Introduction to Fungi, Third Edition

23
Ustilaginomycetes: smut fungi and their allies
23.1 Ustilaginomycetes
The Ustilaginomycetes are one of the four main
classes of Basidiomycota and contain about 1500
species (Kirk et al., 2001). In its present form as
circumscribed by Begerow et al. (1997) and Bauer
et al. (1997, 2001), this group is monophyletic.
Hypha-producing Ustilaginomycetes are united
by their lifestyle as ecologically obligate plant
pathogens, often with an additional free-living
(saprotrophic) yeast phase. They can be distinguished
from the rust fungi in that haustoria are
either altogether absent or, where present, take
the shape of simple intracellular hyphae or
hyphal extensions which invaginate the host
plasmalemma but are not differentiated into a
narrow neck and a wider haustorial body.
Further, intracellular hyphae of Ustilaginomycetes
usually secrete a thick sheath which is
readily visible by transmission electron microscopy
(see Figs. 23.6 and 23.17). The septa either
lack perforations or contain simple pores or
dolipores which are similar to those in the
Urediniomycetes in lacking parenthesomes.
True clamp connections are not usually found.
The basidia of smut fungi produce numerous
basidiospores whereas those of rust fungi usually
produce only four.
The class Ustilaginomycetes has been divided
into three subclasses by Begerow et al. (1997,
2000). We shall consider representatives of two
of these. The Ustilaginomycetidae (Section 23.2)
are the most important plant-pathogenic
Ustilaginomycetes, causing smut-like symptoms.
Typical members of the Exobasidiomycetidae
(Section 23.4) cause other biotrophic diseases
and are distinguished from the former by
producing basidia directly from parasitic
mycelium, not from teliospores. An exception
is Tilletia, which is so similar to smuts in the
Ustilaginomycetidae that we discuss it alongside
them (p. 650). The third group, Entorrhizomycetidae,
contains fungi which produce galls on the
roots of Cyperaceae and Juncaceae (Vánky, 1994).
Numerous Ustilaginomycetes live exclusively
or predominantly as yeasts, and an example of
this lifestyle is the order Malasseziales described
on p. 670.
Fungi causing smut-like disease symptoms
(Fig. 23.7, Plate 12h) have arisen at least
twice within the Basidiomycota, namely in the
Ustilaginomycetes and the Urediniomycetes.
Urediniomycetous smuts are found in the Microbotryales,
and since these share many biological
features with the ‘true’ smuts in the Ustilaginomycetes
and have traditionally been studied by
smut specialists, we shall consider the Microbotryales
in this chapter (Section 23.3). Therefore,
in our usage the term ‘smut fungus’ has a
biological rather than a taxonomic meaning.
23.2 The‘true’ smut fungi
(Ustilaginomycetes)
The word ‘smut’ describes the causal fungus or
the symptoms of a particular group of plant
diseases in which loose masses (sori) of dark