Introduction to Fungi, Third Edition

THE ‘TRUE’ SMUT FUNGI (USTILAGINOMYCETES)
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entire infected host organs or galls may be
dispersed, and teliospores or mycelium of smut
fungi may be borne on or in seeds.
The most common dispersal route in the
smut fungi is by wind-blown teliospores, and
the transatlantic wind-borne passage of African
sugarcane smut, Sporisorium scitamineum, has
been proposed as the most likely route of entry
into the Caribbean. Teliospores of many smuts
may adhere to the seeds of their host plants
and germinate with them. Certain species causing
loose smuts of cereals (see p. 639) systemically
infect the living embryo of the cereal grain.
Grains contaminated by external spore dusts or
systemic infections can be spread by human
transport. Insect dispersal is important in the
anther smut, Microbotryum violaceum, which is
related to the rust fungi (see p. 652).
Fig 23.2 Teliospore surfaces of Ustilago spp. (a) Smooth
surface in U. hordei. (b) Spiny surface in U. nuda.From
Va¤nky (1994); original prints kindly provided by K.Va¤nky.
23.2.3 Teliospore release and dispersal
Piepenbring et al. (1998c) have written an
excellent review of the teliospore as a dispersal
unit in a wide range of smut fungi, and we can
only summarize a few salient features here.
Teliospores of most species are produced in a
sorus enclosed by a thin layer of host tissue. They
are generally released dry. Spore release is often
by the simple rupture of the host epidermis
surrounding the sorus, possibly as a result of
pressure exerted by the expanding teliospores.
The host epidermis may aid in the release of
teliospores, e.g. if it is hit by water drops. The
mechanism in this case is similar to that found
in puffballs (see p. 578). Gusts of wind which
shake infected host organs may also be effective
in releasing smut spores over time. Alternatively,
23.2.4 Ustilago species on
monocotyledonous hosts
Several Ustilago spp. cause diseases on grasses
and cereals. Sori of the agriculturally most
important species, U. hordei, U. nuda, U. tritici
and U. avenae, are produced in place of the
developing seeds. Other Ustilago spp. affect the
leaves of their hosts, e.g. U. filiformis (formerly
U. longissima), which causes leaf stripe smut
on Glyceria spp. Ustilago maydis infects both
vegetative and reproductive organs of its host
(Plate 12h), causing gall-like deformations. This
species is considered in detail on pp. 643 647.
Many Ustilago spp. have also been described from
dicotyledonous hosts, but it now seems that all
of them belong to Microbotryum (Vánky, 1998,
1999; Almaraz et al., 2002; see p. 652).
Phylogenetic studies have shown U. hordei,
U. nuda, U. tritici and U. avenae to be closely
related to each other, to the point where it
becomes difficult to distinguish individual
species by DNA sequences and microscopy. They
are regarded as the core species of Ustilago (Stoll
et al., 2003). Since these species can hybridize
with each other, it has been proposed that they
should be merged into one taxon, U. segetum
(see Bakkeren et al., 2000). However, because
the existing names are so well-established and
because well-known plant diseases are caused by