Introduction to Fungi, Third Edition

BLUMERIA GRAMINIS
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Fig13.3 An example of the use of scanning electron microscopy in fungal taxonomy, separating Blumeria graminis (a,b) from other
members of the Erysiphales (c,d). (a) Whole conidium of B. graminis showing the spiny surface of the wall. (b) The‘annular’ septum
of the conidium.The actual septum is raised and warty, but it is surrounded by a depressed annulus. (c) Conidium of Golovinomyces
(Erysiphe) cynoglossi showing a slightly roughened wall. (d) Conidium of Neoerysiphe (Erysiphe) galeopsidis with its striate conidial wall.
Micrographs kindly provided by R.T. A.Cook. Reprinted from Cook et al. (1997), British Crown copyright1997.
The genus Blumeria contains only one species,
B. graminis, which is separable into several
formae speciales distinguishable by their different
grass (Agropyron, Bromus, Poa) or cereal hosts
(barley, oat, rye, wheat). Thus, B. graminis f. sp.
tritici infects wheat (Triticum) but not barley,
whilst B. graminis f. sp. hordei infects barley
(Hordeum) but not wheat. Blumeria graminis is heterothallic
and hybrids between certain formae
speciales may arise, especially if the hosts themselves
can hybridize. For example, it has been
shown that hybridization between B. graminis ff.
spp. agropyri, tritici and secalis can occur, resulting
in viable ascospores. The three host genera
Agropyron (couch grass), Triticum (wheat) and
Secale (rye) can also hybridize (Hiura, 1978).
Recently, it has become obvious that the definition
of formae speciales is less clear-cut than previously
thought. Extensive overlaps in host spectra
occur especially on wild hosts in the Middle East
where Hordeum spontaneum, the presumed ancestor
species of cultivated barley, is endemic and
where B. graminis probably originated (Clarke &
Akhkha, 2002; Wyand & Brown, 2003). Further,
within a given forma specialis, numerous races
can be distinguished by specific features such
as resistance to fungicides or ability to infect
a given host cultivar.
13.3.1 The infection process
A conidium of B. graminis alighting on the surface
of a grass or cereal leaf will initially come to rest
by the tips of its spines. Within seconds of contact
with hydrophobic (but not with hydrophilic)
surfaces, an extracellular matrix is released by
those spines touching the surface. The matrix
contains proteins, including enzymes such as
cutinases and non-specific esterases, and it may
serve in the initial attachment of the conidium to
the surface (Carver et al., 1999; Nielsen et al., 2000).