Introduction to Fungi, Third Edition

18
Basidiomycota
18.1 Introduction
The Basidiomycota (colloquially basidiomycetes)
are a large group of fungi with over 30 000
species. They include many familiar mushrooms
and toadstools, bracket fungi, puffballs, earth
balls, earth stars, stinkhorns, false truffles, jelly
fungi and some less familiar forms. Also classified
here are the rust and smut fungi, which are
pathogens of higher plants and may cause
serious crop diseases. Most basidiomycetes are
terrestrial with wind-dispersed spores, but some
grow in freshwater or marine habitats. Many are
saprotrophic and are involved in litter and wood
decay, but there are also pathogens of trees such
as the honey fungus, Armillaria, which attacks
numerous tree species, and Heterobasidion annosum,
which can seriously damage conifer plantations.
Common woodland mushrooms such as
species of Amanita, Boletus and their allies grow
in a mutually symbiotic relationship with the
roots of trees, forming ectotrophic (sheathing)
mycorrhiza. Species of Rhizoctonia, representing
mycelial forms of basidiomycetes, behave as
pathogens towards a wide range of plants but
are mycorrhizal associates of orchids. As saprotrophs,
basidiomycetes play a vital role in
recycling nutrients but they also cause severe
damage as agents of timber decay, e.g. dry rot of
house timbers by Serpula lacrymans. The fruit
bodies (basidiocarps) of many mushrooms are
edible, and some are grown commercially for
food, notably Agaricus bisporus (¼ A. brunnescens,
the white button mushroom), Pleurotus spp.
(oyster mushrooms) and Lentinula edodes (shiitake).
It is also well known that the basidiocarps
of certain mushrooms are poisonous to eat, e.g.
Amanita phalloides (the death cap). Some species
have basidiocarps which are hallucinogenic,
e.g. Amanita muscaria (the fly agaric) and
Psilocybe spp. (‘magic mushrooms’).
The mycelium of basidiomycetes may be very
long-lived. Estimates based on the rate of growth
and the diameter of circles of the fairy ring
fungus Marasmius oreades growing in permanent
pasture show that they may be centuries old. It
has been estimated that the age of an individual
mycelium of Armillaria in a Canadian forest is
at least 1500 years, with an extent of 15 hectares
and a probable biomass in excess of 10 tonnes,
making it one of the largest organisms on earth
(Smith et al., 1992).
Not all basidiomycetes grow in the mycelial
form; some are yeast-like and others are dimorphic,
i.e. capable of switching between mycelial
and yeast-like growth. A dimorphic species which
is a dangerous human pathogen to immunocompromised
patients is Filobasidiella (¼ Cryptococcus)
neoformans causing cryptococcosis, a fatal disease
of the brain (see pp. 661 664).
18.2 Basidium morphology
The characteristic structure of sexually reproducing
basidiomycetes is the basidium. It is a
spore-bearing cell which produces basidiospores
externally through curved, tapering sterigmata