Introduction to Fungi, Third Edition

THE ‘TRUE’ SMUT FUNGI (USTILAGINOMYCETES)
651
Teliospores of U. tritici survive in the soil and
attached to seeds (Fuentes-Dávila et al., 2002).
Urocystis agropyri is common on wild grasses, and
another common species is U. anemones which
causes a leaf smut on Anemone and Ranunculus
leaves.
23.2.8 Control of smut diseases
Control of loose and covered smuts presents very
different problems. Whilst the surface of the
grain is merely contaminated with the spores of
covered smuts, in the case of loose smuts the ripe
grain is already infected by a mycelium within the
embryo. The control of covered smuts by means
of fungicidal seed dressings is therefore simple,
and it is standard practice for seed grains to be
treated by seed merchants in this way. The first
effective seed treatment was the steeping of seed
grains in a dilute copper sulphate solution prior
to sowing (Large, 1940). In the middle of the
twentieth century, seed dressings containing
organic mercury compounds were widely used,
but these are now banned due to their high
general toxicity. Instead, cocktails of fungicides
are employed. In most countries with a welldeveloped
agriculture, bunt of wheat is now a
rare disease. For example, the incidence of bunt
balls in seed samples sent to the Official Seed
Testing Station at Cambridge fell from 12 33%
in 1921 1925 to 0.2 0.3% in 1955 1957
(Marshall, 1960).
Control of the loose cereal smuts was impossible
until the Danish plant pathologist Jens
Ludwig Jensen invented the hot water treatment
method in the 1880s (Large, 1940; Ainsworth,
1981). This was based on the observation that
Ustilago spp. are less heat-tolerant than cereals,
and infected seeds could be effectively disinfected
by soaking them first in cold water for
5 h followed by a dip in hot water; 10 min at
54°C for wheat and 15 min at 52°C for barley
(Fischer & Holton, 1957; Ainsworth, 1981).
Today, effective fungicidal seed dressings are
available and these usually combine systemic
fungicides such as carboxin (Fig. 23.13) or its
derivatives with protectant fungicides such
as captan, maneb or pentachloronitrobenzene
(Kulka & von Schmeling, 1995). Such seed
dressings control loose and covered smuts, in
addition to numerous other fungal diseases.
Since infection of next season’s grain with
loose smuts occurs at flowering, one obvious
method of control is to inspect crops grown for
seed at flowering time and to assess the
incidence of smutted heads. In these so-called
seed certification schemes, only crops which
contain fewer than a defined limit, e.g. one
smutted ear in 10 000 ears, are approved for use
as seed stocks (Doling, 1966). It is also possible to
detect the presence of loose smut mycelium
within the embryos by microscopic examination
(Morton, 1961) or PCR-based methods (Pearce,
1998). The latter can also detect the spores of any
other fungus of interest on the seed coat,
including covered smuts such as T. caries,
T. controversa and T. indica. PCR-based methods
are being developed rapidly, partly because
of the need for rapid testing of exported or
imported agricultural produce for contamination,
and partly for the purpose of thwarting
bioterrorist attacks (Schaad et al., 2003).
If all else fails, many systemic fungicides,
e.g. sterol demethylation inhibitors (see p. 410),
are effective against the systemic smuts when
sprayed onto the growing crop. In practice, these
fungicides are often applied to control infections
caused by non-smut cereal pathogens, with the
suppression of Ustilago spp. as an additional
bonus which often goes unnoticed by the
farmer (Jones, 1999).
Although many crop plants and their wild
relatives possess resistance genes against smut
fungi, smut-resistant cultivars are less important
in an agricultural context than those with
resistance against other biotrophic pathogens,
e.g. rusts (see pp. 625 and 627) or powdery
Fig 23.13 Molecular structure of carboxin, a systemic
fungicide commonly applied as a seed dressing.Carboxin acts
by inhibiting the enzyme succinate dehydrogenase (complex II)
in fungal mitochondrial respiration (see Uesugi,1998).