Introduction to Fungi, Third Edition

XYLARIALES
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been placed in several anamorph genera, including
Geniculosporium and Nodulisporium (Rogers,
1979, 2000; Whalley, 1996). Some develop as a
thin covering of the perithecial stroma. The
conidia are dry and are dispersed by air currents,
rain splash or insects.
There are about 40 genera in the Xylariales.
Most species (e.g. Xylaria, Hypoxylon, Daldinia) are
hemi-saprotrophic or saprotrophic, fruiting on
wood. These forms are ligninolytic and cause
white-rot of their substrate (Rayner & Boddy,
1988; Eaton & Hale, 1993). The limits of the
mycelial colonies in decaying wood are demarcated
by black zone lines (pseudosclerotial
plates) made up of brown bladder-like fungal
cells which fill the wood tissue (Campbell, 1933).
Other genera fruit on herbivore dung, e.g.
Hypocopra, Poronia, Podosordaria and Wawelia.
Some are serious plant pathogens; for example,
Hypoxylon mammatum (H. pruinatum) causes
canker of aspen (Populus tremuloides and other
Populus species) in North America (Manion &
Griffin, 1986), Biscogniauxia mediterranea causes
canker of cork oak (Quercus suber), and B.
nummularia causes strip canker of beech (Fagus
sylvatica). Kretzschmaria (Ustulina) deusta causes a
fatal butt rot of beech, elm (Ulmus) and horse
chestnut (Aesculus hippocastanum), whereas
Rosellinia necatrix is a plurivorous root pathogen
known to attack over 130 plant species. Members
of the Xylariaceae have been isolated as endophytes
(symptomless symbionts or commensals)
from a range of plants on which they do not fruit
so that their host range and distribution may
extend far beyond that which would have been
inferred from the occurrence of their ascocarps
(Petrini & Petrini, 1985; Petrini et al., 1995).
Although some species are plurivorous, others
have a restricted host range, e.g. H. fragiforme
which generally fruits on dead beech twigs
(Fagus) and Xylaria carpophila which fruits only
on fallen beech cupules.
The development of perithecia in the family
conforms to what Luttrell (1951) has termed the
‘XyIaria’ type:
The ascogonia are produced free upon the mycelium
or more commonly within a stroma. Branches
from the stalk cells of the ascogonium or from
neighbouring vegetative hyphae surround the
ascogonium and form the perithecial wall. Hyphal
branches with free tips (paraphyses) grow upward
and inward from the inner surface of the wall over
the base and sides of the perithecium. Pressure
exerted by the growth of opposed paraphyses
expands the perithecium and creates a central
cavity. The perithecium becomes pyriform as a result
of growth of hyphae in the apical region of the wall
to form a neck. The layer of inward-growing hyphae
is continuous up the sides and into the perithecial
neck. Growth of these hyphae within the neck
produces a schizogenous ostiole lined with free
hyphal tips (periphyses). The ascogonium produces
ascogenous hyphae which typically grow out along
the inner wall over the base and sides of the
perithecium. Asci derived from the ascogenous
hyphae grow among the paraphyses to form a
continuous hymenium of asci and more or less
persistent paraphyses lining the perithecial cavity. In
some forms the paraphyses are evanescent, and the
ascogenous hyphae form a plexus in the base of the
perithecium. The asci then arise in a single
aparaphysate cluster.
The cytology of ascus development in Xylaria
(Beckett & Crawford, 1973; Rogers, 1975a) and in
the related genus Hypoxylon (Rogers, 1965, 1975b)
follows the usual pattern in most cases. The
ascospores may be uninucleate, binucleate,
or occasionally multinucleate (Rogers, 1979).
In X. polymorpha and H. serpens, immature
ascospores are divided by a septum near the
base which cuts off a small appendage. The
appendage disappears in the mature ascospore,
leaving a truncate base. The ascospore wall of
Daldinia concentrica, as seen with the transmission
electron microscope, consists of five
recognizable layers numbered progressively
from the outside inwards as wall layers
W1 W5 (Beckett, 1976a). The thin, non-pigmented
outer layer W1 acts as a sheath which
completely encloses the spore. Before germination
this layer is sloughed off (see below). The
wall of the mature ascospore is dark, probably
due to the pigment melanin located in an inner
wall layer (W4). There is a hyaline germ slit
in W4 running the length of the ascospore
(Fig. 12.10a). Germ slits are microfibrillar in
construction but their structure varies within
the family (Beckett, 1979a,b). The germ slits