Introduction to Fungi, Third Edition

DIAPORTHALES
373
Aspects of pathogenicity
Although Ceratocystis spp. are spread by insects,
they are not normally associated with the
tunnels of bark-boring beetles. Instead, they
infect through pruning wounds or the bark,
and may be introduced by sap-feeding insects.
Transmission from infected to adjacent healthy
trees may also occur via root contact. Ceratocystis
fagacearum causes oak wilt symptoms by invading
the xylem and blocking the conducting
vessels with mycelium and a gel- or gum-like
substance. Additionally, toxins are produced by
C. fagacearum and other wilt-causing species,
and these can reproduce many wilt symptoms
in the absence of the fungus (Pazzagli et al., 1999).
Upon death of the host, C. fagacearum forms
compact mycelial mats underneath the bark.
The hyphae of the mycelial mat produce conidia
which are dispersed by beetles attracted by the
fruity smell. The fungus is heterothallic and the
conidia double up as spermatia. When spermatia
of compatible mating types are carried to a
mycelial mat, perithecia are formed.
There are several Ceratocystis spp. which are
associated with coniferous trees, partly as pathogens
but also as sap-staining fungi (Harrington &
Wingfield, 1998). These belong to a phylogenetically
clearly defined group around C. coerulescens
(Witthuhn et al., 1998). Because conifers are
unusual hosts for Ceratocystis and all known
species associated with them are closely related,
it is assumed that Ceratocystis has only recently
switched hosts from broad-leaved trees.
Ceratocystis versus Ophiostoma
Much confusion has arisen in the past between
the genera Ceratocystis and Ophiostoma which
are now known not to be closely related phylogenetically
(Spatafora & Blackwell, 1994).
Alexopoulos et al. (1996) have summarized the
main differences, which are as follows. (1) The
anamorphic state of Ophiostoma is annellidic
(Graphium; see Fig. 12.41) whereas that of
Ceratocystis and its allies is phialidic
(Thielaviopsis, Gabarnaudia; Fig. 12.42). (2)
Ophiostoma spp. infect through insect tunnels
whereas Ceratocystis spp. infect through wounds.
(3) The cell wall of Ophiostoma spp. contains
rhamnose-based polymers and cellulose, but
neither kind is found in Ceratocystis. (4)
Ophiostoma spp. are insensitive to cycloheximide
whereas Ceratocystis spp. are sensitive.
12.8 Diaporthales
The order Diaporthales currently includes some
447 species in 94 genera (Kirk et al., 2001) and
is well separated phylogenetically from the
other orders of perithecial ascomycetes (Zhang
& Blackwell, 2001). Several clades can be resolved
within the Diaporthales, although the naming of
families is problematic at present (Castlebury
et al., 2002). Members of the Diaporthales grow
mainly in the bark of trees as saprotrophs or
parasites. Discula destructiva infects all aboveground
organs of dogwood (Cornus spp.) causing
anthracnose (leaf blight, twig dieback, stem
cankers) which is devastating the native Cornus
populations of North America (Redlin, 1991).
Two other important plant-pathogenic genera
are Diaporthe and Cryphonectria, and these will be
considered in more detail below.
Members of the order Diaporthales are
characterized by perithecia which are produced
in clusters or stromata, often embedded in the
host tissue with their long necks protruding
beyond the surface. The asci are unitunicate
and contain eight ascospores which have one or
more septa. Diaporthales may be homothallic or
heterothallic, in the latter case with a bipolar
mating type system. The conidia are produced by
phialide-like structures lining the inside surface
of pycnidia, i.e. anamorphic Diaporthales were
formerly classified as coelomycetes. Pycnidia
are usually dark-walled (melanized) and have
one or more openings through which the
conidia exude in slimy drops. They are more
commonly encountered than the sexual state
(see Fig. 12.43).
12.8.1 Diaporthe and its anamorph
Phomopsis
Developmental aspects
Perhaps the only detailed developmental study
in Diaporthe was carried out by Jensen (1983) on
D. phaseolorum var. sojae, which is homothallic