Introduction to Fungi, Third Edition

550 HOMOBASIDIOMYCETES
agg. (Fig. 19.18d). Pegler (2000) has given a key to
the European species. Basidiocarps may occur
singly, but often grow in clumps of dozens or
even hundreds on the stumps of dead trees. The
fruit bodies are reported to cause gastro-intestinal
upsets if eaten raw, but are edible if they are
cooked and the cooking water is discarded. Most
species are annulate (i.e. with a ring on the stem),
but in A. tabescens a ring is lacking. Serious
pathogens include A. luteobubalina and A. mellea
sensu stricto on a very broad range of hosts, and
A. ostoyae on conifers (Larix, Pinus, Pseudotsuga,
Picea) and birch (Betula). Armillaria cepistipes and
A. lutea are mainly saprotrophic.
In infected trees, sheets of white mycelium
grow between the wood and bark, destroying the
phloem and cambium. In plantations, infections
arise when air-borne basidiospores colonize the
cut ends of thinning stumps. Spread of infections
is by root-to-root contact from diseased to
healthy trees and by means of bootlace-like
rhizomorphs (see and Fig. 18.13b). In this way
adjacent trees in plantations become infected,
resulting in group dying. A considerable area of
forest may be affected by Armillaria, with an
individual clone of A. bulbosa shown to extend
over several hectares (Smith et al., 1992). The
mycelium of Armillaria continues to grow saprotrophically
after the death of an infected tree,
and the zone of infected wood within the tree
may be surrounded by dark, melanized, pseudosclerotial
plates. Rhizomorphs may survive there
for up to 40 years and continue to support
fruiting over several years. Infected wood is
often bioluminescent. Survival of the mycelium
and its protection by pseudosclerotial plates
makes the control of diseases caused by
Armillaria difficult and expensive, although
fungicidal treatment has been attempted (West,
2000). Alternative biological control measures
using fungal antagonists such as Trichoderma spp.
also hold promise. Integrated control based on a
combination of fungicidal treatments and biological
antagonists (fungi and nematodes) has
had a limited measure of success (Raziq, 2000).
Although A. mellea agg. is generally regarded
as a pathogen, it forms endotrophic mycorrhiza
with several genera of chlorophyllous
orchids in the tropics, and also with the
colourless (achlorophyllous) orchid Gastrodia
elata. As in other orchids (see p. 597) the seedlings
of Gastrodia only become established following
infection by the haploid basidiomycete
Rhizoctonia. However, the Rhizoctonia infection is
only a primary phase of limited duration, and
secondary infection by Armillaria is essential
for successful further growth of the orchid
protocorm. The Armillaria mycelium within the
protocorm is connected to mycelium growing
parasitically on adjacent trees (Smith & Read,
1997).
Crinipellis (75 spp.)
Crinipellis perniciosa is the cause of a severe
witches’ broom disease of cocoa (Theobroma
cacao) especially in South America (Purdy &
Schmidt, 1996; Griffith et al., 2003). Young
shoots are induced to proliferate and become
swollen (Fig. 19.18e). There is considerable
reduction in the yield of cocoa pods, and crop
losses of up to 80% have been reported. Crinipellis
perniciosa is hemibiotrophic; its monokaryotic
mycelium is biotrophic but its dikaryotic mycelium
is necrotrophic. It forms groups of small
(pileus diameter: 5 15 mm) crimson to pink
basidiocarps on dead cocoa twigs (Fig. 19.18f).
Infection is by air-borne basidiospores which
germinate under conditions of high humidity
(e.g. dew) on meristematic tissues such as
flowering cushions, young leaves and shoots.
Germ tubes penetrating stomata or wounded
tissue establish a monokaryotic mycelium which
pervades the proliferating hypertrophied shoots.
Infection is not systemic. The monokaryotic
phase does not grow in agar culture, but
growth can be induced in cocoa or potato
callus tissue cultures. There are several biotypes
of C. perniciosa infecting different host plants.
The C (cacao) biotype is primarily homothallic.
Infected host twigs and pods eventually become
necrotic and death of host tissues is associated
with a change in the nuclear condition of the
mycelium to the dikaryotic state. Basidiocarps
develop on the dikaryon in the dead twigs. The
dikaryotic mycelium can be grown in culture,
and fruit body development is also possible on
agar or on a bran vermiculite medium covered
with a peat-based casing soil.