungus has not damaged the living sapwood. The final image (right-hand side) shows a fruitbody on one of the trunks.

Other wood-rot fungi In addition to the root-rot pathogens and heartrot fungi, a wide range of essentially saprotrophic wood-decay fungi are found on dead trunks, fallen trees and cut stumps. Figures L and M show two of the jelly fungi - basidiomycota with fruitbodies that have a rubbery or jelly-like consistency. Auricularia auricula (Figure L) is common on the branches of elder (Sambucus) trees, which it infects as a weak parasite. It is known as "Jew's ear", because Judas is supposed to have hanged himself on an elder tree. Figure M shows a Tremella species (probably T. mesenterica) on a fallen oak stem. This fungus often grows in association with another wood-rot fungus Stereum hirsutum.
Figure L. Fruitbodies of Jew's ear, Auricularia auricula, a specialised wood-rot fungus of elder trees. The larger fruitbodies are about 5 cm diameter. Figure M. Jelly-like lobes of Tremella on a fallen oak trunk; the cluster is about 8 cm diameter.
The white-rot fungus Coriolus versicolor (Figure N) is commonly seen as clusters of thin, leathery fruitbodies on logs and stump surfaces of broadleaved trees. It colonises from airborne spores and, unlike most of the fungi discussed above, establishes many separate colonies from individual basidiospores on a stump surface. When the hyphae of these colonies meet they can show an incompatibility reaction, because there are several "vegetative" compatibility genes that segregate during meiosis to produce the basidiospores. So the junction of any two colonies can be marked by interaction zone lines (Figure O) where the hyphae have died and polymerised phenolics (e.g. melanin) have been produced. Fungi isolated into pure culture from either side of these zone lines can belong to the same (or different) species of wood-rot fungi and they show mutual antagonism on agar plates. Wood containing these zone lines is becoming popular as decorative objects (Figure P).
Figure N. Coriolus versicolor on a fallen oak trunk. Each fruitbody is about 5 cm diameter.
Figure O. Part of a beech stump showing interaction zone lines between colonies of different (mutually antagonistic) strains of wood-rot fungi, including Coriolus versicolor. Figure P. Decorative bowl made from fungal-colonised beech wood.
Coriolus versicolor has been studied intensively in laboratory culture and shown to tolerate very low nitrogen levels. It grows well on media with a ratio of carbon to nitrogen (C:N ratio, expressed in terms of the amounts of these elements) as high as 1600:1, whereas standard laboratory media have a C:N about 32:1. In nitrogen-poor conditions this fungus seems to allocate nitrogen preferentially to production of enzymes and vital cellular components, and it might also have an efficient mechanism for recycling the cellular nitrogen. But recent studies have shown that nitrogen-fixing bacteria also colonise wood and might provide nitrogen to support the wood-rot fungi. It is known that many basidiomycota can use either living or heat-killed bacteria as the sole source of nitrogen in laboratory culture conditions (see Thermophilic microorganisms). In addition to the basidiomycota, some common white rots are caused by ascomycota. Two such fungi are shown below - Xylaria hypoxylon and X. polymorpha.
Figure Q, Xylaria hypoxylon (candle-snuff fungus). Figure R, Xylaria polymorpha (dead man's fingers).

Enzymology of wood decay

Cellulose and hemicelluloses are the two principal substrates for wood-decay fungi. In the soft-rots and white-rots they are degraded by conventional enzymes released from the fungal hyphae. This is illustrated below for cellulose, a straight-chain beta-1,4-linked polymer of glucose. Three types of enzyme are required:

cellobiohydrolase (CBH), which acts at the end of the molecule, successively cleaving off the disaccharide cellobiose
endo-beta-1,4-glucanase, which acts within the chain, breaking it into smaller units and providing more "ends" for CBH to act on
beta-glucosidase (or cellobiase) which cleaves cellobiose to two glucose units.

By acting together, these enzymes provide the fungus with glucose as a main energy source. However, the main chain-splitting enzymes are quite large (ranging from about 15,000 to more than 40,000 Daltons) so they do not diffuse far from the hyphae. Therefore, the soft-rot and white-rot fungi tend to cause localised decay of wood around their hyphae.

Outline of the structure and enzymic breakdown of cellulose. From Deacon (1997).

Lignin is a large, complex, three dimensional polymer, composed of three main types of phenyl-propane unit, linked by various types of bond. It is shown below in highly simplified form with just a few residues and bonds. Only the white-rot fungi can degrade it, and they do this by a remarkable process termed enzymatic combustion, resembling any other type of combustion.

Essentially, this process is an enzyme-mediated oxidation, involving the initial transfer of single electrons to the intact lignin molecule. Then these electrons are transferred to other parts of the molecule in uncontrolled chain reactions, leading to breakdown of the polymer.

White rot fungi produce several types of enzyme - some to generate hydrogen peroxide as an oxidant, and others to transfer the electrons. They include:

lignin peroxidase (previously known as ligninase). This is an iron-containing enzyme which accepts two electrons from hydrogen peroxide (H₂O₂), then passes them as single electrons to the lignin molecule.
manganese peroxidase, which acts in a similar way to lignin peroxidase but oxidises manganese (from H₂O₂) as an intermediate in the transfer of electrons to lignin.
laccase, a phenol oxidase which directly oxidises the lignin molecule
several hydrogen-peroxide-generating enzymes - for example glucose oxidase which generates H₂O₂from glucose (a product of cellulose or hemicellulose breakdown).

Simplified structure of lignin, showing the three main phenyl-propane units and 3 of the many types of bond. The whole molecule is a complex, three-dimensional framework containing thousands of phenyl-propane units. From Deacon (1997)

The special case of brown-rot fungi. Brown-rot fungi are unique, because they cause a very generalised rot which extends far beyond the fungal hyphae. This would be difficult to explain by the actions of "conventional" cellulase enzymes, and recent work suggests that the cellulase enzymes produced by these fungi have very little effect on cellulose in laboratory culture. Instead, the brown-rot fungi act by producing the enzyme glucose oxidase which generates hydrogen peroxide (H₂O₂) from glucose in the hemicelluloses. The H₂O₂ then oxidises cellulose and also modifies (but does not degrade) the lignin, leaving this as a brown framework. Being a small molecule, H₂O₂ can diffuse freely through the wood, causing generalised breakdown. In fact, the characteristic decay pattern caused by brown-rot fungi can be reproduced by treating wood with H₂O₂ alone. This decay pattern typically involves brick-like cracking of the wood (as in the case of dry-rot, Figure A) where it dries and splits along planes of weakness between heavily decayed and less heavily decayed regions.

This site is no longer maintained and has been left for archival purposes

Text and links may be out of date

This site is no longer maintained and has been left for archival purposes

Text and links may be out of date