Introduction to Fungi, Third Edition

642 USTILAGINOMYCETES: SMUT FUNGI AND THEIR ALLIES
Fig 23.6 TEM of Ustilago hordei infecting
a compatible barley cultivar.The host cell
consists of a large vacuole (V) surrounded
by a thin layer of cortical cytoplasm.
The intracellular fungal hypha (F) has
penetrated the plant cell wall (PW) and
invaginated the host plasmalemma (single
arrow); note the thick electron-dense
sheath (Sh) between the hyphal wall and
the host plasmalemma.The double arrow
points to vesicles in the intercellular space
(IS). Photomicrograph taken by G. Hu;
reprinted from Hu et al.(2003),with
permission from Elsevier.
Fig 23.7 Loose smuts. (a) Infection of Ustilago avenae on oats.
All seeds on the affected inflorescence have been replaced by
teliospore sori. (b) Ustilago tritici causing loose smut on wheat.
A healthy wheat ear is shown on the left.
with each other again at a later stage (Nielsen,
1988). In U. tritici there are also no sporidia,
but here each uninucleate cell of the septate
promycelium gives rise to a germ tube, and the
dikaryophase is established by fusion of these
germ tubes (Malik, 1974). Ustilago filiformis has
teliospores which, on germination, do not
produce an obvious promycelium, but merely a
short tube from which sporidia are budded off
successively (Fig. 23.5). All of the above fusion
events ultimately result in a dikaryotic hypha
which infects the host plant.
In contrast to U. maydis, many small-grain
cereal smuts entertain a gene-for-gene relationship
with their hosts. In incompatible interactions,
the infection hypha is encased in an
unusually thick sheath as soon as it penetrates
the first epidermal cell. This is closely followed
by the death of the infected cell through a
hypersensitive response (Hu et al., 2003). The
breeding of resistant cultivars can be an
effective means to control these pathogens
(Smith et al., 1988). As we have already seen in
the rust fungi (p. 625), the evolution of numerous
races of smut fungi causes difficulties in
cultivating cereals carrying major resistance
genes. The spread of these races can be monitored
in time and space by screening field
isolates against a differential of cereal cultivars
(Menzies et al., 2003). Distinct smut races show a
high tendency to hybridize under natural conditions,
so that new races can arise rapidly
(Thomas, 1984).