Introduction to Fungi, Third Edition

OPHIOSTOMATALES
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Fig12.37 Asexual reproduction in Ophiostoma ulmi. (a,b) Synnematous conidiophores (Graphium ulmi). Parallel bundles of dark
hyphae branch at their tips to produce holoblastic conidia which accumulate in a sticky drop. (c,d) Mononematous synanamorph
terminating in conidiogenous cells with a succession of holoblastic conidia which, on detachment, leave a protruding scar or denticle.
In d some of the conidia show yeast-like budding. (e) An older hypha within which a new hypha and endoconidia have developed
(from Harris & Taber,1973).
based on their vegetative incompatibility characteristics,
showed that they can be separated
into two distinct vc (vegetative compatibility)
supergroups. One biotype, centred on the
Romania Moldova Ukraine region of Europe,
was designated the Eurasian or EAN vc supergroup,
whilst the other, centred on the Southern
Great Lakes region of North America, has
been designated the North American or NAN vc
supergroup. They have been formally recognized
as distinct subspecies, O. novo-ulmi subsp. novoulmi
for the EAN strains and subsp. americana
for the NAN strains. There are morphological
differences between the perithecia of the two
subspecies, those of subspecies novo-ulmi having
longer necks than those of subspecies americana
(Brasier & Kirk, 2001). Naturally occurring
hybrids between the two subspecies have been
detected. Rare interspecific hybrids between
O. ulmi and O. novo-ulmi also occur where the
former species is being replaced by the latter
(Brasier et al., 1998). Hybridization between introduced
pathogens permits their rapid evolution
(Brasier, 2001). Ophiostoma himal-ulmi is endemic
to the Himalayas and has been distinguished as
a third species (Brasier & Mehrotra, 1995).
Fertilized females of bark-boring beetles,
possibly carrying ascospores or conidia, excavate
tunnels beneath the bark of living trees, often
those already weakened by the disease, to lay
a cluster of eggs. After hatching, the developing
larvae also make tunnels beneath the bark
radiating outwards from the egg chamber. They
feed on the infected wood, and the galleries
which they excavate are often lined by synnemata
or perithecia, so that conidia and ascospores
become attached to their mouthparts and
bodies. Young, sexually immature adults emerge
in the spring and summer. They fly to the
crotches (branch points) of young twigs where
they feed on bark before maturation and
mating. During this twig-feeding stage spores
of the fungal pathogen may be introduced into
the host sapwood, and from these multiple
inoculation points the fungus may spread into
host tissues by mycelial growth or by movement