Introduction to Fungi, Third Edition

13
Hymenoascomycetes: Erysiphales
13.1 Introduction
The Erysiphales are a clearly defined, monophyletic
order of about 500 species, all of which are
obligately biotrophic pathogens of plants. Braun
(1987, 1995) has provided thorough monographic
treatments of this group, including descriptions
of most known species. There is only one family,
the Erysiphaceae. The species grouped here cause
symptoms readily recognized as ‘powdery mildews’
because the conidia produced in abundance
on the shoots of infected host plants give
them a whitish powdery appearance. The term
‘powdery mildews’ is often also applied to the
organisms causing them. Only angiosperms are
attacked, almost always belonging to the dicotyledons.
One notable exception is the powdery
mildew of cereals and grasses caused by Blumeria
graminis (formerly Erysiphe graminis). Other
economically relevant species are Podosphaera
leucotricha causing apple mildew, Podosphaera
(formerly Sphaerotheca) mors-uvae causing
American gooseberry mildew, and Uncinula necator
(now Erysiphe necator) causing the powdery
mildew of grapes. Many other species produce
less destructive infections and are ubiquitous in
nature, e.g. Microsphaera alphitoides (now Erysiphe
alphitoides) on the leaves of oak (Quercus spp.) and
Phyllactinia guttata on hazel and many other
broad-leaved trees. Being obligate biotrophs,
powdery mildews cannot be kept in axenic
culture, although Arabi and Jawhar (2002) have
recently grown B. graminis on agar augmented
with shredded barley leaves. The possibility of
infecting Arabidopsis thaliana with several different
powdery mildew species holds promise for
future investigations (Vogel & Somerville, 2002).
In the Erysiphales, the mycelium generally
consists of uninucleate haploid segments. It is
almost always confined to the leaf surface, and
infections are limited to the epidermal cells
which are penetrated from the outside following
the formation of appressoria. Inside the host cell,
haustoria are formed which provide a large area
of contact with the host. Only relatively few
powdery mildews (e.g. Phyllactinia spp.) are able
to penetrate more deeply into the host tissue.
Either way, soon after infection conidia are
produced at the infected leaf surface from a
foot cell, either in basipetal chains or singly as in
Erysiphe if a conidium is released before the next
one is formed. Conidial states of the Erysiphales
are referable to the anamorphic genus Oidium,
barring a few specialized genera such as the
Ovulariopsis state of Phyllactinia. A division of
Oidium into subgenera has been proposed (Cook
et al., 1997; Braun et al., 2002; see Fig. 13.1). The
conidia of Erysiphales are generally described as
meristem arthroconidia because the conidiogenous
cell is not homologous to a rudimentary
phialide (Hughes, 1953). Numerous crops of
conidia can be produced in a growing season,
and they are the main carriers of infection.
The conidia of Erysiphales are unusual
because unlike most fungal spores they are
fully hydrated. Germination does not require
the uptake of exogenous water and can proceed
even in atmospheres with low relative humidity
(Somers & Horsfall, 1966). In fact, germination