Introduction to Fungi, Third Edition

610 UREDINIOMYCETES: UREDINALES (RUST FUNGI)
genetic adaptability by fragmenting into several
forms which infect non-overlapping spectra of
host species and thereby become reproductively
isolated. Because of the identity of the host
family as an additional taxonomic feature, species
of rust fungi are relatively easily grouped
into genera which are often monophyletic. At the
higher taxonomic level, two large groups can be
resolved weakly by phylogenetic analyses (Maier
et al., 2003; Wingfield et al., 2004). They coincide
with the Pucciniaceae and Melampsoraceae of
earlier concepts (Dietel, 1928) in which these two
families were distinguished by their teliospore
or probasidium being stalked (Pucciniaceae) or
unstalked (Melampsoraceae). Another point of
distinction is that a particular spore-producing
structure, the aecial stage (see below), is produced
on angiosperm hosts by Pucciniaceae but
on gymnosperms by Melampsoraceae (Wingfield
et al., 2004). Dietel’s two broad groups are usually
split up into about 13 14 smaller families
(Kirk et al., 2001; Cummins & Hiratsuka, 2003),
but since the boundary lines are still being
re-drawn from time to time, we prefer to adhere
to the original concept for the purposes of this
book.
The popular name ‘rust fungi’ refers to the
reddish-brown colour of some of the spores
which are produced in dense pustules on crop
plants, giving them a ‘rusted’ appearance. This
is especially true of the rusts on cereals, which
have probably caused crop losses since the beginning
of agriculture. Archaeological excavations
have uncovered rusted cereal remains dating
back to the Bronze Age (Kislev, 1982), and it is
well known that the ancient Romans held a
special festival, the Robigalia, to appease their
rust gods. This took place on 25 April, i.e. at a
time when the crop was particularly vulnerable
to attack (Large, 1940). The Robigalia may be the
origin of Rogation Sunday, a day of blessing of
the crops which is still observed by some
Christian churches every year in late April
(Schuman, 1991).
Rusts can also cause serious economic damage
on non-cereal crops, and examples are given in
later sections of this chapter. Because of the
immense economic importance of rust fungi,
an enormous body of literature has been written,
and the wheat Puccinia graminis system is probably
the most thoroughly examined of all
host pathogen interactions involving fungi.
None the less, no substantial integrated treatment
of the rust fungi has appeared in the past
two decades, the two-volume set on cereal rusts
(Bushnell & Roelfs, 1984; Roelfs & Bushnell, 1985)
having been the last major effort. However, good
keys and species descriptions are available, e.g.
in Grove (1913), Gäumann (1959), Wilson and
Henderson (1966) and Cummins and Hiratsuka
(2003).
22.2.1 The basic life cycle of rusts
The classical example of a rust fungus, Puccinia
graminis, is the cause of black stem rust on wheat
and other cereals (Fig. 22.1), which is described
more fully in Section 22.3. The life cycle of rusts
is homologous with that of the Homobasidiomycetes
(Fig. 18.4) in being divided into stages of
primary (homo- and mono-karyotic) and secondary
(hetero- and di-karyotic) mycelium. The
heterokaryotic phase is the main period in the
life cycle of rusts and it is the only one in which
some rusts can survive indefinitely under suitable
conditions. The host plant species on which
this stage is produced is therefore termed the
principal host, with that bearing the homokaryotic
mycelium called the alternate host.
On leaves of the principal host, P. graminis
produces urediniospores in pustules called
uredinia, and they rapidly spread the infection
because they are capable of re-infecting the same
host species. Urediniospores are produced on
stalks from which they break off at maturity,
being released and distributed passively by wind.
Urediniospores of rust fungi have a relatively
thick wall which is often pigmented and typically
spiny. They are capable of surviving airborne
for several weeks or months, which accounts for
their long-distance dispersal sometimes over
hundreds or even thousands of miles. Each
urediniospore is binucleate, with the two nuclei
being of opposite mating type. In temperate
climates, uredinia of many rusts are gradually
replaced by telia in autumn, especially on leaf
sheaths and stems. Teliospores (¼ probasidia)
of Puccinia are two-celled and thick-walled.