Annosus Root Disease

Description

Annosus root disease fungus (Heterobasidion annosum) is a hard, woody fungus growing right at ground level, on stumps, root collars and exposed roots of conifers. The fruiting body is shaped like a bracket or an irregular crust, usually 5–10 cm across, reddish-brown to dark brown on top, matt and knobbly, with a white margin. The underside is white-cream, with very small pores that turn brownish when damaged. The flesh is fibrous-corky and becomes woody with age.

The fungus grows in coniferous and mixed forests, all year round, because the fruiting bodies are perennial. It is one of the most dangerous tree parasites in forests: it causes root and stem rot, and a tree on which fruiting bodies have appeared is usually already doomed. It often occurs in patches of dying trees, especially in spruce stands.

It is distinguished from other bracket fungi of similar appearance by its perennial, hard fruiting body with a white margin, its small pores, and its growing place at the roots of conifers. Two related species of Heterobasidion, growing mainly on fir and spruce, are almost impossible to tell apart with the naked eye. The species is inedible.

Identification features

Cap

Fruiting body perennial, persistent, bracket-shaped, shelf-like, resupinate-reflexed or irregular, often wavy; attached laterally or by the base. Grows singly or in small groups, fused at the bases. Diameter usually 5–10 cm (descriptions also report specimens up to about 40 cm wide and 3.5 cm thick). Surface matt, forming a thin crust, often knobbly and uneven; colour reddish-brown to dark reddish-brown, blackish-brown in the central part, turning black with age. Growing margin white. A resupinate (spread-out) form may also occur, consisting only of the white crust corresponding to the underside of the fruiting body.

Hymenophore (underside of the cap)

Tubes white-cream, arranged in layers in older specimens. Pores small, roundish, decurrent, 0.2–0.4 mm in diameter (about 3–4 per mm), white, readily turning brownish when damaged.

Stem

No stem; the fruiting body is sessile, attached to the substrate laterally or by the base.

Flesh

Thickness 0.5–3 cm, light wood-coloured, fibrous-corky; elastic when young, woody when older. Smell sweetish or distinctly mushroomy (descriptions differ). Do not taste the flesh.

Spore print and spores

Spore print white. Spores non-amyloid, roundish-elliptical, finely dotted, 5–6 × 4–4.5 µm.

Habitat and distribution

Ecology. A dangerous parasite and saprotroph of trees, mainly conifers (spruce, pine, fir), less often broadleaved trees (birch, beech, oak). Causes intense rot of the roots and stem base (white, mottled and pitted rot). Infection occurs through wounds and freshly cut stump surfaces, which wind-borne spores reach, and then through root grafts and contact between a healthy and a diseased root; the mycelium spreads in the roots by about 0.1–2 m per year. It does the most damage in spruce stands on formerly farmed soils. It prefers neutral or alkaline soils and is rare on acidic ones. Fruiting bodies usually form after the tree has died, and on fertile sites they may appear on living trees.

Distribution. Widespread in coniferous and mixed forests of the Northern Hemisphere (Europe, Asia, North America); according to some descriptions also in Central America, Africa and Australia. In Poland it occurs in coniferous and mixed forests. Fruiting bodies grow low down: on root collars, stumps, the base of trunks and exposed roots. It is not legally protected in Poland.

Fruiting. Fruiting bodies are perennial, so they can be found all year round; sexual spores are released mainly in summer. There is no precise data on months. Fruiting bodies appear only 1.5–3 years after infection.

What can it be confused with?

• Heterobasidion abietinum (fir root rot fungus) — Grows mainly on fir, rarely on spruce; the fruiting bodies look very similar, and the host and specialist testing are needed for a reliable distinction.
• Heterobasidion parviporum (small-pored root rot fungus) — Grows mainly on spruce, rarely on fir; the fruiting bodies are hard to tell apart macroscopically.
• Neoantrodia serialis (serried cinder conk) — Fruiting bodies paler and yellowish, rather than reddish-brown to dark brown with a white margin.

Never rely on a single feature. If you are not sure — do not pick it; show the mushroom to a qualified mushroom expert.

Interesting facts

• Elias Fries described the species in 1821 as Polyporus annosus; in 1874 Robert Hartig linked it to a disease of conifers, and the current genus name (Heterobasidion, "different basidia") was given in 1888 by Julius Oscar Brefeld.
• The disease caused by this fungus, known in the literature as root rot (annosus root rot), is considered one of the most dangerous diseases of conifers in the Northern Hemisphere.
• Infected trees usually die in patches: seen from above, it looks like rings of dead trees in various stages of decay, the oldest in the centre and ever younger ones towards the outside.
• Stump infections are prevented with urea or biologically: the cut surface of the stump is covered with a preparation made from the mycelium of Phlebiopsis gigantea, a competitor of Heterobasidion that is harmless to healthy trees.