Introduction to Fungi, Third Edition

392 HYMENOASCOMYCETES: ERYSIPHALES
in some species is inhibited by free water. Ultrastructurally,
conidia are highly vacuolated (see
Fig. 13.11). The major carbon and energy reserve
seems to be glycogen (McKeen et al., 1967; Roberts
et al., 1996), although lipid droplets have also
been reported, and lipids may contribute about
10% of the dry weight of conidia (Lösel, 1988).
The conidia of Erysiphales are uninucleate.
Ascocarps are usually formed late in the vegetation
period. These have traditionally been
termed cleistothecia, although they differ fundamentally
from those of the Plectomycetes because
the asci of Erysiphales are club-shaped, not globose,
and are formed at one level at the bottom of
the ascocarp rather than being scattered throughout.
Further, at maturity the ascocarp breaks
open by a pre-determined line of weakness,
exposing the asci which forcibly discharge their
spores by a squirt mechanism. In contrast, the
ascospores of Plectomycetes are released passively
when the ascus wall disintegrates. Braun et al.
(2002) have proposed the term chasmothecium
(Gr. chasma ¼ an opening, open mouth) for the
ascocarp of the Erysiphales. Chasmothecia are
brown globose bodies which have no ostiole.
Depending on species, they may contain one or
several asci, and their line of weakness may run
around the equator of the chasmothecium, or
through its apex. Chasmothecia are often ornamented
by highly characteristic appendages
(Fig. 13.1) which are usually sufficient to permit
unambiguous species identification, together
with the number of asci in the ascocarp (one or
several), the number of ascospores per ascus, and
the identity of the host plant. However, the
phylogenetic value of chasmothecial appendages
appears limited (see Fig. 13.1). In Northern
European climates, the asci are usually fully
formed in late autumn, but chasmothecia do
not open until the following spring when the
host plants begin to grow. They are therefore
thought of primarily as overwintering structures,
even if their viability may be low. In countries
with dry hot summers, chasmothecia may serve as
oversummering structures, being formed in late
spring and releasing ascospores in the autumn.
The developmental events taking place during
chasmothecium formation are immensely complex
and have been described by Luttrell (1951)
and Gordon (1966). They are probably similar in
most species. Initially, two superficial uninucleate
hyphae meet and one encircles the other.
The central cell receives a nucleus and enlarges
somewhat. The central cell has been termed the
pseudoascogonial cell because it does not seem
to play any direct role in ascus formation. The
pseudoantheridial cells which encircle the pseudoascogonium
divide to form the peripheral cells
of the ascocarp. Some outer peripheral cells
(‘mother cells’) develop short septate receptive
hyphae which make contact with vegetative
hyphae on the host surface. Following plasmogamy,
one nucleus is taken up by the receptive
hypha and divides in each segment of the receptive
hypha until one nucleus derived from the
vegetative hypha reaches the mother cell. The
mother cell then divides repeatedly.
At this stage, the immature ascocarp consists
of a pseudoparenchymatous centrum composed
largely of binucleate cells derived from the
mother cells intermixed with some uninucleate
cells, and surrounded by a peridium, some 4 6
cell layers thick. The peridium becomes darkly
pigmented. Uninucleate and binucleate cells
above the middle part of the centrum lyse. Karyogamy
occurs only within certain of the binucleate
cells which are more or less isolated from
the surrounding cells by lysis. These cells then
enlarge to form asci. The asci appear to grow at
the expense of the uninucleate and binucleate
cells of the centrum, so that eventually the asci
(or a single ascus, depending on the genus)
occupy almost the entire centrum. Meiosis of the
fusion nucleus in developing asci is usually
delayed until the centrum cells have all been
absorbed, although it tends to be completed
before the winter dormancy.
13.2 Phylogenetic aspects
The Erysiphales are clearly delimited and defined
as a group, but the question where to position
this order within the Ascomycota has aroused
considerable controversy over the past 150 years
or so and is still undecided. Braun et al. (2002)
have given an overview of the taxonomic history