Introduction to Fungi, Third Edition

THE ‘TRUE’ SMUT FUNGI (USTILAGINOMYCETES)
649
overwintering plants under snow cover from soilborne
teliospores (Purdy et al., 1963). In contrast
to other Tilletia spp., spores dusted onto the seed
surface are unimportant as inoculum because
germination of winter wheat seeds precedes
teliospore germination by a few months. The
disease caused by T. controversa is called ‘dwarf
bunt’ because infected plants show stunted
growth. Like T. caries, T. controversa has a genefor-gene
relationship with its host, and breeding
for resistance is an efficient control method
(Fuentes-Dávila et al., 2002). The life cycle of
T. controversa is similar to that of T. caries, except
that T. controversa has a bipolar multiallelic
mating system whereas that of T. caries is bipolar
with only two mating type alleles.
Tilletia controversa is cosmopolitan but is not in
itself a serious pathogen. However, it has
acquired notoriety by being used as the reason
for establishing import restrictions on wheat
imports from the United States by China during
the 1970s to 1990s (Mathre, 1996).
Tilletia indica
The disease caused by T. indica is called Karnal
bunt, named after the city in India where it was
first described (Mitra, 1931). It is also called
partial bunt because the teliospore sori often fill
only part of the infected wheat grain, leaving the
embryo unaffected. In contrast to T. caries and
T. controversa, T. indica does not appear to grow
systemically. Instead, teliospores germinate by
producing numerous (up to 180) haploid primary
needle-shaped sporidia from the tip of the
aseptate promycelium. The primary sporidia
germinate by monokaryotic haploid hyphae,
and these in turn produce a further crop of
haploid monokaryotic secondary sporidia. Two
types may be produced, namely a repetition of
the needle-shaped form or a sausage-shaped
sporidium liberated actively by the surfacetension
catapult mechanism. In contrast to
T. caries and T. controversa, H-shaped fusion cells
are not formed and both types of secondary
sporidium are monokaryotic and haploid.
Secondary sporidia can germinate to produce a
mycelium giving rise to further sporidia of either
type (Dhaliwal, 1989). The actively liberated form
enables the fungus to work its way up on the
outside of the leaves of growing wheat plants
until it reaches the flag leaf. There, fusion
of compatible monokaryotic haploid hyphal
segments may occur, with the resulting heterokaryotic
mycelium causing infections of individual
florets of the immature wheat ear (Goates,
1988; Nagarajan et al., 1997).
Occurrence and severity of Karnal bunt have
increased in high-yielding crop systems, but the
disease is not in itself serious, causing crop losses
less than 1% per annum even in severe epidemics
(Nagarajan et al., 1997). In addition, several
resistant cultivars are available. Although bunt
balls contain trimethylamine, flour made from
crops with up to 4% infected grains is still fit for
human consumption (Fuentes-Dávila et al, 2002).
As in the case of T. controversa, the major threat
posed by T. indica is a legal one. Countries as yet
free from the disease may ban wheat imports
from those affected by T. indica. For example, the
United States imposed quarantine regulations
to prevent the spread of Karnal bunt to North
America, only to find bans imposed on US
exports by other countries when T. indica was
eventually discovered in Arizona in 1996 (Palm,
1999). Rush et al. (2005) have given a fascinating
account of the complex interactions between
agriculture, politics, international trade and
research in dealing with Karnal bunt in the
United States, concluding that the threat posed
by this disease was initially overstated.
23.2.7 Urocystis
The distinguishing feature of the genus Urocystis
is that the teliospore consists of one or more
melanized fertile cells surrounded by several
sterile cells (Figs. 23.11, 23.12). This type of
compound teliospore is called a spore ball.
There are about 140 species (Vánky, 1994). An
important species is U. tritici which causes leaf
stripe-smut or flag smut on wheat in warm
climates. This was formerly called U. agropyri, a
name now applied in a more restricted sense to
forms on wild grasses such as Agropyron and
Elymus spp. (Fig. 23.11). The fertile cell of a spore
ball of Urocystis germinates by producing an
aseptate promycelium which gives rise to about
four primary sporidia (Fig. 23.12). These sporidia