Introduction to Fungi, Third Edition

368 HYMENOASCOMYCETES: PYRENOMYCETES
of conidia in xylem vessels (Webber & Brasier,
1984; Webber & Gibbs, 1989). By the latter
method it has been estimated that movement
can be as much as 10 cm day 1 . In addition to
being transmitted by insect vectors, the pathogen
can be passed from tree to tree by natural
root contact.
The control of Dutch elm disease has been
attempted by the combined use of fungicides and
insecticides but is now based largely on the
breeding of resistant cultivars. Major genes for
resistance have been identified in a group of
Asian species. By crossing some of these with
European elms, cultivars have been bred and
released for commercial sale (Smalley & Guries,
1993). Resistance may be correlated with the
production of phytoalexins (mansonones) by the
host in response to infection (Smalley et al., 1993).
In U. minor a correlation has also been found
between vessel diameter and susceptibility, trees
having vessels of large diameter being more
susceptible to the disease than those with
smaller diameter vessels (Solla & Gil, 2002). An
interesting novel potential method of control
is by the use of hypovirulent strains of the
pathogen infected by cytoplasmically transmissible
virus-like agents called d-factors (d for
disease), now known to consist of doublestranded
mitochondrial RNA elements. Twelve
d-factors have been characterized. Strains
carrying d-factors are hypovirulent and this
is correlated with a reduced capacity in vitro
to produce cerato-ulmin (Rogers et al., 1986;
Sutherland & Brasier, 1995).
12.7 Microascales
The Microascales are a small order currently
containing 67 species (Kirk et al., 2001).
The taxonomy of the Microascales has seen a
turbulent history. Species accommodated here
are characterized by perithecia (rarely cleistothecia)
which usually have a long neck. The fact that
asci are scattered throughout the perithecial
cavity and are not produced by croziers is one
reason why members of the Microascales have
been considered in the past to belong to the
Plectomycetes. Another moot point has been the
great similarity of perithecia and general ecological
features between Ceratocystis (Microascales)
and Ophiostoma (Ophiostomatales), which will be
discussed in more detail below. In its current
shape, the order Microascales is monophyletic by
DNA analyses (Hausner et al., 1993). Because the
two major lineages, the Ceratocystidaceae and
Microascaceae, show considerable differences in
their biology and ecology, we will briefly discuss
members of both groups.
12.7.1 Microascus (Microascaceae)
The family Microascaceae currently contains
43 species in 8 genera. The conidial forms are
hyphomycetous, with conidia produced on
annellides (see Fig. 8.7). The most important
genus is Microascus with 14 species, which have
been described in detail by Barron et al. (1961).
Simple conidiogenous forms of Microascus are
referable to Scopulariopsis (Fig. 12.38), whereas
more complex, synnematous forms belong
to Cephalotrichum (formerly called Doratomyces;
Fig. 12.39) or Trichurus (Fig. 12.40). In agar culture,
both Cephalotrichum and Trichurus produce
simple Scopulariopsis-like forms in addition to
the striking synnemata. The connection between
these annellidic forms and Microascus has
now been confirmed by phylogenetic analyses
(Issakainen et al., 2003). Just to confuse matters,
Cephalotrichum stemonitis produces, in addition to
annelloconidia, a distinct Echinobotryum conidial
state (Fig. 12.39e). Graphium is another formgenus
producing synnemata which form annelloconidia
at their apex (Fig. 12.41), but here the
conidia are held in a drop of mucilage at the
tip of the synnema. Certain Graphium-like forms,
e.g. G. penicillioides (Fig. 12.41a), have affinity with
Microascus, although others seem to belong to
the Ophiostomatales (Okada et al., 2000).
Microascus spp. and members of related
genera (e.g. Pseudoallescheria, Petriella) are capable
of metabolizing a wide range of carbon sources,
including keratin, crude oil, cellulose and even
phenol, and they are tolerant of wide-ranging
environmental conditions. Consequently, they
are found from the Arctic to hot desert soil, in
salt marshes, bat guano, herbivore dung, food,