Introduction to Fungi, Third Edition

630 UREDINIOMYCETES: UREDINALES (RUST FUNGI)
that he was dealing with infected Sorghum
material. It is, however, a pathogen of maize
and can be encountered wherever that crop is
grown. Since maize is native to Central America,
the pathogen probably also originated there.
In Europe, crop losses are not generally sufficiently
severe to warrant control measures
(Smith et al., 1988), but elsewhere breeding programmes
are in operation and fungicides are
also occasionally used (Hooker, 1985). The fungus
produces numerous chocolate-brown uredinia
on the leaves of grain and fodder maize. It is
macrocyclic and heteroecious, alternating with
Oxalis spp.
Several macrocyclic Puccinia spp. not causing
agriculturally significant disease are frequently
encountered in the field. An example is P. caricina
which has sedges (Carex spp.) as its principal host
and produces spermogonia and aecia on
Urtica (stinging nettle). The latter are associated
with growth deformations (Plate 12b). Another
ubiquitous species is P. poarum, with grasses of
the genus Poa as principal and Tussilago farfara
(coltsfoot) as alternate hosts. This species is
unusual in completing two life cycles per
growing season, with aecia appearing in May
and August (Wilson & Henderson, 1966). Mint
rust (P. menthae) is autoecious and macrocyclic.
All the above species are abundant in Europe and
on most other continents.
Puccinia punctiformis is a systemic autoecious
rust attacking Cirsium arvense (creeping
thistle). In spring, infected plants are clearly
distinguishable by their yellowish appearance
and appressed leaves, and by the strong sweet
smell associated with numerous spermogonia
which develop all over the infected shoots.
These aromatic substances include common
fragrance molecules such as benzaldehyde,
2-phenylethanol, indole and phenylacetaldehyde
(Connick & French, 1991), and it is possible to
smell infected thistles from a distance of several
metres. All these substances are produced by
a wide range of fungi and, with the exception
of indole, also by the host plant during
flowering. The infections develop from a dikaryotic
mycelium overwintering systemically in
the rootstock. The haploid condition of the
spermogonial tissue is probably established by
de-dikaryotization as new shoots are infected in
spring. Transfer of spermatia to compatible
flexuous hyphae results in dikaryotization, and
the next structures to form resemble uredinia
with chocolate-brown spores. They are sometimes
called uredinoid aecia. The urediniospores
can infect healthy thistles and normal uredinia
develop on these. Later in the season, teliospores
develop (Buller, 1950).
A yet more specialized way to attract insect
pollinators has been developed by a complex of
rusts attacking Brassicaceae (Arabis spp.). Infected
plants produce the same fragrances as Cirsium
infected by P. punctiformis, plus many more
(Raguso & Roy, 1998). Further, infected plant
organs are morphologically modified to form
flower-like structures called pseudoflowers.
Intriguingly, these do not resemble the flowers
of the host, but those of taxonomically unrelated
plant species such as buttercups (Ranunculus spp.),
which often grow in the same habitats (Roy, 1994).
The combination of scent, shape, colour and
nectar in this floral mimicry is necessary to
attract a wide range of insects (Roy, 1994; Roy &
Raguso, 1997). Several rust species can stimulate
the production of such pseudoflowers on Arabis
spp., including the macrocyclic heteroecious
P. monoica which alternates with grasses, and
correlated autoecious and microcyclic species
(Roy et al., 1998).
22.5.2 Uromyces
Common species of Uromyces are U. ficariae, a
microcyclic species forming brown telia on
Ranunculus ficaria, and U. dactylidis with urediniospores
and teliospores on grasses (Dactylis,
Festuca and Poa) and aecia on Ranunculus spp.
Uromyces dianthi produces uredinia and telia on
Dianthus (carnation) and other ornamental flowers,
with Euphorbia spp. as the alternate host.
However, the most important species of Uromyces
are those infecting legumes (Fabaceae), and
because of the ease with which legumes such as
Pisum, Phaseolus and Medicago can be cultivated in
the laboratory, their rusts have been used for
numerous studies of fundamental physiological
aspects such as differentiation of infection
structures and haustorial functioning, which