Introduction to Fungi, Third Edition

404 HYMENOASCOMYCETES: ERYSIPHALES
13.5 Podosphaera and Sphaerotheca
These two genera, although easily distinguished
from each other with the light microscope by
their chasmothecial appendages, have now been
grouped together, and Podosphaera takes precedence
over Sphaerotheca (Braun et al., 2002). Both
produce chasmothecia containing only a single
ascus. The important features of the conidia
of all species grouped here are that they are
catenate and contain conspicuous fibrosin
bodies. These are also seen in some other
genera, e.g. Sawadaea (see p. 405). Fibrosin
bodies are filamentous organelles up to 8 mm
long which appear highly light-refractile or
sparkling when viewed with the light microscope
(Fig. 13.11). Their biochemical composition
and function appear to be unknown.
Fig13.10 Chasmothecial appendages of Erysiphales.
(a) Flattened dichotomously branched appendages of
Podosphaera clandestina. (b) Branched appendages of Sawadaea
(formerly Uncinula), bicornis with recurved tips.
stated to be attacked by the plurivorous species
are those belonging to Solanaceae (e.g. tobacco)
and Cucurbitaceae (especially melon, cucumber,
squash and pumpkin). However, it is unclear
whether the same species attacks both families.
On cucurbits it has been described as G. cucurbitacearum
but here it seems to be of lesser significance
than Sphaerotheca fuliginea (¼ Podosphaera
xanthii) and might have been confused with it in
the past (Jahn et al., 2002). Molecular data indicate
that both Golovinomyces spp. are polyphyletic,
and that the clades do not obviously relate to
their host ranges. Another catenate species,
Neoerysiphe (formerly Erysiphe) galeopsidis affecting
mainly Lamiaceae differs from Golovinomyces in
having conidia with a minutely striated conidial
surface (Fig. 13.3d).
13.5.1 Sphaerotheca (now Podosphaera)
Common species are S. fuliginea on dandelion
(Taraxacum officinale) and other Asteraceae,
S. macularis causing powdery mildew of hops
(Humulus lupulus), S. mors-uvae (American gooseberry
mildew) and S. pannosa, the common rose
mildew. The chasmothecial structure of Sphaerotheca
closely resembles that of Erysiphe with its
simple appendages, the only notable microscopic
difference being that the chasmothecium contains
only one ascus in Sphaerotheca (Fig. 13.12).
The fine structure of developing and mature
chasmothecia of S. mors-uvae has been studied by
Martin et al. (1976). They have shown that the
darker melanized cells forming the peridium
are, like those of vegetative hyphae, uninucleate.
Most of the inner cells of the chasmothecium are
binucleate, suggesting that they may have arisen
from a binucleate ascogonial fusion cell. Another
interesting discovery was that fibrosin bodies,
previously reported from conidia, are also
present in the ascospores.
The mycelium and conidia of S. pannosa are
common on leaves and shoots of cultivated and
wild roses. Chasmothecia are formed on twigs,
embedded in a dense mycelial felt. Overwintering
is not only by means of ascospores, but
particularly as mycelium within dormant