Molecular Plant Pathology - Pathogen Profiles
Conifer root and butt rot caused by Heterobasidion
annosum (Fr.) Bref. s.l.
FRED O. ASIEGBU, ALEKSANDRA ADOMAS and JAN STENLID
Department of Forest Mycology & Pathology, Swedish University of
Agricultural Sciences, Box 7026, 750 07 Uppsala, Sweden
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.
Heterobasidion annosum (Fr.) Bref., H. parviporum
Niemelä & Korhonen and H. abietinum Niemelä & Korhonen;
kingdom Fungi; class Basidiomycotina; order Aphyllophorales; family
Bondarzewiaceae; genus Heterobasidion.
Presence of the fungus fruit bodies, basidiocarps whitish in the margins,
upper surface is tan to dark brown, usually irregular shaped, 3.5 (-7) mm2
and up to 40 cm in diameter; pores 5-22 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 x
2.8-5.0 µm in size. Mating tests are necessary for identification of
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).
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.
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
A dead pine tree killed by Heterobasidion annosum. (b) Fruit
body of H. annosum at the base of the trunk. (c) Enlarged fruit
body of H. annosum. (d) Decayed tree trunk due to H. annosum
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