The root and butt rot caused by Heterobasidon annosum is one of the most destructive diseases of conifers in the northern temperate regions of the world, particularly in Europe. Economic losses attributable to Heterobasidion infection in Europe are estimated at 800 million euros annually. The fungus has been classified into three separate European intersterile species P (H. annosum), S (H. parviporum) and F (H. abietinum) based on their main host preferences: pine, spruce and fir, respectively. In North America, two intersterile groups are present, P and S/F, but these have not been given scientific names. The ecology of the disease spread has been intensively studied but the genetics, biochemistry and molecular aspects of pathogen virulence have been relatively little examined. Recent advances in transcript profiling, molecular characterization of pathogenicity factors and establishment of DNA-transformation systems have paved the way for future advances in our understanding of this pathosystem.

Taxonomy:  Heterobasidion annosum (Fr.) Bref., H. parviporum Niemelä & Korhonen and H. abietinum Niemelä & Korhonen; kingdom Fungi; class Basidiomycotina; order Aphyllophorales; family Bondarzewiaceae; genus Heterobasidion.

Identification:  presence of the fungus fruit bodies, basidiocarps whitish in the margins, upper surface is tan to dark brown, usually irregular shaped, 3.5 (−7) cm thick and up to 40 cm in diameter; pores 5–19, 7–22 and 13–26 mm2 for the P, F and S groups, respectively. Small brownish non-sporulating postules develop on the outside of infected roots. Asexual spores (conidiospores) are 3.8–6.6 × 2.8–5.0 µm in size. Mating tests are necessary for identification of intersterility groups.

Host range:  The fungus attacks many coniferous tree species. In Europe, particularly trees of the genera Pinus and Juniperus (P), Picea (S), Abies (F) and in North America Pinus (P) and Picea, Tsuga and Abies (S/F). To a lesser extent it causes root rot on some decidous trees (Betula and Quercus).

Disease symptoms:  symptoms (e.g. exhudation of resin, crown deterioration) due to Heterobasidion root rot in living trees are not particularly characteristic and in most cases cannot be distinguished from those caused by other root pathogens. Heterobasidion annosum s.l. is a white rot fungus. Initial growth in wood causes a stain that varies in colour depending on host tree species. Incipient decay is normally pale yellow and it develops into a light brown decay to become a white pocket rot with black flecks in its advanced stage.

Control:  silvicultural methods (e.g. stump removal), chemicals (urea, borates) and biological control agent (Phlebiopsis gigantea, marketed as PG Suspension® in the UK, PG IBL® in Poland and Rotstop® in Fennoscandia) are commonly used approaches for minimizing the disease spread.