Introduction to Fungi, Third Edition

BLUMERIA GRAMINIS
399
Fig13.5 Interpretation of the
fine structure of the haustorium
of B. graminis f. sp. hordei
(redrawn from Bracker,1968b).
(a) Section of host leaf at point
of penetration.The body of the
haustorium (bo) containing
asinglenucleus(nu)lies
immediately beneath the point
of penetration.The body of the
haustorium is enclosed in a sheath
with extensive matrix (MAT).The
sheath membrane (SM) is close
to the host tonoplast (T).
The sheath membrane bears
invaginations (SI). A single lobe
(lo) of another haustorium
enclosed in an extension of the
sheath is shown to the left of the
diagram, and four lobes enclosed
in a common sheath to the right.
(b) Enlargement of the neck of a
haustorium. Note the thickened
collar (C) deposited on the host
cell wall (W).The sheath
membrane (SM) is continuous
with the host plasmalemma (E).
Other abbreviations: XW
(cross-wall),CH (channel),
H (host cytoplasm).
The difference is that other fungi are capable of
hydrolysing sucrose externally by the activity of
their own secreted invertases.
13.3.5 Life cycle and epidemiology of
B. graminis
About 7 10 days after infection by B. graminis, the
haploid surface mycelium can produce conidia
under field conditions. Conidia develop from
a flask-shaped foot cell within which mitotic
nuclear division occurs. The foot cell elongates
away from the host leaf and a cross-wall cuts off
the hyphal tip. Further cross-walls develop so that
a chain of cells is formed, increasing in length
at its base by further divisions of the foot cell.
Such conidia are said to be catenate (Fig. 13.2d).
Each conidium is uninucleate. In successful infections,
a dense stand of conidiophores is produced
so that the lesion appears as a white powdery
pustule (Fig. 13.6a). Aust (1981) has estimated
that a single pustule can release about 1.5 10 4
conidia day 1 . A high spore inoculum can be built
up very quickly, and numerous infection cycles
can occur during one growing season. Dispersal
of powdery mildew conidia is mainly by wind,
whereby short gusts of wind are ideal for spore