Introduction to Fungi, Third Edition

374 HYMENOASCOMYCETES: PYRENOMYCETES
Fig12.43 Phomopsis phaseoli.(a)Surfaceviewofa
conidium-producing stroma formed in agar culture.
(b) Phialides producing b-conidia. (c) Ovoid a-conidium and
elongated b-conidium. Both types of conidium are produced
from similar phialides. (b) and (c) to same scale.
like all species of Diaporthe examined to date.
In culture, the mycelium consists of narrow
hyphae containing 3 4 nuclei per segment, and
wider ones with up to 15 nuclei per segment.
Hyphae aggregate and swell to form a pseudoparenchymatous
stroma which produces both
pycnidia and perithecia. In nature, the pycnidial
Phomopsis state is produced earlier than the
Diaporthe state which is most often seen on
dead plant material. Conidial locules form in the
upper region of the stroma (ectostroma) when
two opposing palisades of hyphae press against
each other, accompanied by lysis of hyphae
bordering the developing slit. In consequence,
the slit becomes convoluted and lined
by hymenium. Ectostromatic cells proliferate to
form a neck, resulting in the typical shape of the
pycnidium consisting of a more or less globose
structure within which the lobed or folded
hymenium is located, and one or several necks
through which conidia are exuded (Fig. 12.43a).
The conidiogenous cells are interpreted as
phialides. They are awl-shaped, 20 mm long and
tapering from 2 3 mm at their base to 1 mm at
their apex (Fig. 12.43b). Two types of conidia are
produced, often within the same conidioma
(Fig. 12.43c). The ovoid a-conidia contain two
lipid droplets and germinate readily in culture,
whereas b-conidia are highly elongated and do
not usually germinate in P. phaseoli or other
Phomopsis spp. They are therefore interpreted as
spermatia (Jensen, 1983).
According to Uecker (1988), Phomopsis typically
has a dark stroma producing a- and b-conidia,
the teleomorph being Diaporthe. Unfortunately,
few typical Phomopsis species exist since many
produce only either a- or b-conidia, and/or
lack the Diaporthe state. Further, there are few
distinguishing features in conidial shape
between different Phomopsis species, and species
identification is based mainly on the host species
with which a given strain is associated. This
poses problems because the delimitations of host
ranges are not precisely characterized. Thus, the
taxonomy of Phomopsis is in a state of confusion,
with many synonyms probably in existence.
Uecker (1988) has compiled over 800 Phomopsis
names in current use, but Kirk et al. (2001)
have estimated that only about 100 species of
Phomopsis exist. The genus is thus in urgent need
of an up-to-date monographic treatment, Grove
(1935) having been the last mycologist to rise
to this formidable task.
In D. phaseolorum, sexual reproduction is
initiated by the formation of ascogonial coils in
the lower region of the stroma, termed entostroma
by Jensen (1983). These coils become