Introduction to Fungi, Third Edition

THE ‘TRUE’ SMUT FUNGI (USTILAGINOMYCETES)
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to its own toxin. In contrast to the S. cerevisiae
system where the unprocessed toxin precursors
bestow resistance, in toxin-producing U. maydis
strains resistance is encoded by nuclear genes.
Resistance is specific, i.e. a KP1-producing strain
is sensitive to KP4 and KP6. All three toxins are
also effective against other members of the
Ustilaginales (Koltin & Day, 1975).
Attempts have been made to transform
wheat plants with the KP4 toxin gene of the
U. maydis virus P4 in order to engineer cultivars
with resistance against Tilletia caries (Clausen
et al., 2000), but no recent reports seem to have
been published on this subject. Even if cultivars
with good resistance in the field can be
produced, it is unlikely that a transgenic crop
plant expressing a calciseptine-like toxin will
gain acceptance with regulatory authorities or
the general public.
23.2.6 Tilletia
There are about 125 species of Tilletia, all of which
parasitize grasses (Poaceae). In economic terms,
the most important pathogens are T. caries
(¼ T. tritici) and T. indica on wheat, T. controversa
on wheat and other cereals, and T. horrida on rice.
Many other Tilletia spp. infect wild grasses (Vánky,
1994). Teliospores of Tilletia spp. typically bear
reticulate surface ornamentations (Fig. 23.9).
They are long-lived in the soil and on the exterior
of seeds. Infections are systemic and result in
covered smut symptoms in the seeds.
past, but are much less frequent now because the
incidence of common bunt has declined.
Crushed sori have a fishy smell caused by the
presence of trimethylamine. For this reason, the
disease is also known as ‘stinking smut’, and
flour made from contaminated grain is unfit for
human consumption. Since teliospores are not
readily released in the field, Piepenbring et al.
(1998c) have suggested that T. caries may be an
example of a fungus that has adapted to humans
as a dispersal vector.
This fungus was a major pathogen in the past
and occupies a special place in the history of
plant pathology (Large, 1940; Ainsworth, 1981).
In 1752, Mathieu Tillet, by careful experimentation,
demonstrated that the common bunt
disease was associated with dusting the seeds
with bunt spores prior to sowing. He also
reported that incidence of the disease was
somewhat reduced by steeping the seeds in
sea water and lime prior to sowing. In 1807,
Bénédict Prévost observed the process of teliospore
germination with his microscope, and he
proposed that the bunt disease was caused by
a living organism. Further, he discovered that
teliospore germination was inhibited by copper
salts, and that the treatment of wheat seeds with
dilute copper sulphate solution was effective
against infections of T. caries. The combination of
copper and lime became known as the famous
Bordeaux mixture only much later, in about
Tilletia caries
This species is cosmopolitan and causes ‘common
bunt’, the best-known covered smut disease of
wheat. The entire interior of the infected grain
becomes converted to a greenish-brown teliospore
sorus surrounded by the pericarp. Such sori
are called ‘bunt balls’. The teliospores are not
released until threshing, when they are dusted
onto the surface of healthy grains. If heavily
contaminated crops are processed, the teliospore
concentrations in the air can be sufficiently high
to cause dust explosions in mills or storage
facilities. Respiratory allergies among millers
caused by T. caries were also common in the
Fig 23.9 SEM of teliospores of Tilletia caries.FromVa¤nky
(1994); original prints kindly provided by K.Va¤nky.