Introduction to Fungi, Third Edition

BLUMERIA GRAMINIS
393
of the Erysiphales. Affinities with Pyrenomycetes
or Plectomycetes have been proposed in the past
but are not now thought to be true. Instead, the
phylogenetic position of the Erysiphales is fairly
isolated, possibly associated weakly with the
Helotiales (Saenz & Taylor, 1999b).
For many years, the taxonomy of genera
and species was based on the system proposed
by Léveillé (1851) who emphasized the features
of the chasmothecium, especially the number of
asci (one or several) and the type of appendage
which may be simple, uncinate (¼ recurved or
hooked), dichotomously branched, or bulbous
(see Fig. 13.1). Recent phylogenetic studies based
on a variety of DNA sequences have revealed that
in the Erysiphales, unlike most other groups of
organisms, the features of asexual reproduction
are more clearly diagnostic than those associated
with sexual reproduction. There is a good correlation
between the anamorphic state and the
groupings obtained by DNA sequence analysis
(Saenz & Taylor, 1999a), and the gross anamorphic
features also correlate with the surface
ornamentations of the conidia as seen by scanning
electron microscopy (Fig. 13.3; Cook et al.,
1997). Thus, the genera of Erysiphales are currently
defined mainly by their anamorphs. In
contrast, the striking chasmothecial appendages
do not correspond well to the individual groups
because notably the dichotomous and simple
mycelioid appendages are found in more than
one taxon (Saenz & Taylor, 1999a). One casualty
of these findings is the genus Sphaerotheca with
simple appendages and one ascus per chasmothecium,
which has been incorporated into Podosphaera,
formerly comprising only species with
chasmothecia containing one ascus but bearing
appendages with dichotomously branched tips
(Braun et al., 2002).
We have encountered a great plasticity of
appendages on ascocarps before, especially in the
Plectomycetes where they may be involved in
insect dispersal (see p. 289). In contrast, in the
Erysiphales the chasmothecial appendages seem
to be related to the type of host infected.
According to the analyses of Mori et al. (2000),
the most basal species among the Erysiphales is
Parauncinula septata which has uncinate (hooked)
appendages. This species occurs on oak (Fagaceae),
and Mori et al. (2000) have speculated that the
Erysiphales originated on members of the Fagaceae
because that host family hosts by far the
greatest diversity of powdery mildews (Amano,
1986; Braun, 1987, 1995). Together with another
ancestral species possessing chasmothecia with
uncinate appendages, Caespitotheca forestalis,
P. septata may have diverged from other powdery
mildews some 80 90 million years ago
(Takamatsu et al., 2005).
Many of the powdery mildews associated
with the leaves of deciduous trees have hooked
or branched appendages, whereas those on evergreen
or herbaceous plants often have simple
(mycelioid) appendages. It is known that complex
appendages facilitate the attachment of chasmothecia
to twigs or the bark of host trees for
overwintering (see p. 407; Gadoury & Pearson,
1988; Cortesi et al., 1995), and it must be advantageous
for the primary inoculum in spring to be
close to the budding leaves, rather than falling
to the ground. Mori et al. (2000) proposed that no
such selection pressure may hold for species parasitizing
herbaceous plants, which might have
permitted a reduction in the complexity of appendages
to simple mycelioid ones. Such changes
from woody to herbaceous hosts accompanied by
the reduction in the complexity of appendages
may have occurred several times independently
within the Erysiphales (Takamatsu et al., 2000).
13.3 Blumeria graminis
Although Blumeria graminis is somewhat atypical
of a powdery mildew in being a pathogen of
grasses and cereals, we shall describe it in detail
because more is known about it than any other
member of the Erysiphales. Blumeria is separated
from Erysiphe on several grounds. The haustorium
of B. graminis is digitate, i.e. it has striking
finger-like projections (Fig. 13.2c), whereas the
haustoria are knob-like in most members of
the Erysiphales. Upon closer inspection, they
also have lobes, but these are tightly folded
round the main haustorial body, giving it a
globose appearance (Bushnell & Gay, 1978).
A second distinguishing feature of B. graminis