What You Should Know
Heterobasidion annosum is a basidiomycete fungus that is considered to be the most economically important forest pathogen in the Northern Hemisphere. In Europe alone, H. annosum is responsible for the loss of 800 million euros annually (1 billion dollars US), and this pathogen is also widespread in forests in the USA.
This root and butt rot fungus is a destructive pathogen, appearing as a parasite on conifers across the continent. It is especially destructive in conifer plantations and thinned second-growth conifer forests, and it is well known to foresters as "annosus rot." Given enough time, Heterobasidion annosum can kill a tree by rotting its roots and trunkwood, making it very susceptible to windthrow.
Other names: Annosum Root Rot.
Heterobasidion annosum Mushroom Identification
Parasitic on the wood of conifers (rarely on hardwoods) and saprobic on deadwood; causing a white pocket rot of the roots and butt; annual or perennial; growing alone or gregariously at the bases of trees; appearing year-round; widely distributed in North America but rare in the central Rocky Mountains.
Variable: sometimes merely a spreading pore surface; sometimes with a folded-over edge of a cap; sometimes with a poorly to well-developed cap.
When present up to 25 cm across and 15 cm deep; shelf-like to kidney-shaped or irregular; finely velvety or bald; roughened and cracked; brown to dark brown or black, with a whitish margin and a reddish to reddish-brown marginal zone; sometimes hosting moss or algae.
Creamy to pinkish, orangish, or yellowish; not bruising appreciably; with 4-5 circular to angular pores per mm; tube layers often indistinct, to 3 mm thick.
Flesh: White; corky to woody.
Spore Print: Presumably white.
Heterobasidion annosum Host Range and Distribution
This fungus is an economically important pathogen of at least 200 different species in 31 genera of conifers and hardwoods, including Abies, Acer, Larix, Malus, Picea, Pinus, Populus, Prunus, Quercus, Sequoia, and Tsuga. This pathogen is widespread in the Northern Hemisphere, and most frequently occurs on gymnosperms.
Previously, all fungi that shared the ecology and morphology of this fungus were classified as Heterobasidion annosum. However, mating experiments determined that many different intersterile groups were present within this genus, and that these intersterile groups showed a great deal of host specificity. The groups are classified as P-type (host preference for Scots pine), S (Norway spruce), and F-type (silver fir), with some intersterile groups having multiple group types, and different populations existing within intersterile groups.
Genetic analyses of these populations show that the European and North American Heterobasidion annosum populations form different clades, and new species names of Heterobasidion parviporum and Heterobasidion abietinum have been proposed for the European S and F intersterile groups, respectively.
Heterobasidion annosum Life Cycle
In the summertime, basidiospores, the primary infective propagules, are released. These basidiospores are carried long distances by wind currents. They infect trees (usually conifers) through damage such as freshly cut stumps. Once on the stump the fungus colonizes and moves into the root via mycelium.
Heterobasidion annosum moves short distances from the roots of an infected stump through root grafts with other trees. It can also spread through insects that feed on roots. Since this fungus can not move very far through soil, it relies on tree roots to help it infect neighbouring trees. In these roots, it can grow 0.1–2.0 m per year. This results in a spread of the fungus and disease gaps in the forest. These disease gaps are produced when the trees dies and falls, creating gaps in the forest canopy. These gaps affect the moisture and sunlight available, altering the habitats for plants and animals on the forest floor. Spiniger meineckellus, the name for the asexual stage of this fungus, is produced on stumps when the conditions are moist, and the conidiospores that are produced will be able to live in the soil for up to ten months. The role of conidiospores is unknown in the infection process and is not thought to be important.
Heterobasidion annosum Symptoms & Signs of Disease
Symptoms and signs of fungus disease are often found underground. The H. annosum infections cause an abnormal change in structure in the roots that climbs up to the butt of the tree. More than half the tree may be killed before any symptoms appear to the human eye. Basidiocarps can take up to one and a half or even three years to be visible. This infection causes the trees to have abnormal needle growth, pale yellow barks, and the trees to wither and die. This root disease typically causes the tree to have a thin crown from the bottom up and inside out. Trees will eventually die.
A landscape-scale symptom is the rings of dead trees in various stages of decay and death, with the oldest at the center and progressively younger moving outward.
The white-rot fungus found in the roots is the sign of telling whether the tree has been affected by H. annosum. The bark changes colors as the stages progress, they go from pale yellow, to a crusty light brown, and finally, in its advanced stage, it turns white with the signature of Fomes annosus ― a sprinkled streak of black spots. Another sign is the leaking part of the root that causes a compact mass to form between it and the sand.
Heterobasidion annosum Isolation
There are several ways to isolate Heterobasidion annosum. Water agar could be used with infected host tissue to produce conidiophores which a simple or branched part hypha of a fungus to eliminate Heterobasidion annosum. Another way of isolating Heterobasidion annosum is by using the thin disks of living sapwood from Picea abies. By cutting the thin disks into petri dishes which is used to culture bacteria and placing them on moist filter paper, this technique allows spores to be captured from the air, and result in the asexual stage of the fungus forming on the disks.
Heterobasidion annosum Taxonomy and Etymology
When the great Swedish mycologist Elias Magnus Fries described this bracket fungus in 1821 he gave it the binomial scientific name Polyporus annosus. The currently-accepted scientific name of this species dates from an 1888 publication by the German mycologist Julius Oscar Brefeld (1839 - 1925).
Heterobasidion annosum has many synonyms including Boletus cryptarum Bull., Polyporus annosus Fr., Polyporus cryptarum (Bull.) Fr., Poria cryptarum (Bull.) Gray, Fomitopsis annosa (Fr.) P. Karst., Fomes annosus (Fr.) Cooke, Fomes cryptarum (Bull.) Sacc., and Spiniger meineckellus (A. J. Olson) Stalpers.
Heterobasidion, the generic name, means 'with variable basidia'. The specific epithet annosum comes from the Latin annus and means aged (many years old - perennial in other words).
Photo 1 - Author: Michel Langeveld (Creative Commons Attribution-Share Alike 4.0 International)
Photo 2 - Author: Michel Langeveld (Creative Commons Attribution-Share Alike 4.0 International)
Photo 3 - Author: Jerzy Opioła (Creative Commons Attribution-Share Alike 4.0 International)