Introduction to Fungi, Third Edition

376 HYMENOASCOMYCETES: PYRENOMYCETES
rapidly spreading infection of Castanea dentata
(American edible chestnut) in North America,
and in 1938 on C. sativa (European edible chestnut)
in Italy, from where it spread rapidly to
other countries. It is also known in Asia, the
putative centre of origin of C. sativa which was
brought from the Near East to Western Europe
by the Romans. It is possible that the pathogen
was accidentally introduced into the USA and
Europe with seedlings of Asian C. crenata
(Anagnostakis, 1987). Other tree species such as
oak (Quercus spp.) are occasionally attacked,
although only with minor commercial damage.
The fungus is a wound pathogen, colonizing
the bark and cambium tissues as a spreading
mycelium. The host defence reaction produces
cankers (see Fig. 12.18), and branches die if
cankers girdle their circumference. Cankers are
often coloured reddish-brown due to the abundant
formation of conidia which ooze out in
sticky tendrils from the pycnidia producing
them. Conidia are spread by animals and rainsplash.
Perithecia are also formed, embedded
in the bark tissue. The fungus is heterothallic,
with a bipolar mating system. Conidia function
as spermatia but can also germinate directly
to cause fresh infections. The roots are not
normally affected, and sucker shoots may grow
from intact rootstocks or from points proximal
to girdling cankers, but these new shoots also
become infected in due course. Following the
establishment of C. parasitica, the American
chestnut tree has become reduced to a shrublike
habit, not unlike the way in which the elm
tree in Europe has been affected by Dutch elm
disease (see p. 366). The disease caused by
C. parasitica is known as chestnut blight and
has had a dramatic effect especially in the
eastern United States, where C. dentata once
accounted for 50% of the value of hardwood
timber (Agrios, 2005).
Hypovirulence in Cryphonectria parasitica
Similarly severe outbreaks to those in the United
States were noted in Europe, but self-healing
cankers were observed about 15 years after the
first sighting of the disease. Self-healing was
shown by Grente and Sauret (1969) to be
associated with the presence of hypovirulent
C. parasitica isolates, i.e. strains with a much
reduced virulence. Moreover, when the hyphae of
a fully virulent colony were allowed to fuse with
those of a hypovirulent strain by anastomosis,
the former became hypovirulent, too. Eventually
it was discovered that hypovirulence is associated
with the presence of an unusual fungal
virus consisting of double-stranded RNA surrounded
by membranes but without a protein
coat (Anagnostakis & Day, 1979). This new type
of virus was named Hypovirus (see Dawe & Nuss,
2001). There are now three species of Hypovirus
known from Cryphonectria (Smart et al., 2000),
the best-examined being CHV-1 (Cryphonectria
Hypovirus 1). Cryphonectria is also a repository
for mycoviruses belonging to several other
taxonomic groups (Hillman & Suzuki, 2004).
Strains of C. parasitica infected by Hypovirus
have several phenotypic characteristics that
distinguish them from uninfected C. parasitica.
In addition to their hypovirulence on chestnut
trees, these include a pale rather than orange
colony colour on agar, reduced production of an
extracellular laccase and of a cell surface hydrophobin
protein due to reduced transcription of
their genes, poor asexual sporulation, and lack of
sexual reproduction because of female sterility.
Several genes were thus found to be affected by
the presence of the virus. This pleiotropic effect
is thought to be due to the action of the virus on
various signalling cascades, whereby the stimulation
of the cAMP pathway due to an inhibition
of the inhibitory a subunit (CPG-1) of a large
trimeric G protein has been particularly strongly
implicated (Chen et al., 1996).
The CHV-1 virus genome consists of two open
reading frames which produce altogether three
proteins (Shapira et al., 1991). It is not yet clear
how these act to cause the observed symptoms,
except that the p29 protein is responsible for
reducing the pigmentation and asexual sporulation
as well as transcription of the laccase gene.
However, p29 does not seem to cause hypovirulence
(Nuss, 1996). In contrast, when the
CPG-1 protein levels were reduced by the
presence of the wild-type virus or by genetic
manipulation of uninfected C. parasitica, or when