gene-for-gene relationshiptypical of biotrophic plant
pathogens, discussed later in this chapter.
Smut fungi
The smut fungi are members of the Basidiomycota
and gain their name from the fact that they produce
millions of black, sooty spores, known as teliospores.
Over 1000 species of smut fungi are known to para-
sitize plants, but the species of most economic import-
ance are those that attack cereal crops, including loose
smut of wheat (Ustilago nuda; Fig. 14.17), maize smut
(Ustilago maydis), and stinking smut of wheat (Tilletia
caries), so-named because the spores have a fishy smell
due to the presence of trimethylamine. The charac-
teristic feature of smut fungi is that they develop
slowly within the plant tissues, often producing only
small amounts of hyphal growth, but later in the
growing season they produce large numbers of diploid
teliospores, which develop in place of the cereal grain
or other reproductive structures. For example, in loose
smut of cereals (Fig. 14.17) the whole flowering spike
is replaced by a mass of spores, which are exposed
when a thin sheath which contains these spores
breaks down at maturity. Stinking smut is similar, but
the grains appear to be normal because they are sur-
rounded by the seed coat (pericarp) and the spores are
only released when the grain is threshed.
Plants become infected by loose smut when germin-
ating spores penetrate the developing ovary, so that
the resulting seeds already contain the mycelium.
Then, when the seeds germinate the fungus progres-
sively colonizes the stem tissues and eventually enters
the flowering spike where it replaces the grain with a
spore mass. Infection by stinking smut is somewhat
different: the spores can survive for up to 15 years,
and they germinate at the same time as the grain,
penetrate the base of the shoot (coleoptile), and then
progressively colonize the tissues, leading finally to
the production of a spore mass. In other smuts the
process of development is different again. For example,
in Uromyces anemones(a smut fungus that grows on
anemone and buttercup) the spore masses develop
beneath the surface of the stem or leaf before they
mature and erupt through the plant epidermis.
When the teliospores germinate they undergo
meiosis to produce a haploid phase termed the
promycelium. This produces uninucleate sporidia,
equivalent to basidiospores. Then plasmogamy (cell
fusion) of sporidia of compatible mating types gives rise
to a dikaryon, and this will eventually produce the mass
of teliospores. An interesting feature of smut fungi
is that the dikaryotic phase seems to be obligately
parasitic on plants, but the monokaryotic, uninucleate
sporidia multiply as yeast-like budding cells. These are
nonpathogenic and can be cultured on standard labor-
atory media.FUNGI AS PLANT PATHOGENS 297
Fig. 14.17(a,b) Loose smut of wheat caused by Ustilago nuda. At maturity the flowering spikes produce a mass of
black spores in place of the seeds. The image in the center compares a smutted flowering spike with a normal spike.
(c) Warty, ornamented spores (smut spores) about 8μm diameter.
(a) (b) (c)