Introduction to Fungi, Third Edition

628 UREDINIOMYCETES: UREDINALES (RUST FUNGI)
per hectare is sufficient to spark an epidemic.
Even if no lesions are seen on winter wheat, the
rust may still be present, surviving as latent
infections. Crop losses can be severe in the cooler
climates of North-Western Europe and North
America where winter cereals, especially winter
wheat, are widely grown (Stubbs, 1985). Like
most other rusts, P. striiformis is controlled
mainly by resistance breeding (Line, 2002). Chen
(2005) has given a detailed account of recent
epidemics and migrations of P. striiformis.
22.4.3 Puccinia hordei (barley leaf rust or
‘dwarf rust’)
This macrocyclic rust alternates between
Ornithogalum spp. (Hyacinthaceae) as the alternate
host and cultivated and wild barley (Hordeum)
species as principal host. Clifford (1985) considered
this species to be the most important rust on
barley. The uredinial lesions are brownish in
colour and are smaller than those of other rusts,
hence the name ‘dwarf rust’. In agricultural
situations, the alternate host is probably of little
importance as the fungus can overwinter in the
uredinial state on volunteer plants or winter
barley. Its optimum temperatures for infection
(15°C) and sporulation (10 20°C) are almost as
low as those for P. striiformis. In Europe in the
1970s, P. hordei caused major crop losses because
the cultivars sown were highly susceptible.
However, the situation seems to have improved
in recent years, and several cultivars showing
good partial resistance have been bred (Niks et al.,
2000).
22.4.4 Puccinia coronata (crown rust of oats
and forage grasses)
This rust is so named because of the spiny
extensions at the apex of its teliospore (see
Fig. 22.13a). It is the most damaging disease on
oats and has probably been a pathogen since oats
were first cultivated (Simons, 1985). In addition to
oats, numerous other grasses are infected, including
important forage grasses such as the perennial
ryegrass, Lolium perenne, for which separate
resistance breeding programmes have been
initiated (Kimbeng, 1999). Heavily infected
meadows can acquire a distinctly orange hue
due to the urediniospores being produced in
great abundance. There are no formae speciales
other than f. sp. avenae on oats because the
delimitation between strains on different grasses
is not clear cut. Puccinia coronata is macrocyclic,
alternating with Rhamnus (buckthorn) shrubs.
As with P. graminis and P. triticina, a standardized
nomenclature for identifying races of P. coronata
f. sp. avenae has been proposed. This is based on
a differential comprising 16 oat cultivars carrying
single resistance (Pc) genes (Chong et al., 2000).
In total, about 100 resistance genes are known,
and resistance breeding is the main strategy to
control oat rust. Heavy infections of susceptible
cultivars can result in total crop failure.
Even well-known rust species can harbour
surprises upon close inspection, and one of these
was the description of a form of P. coronata
infecting Bromus inermis as its principal host.
Anikster et al. (2003) described that germinating
teliospores of this form produced only two
basidiospores, each of which had four instead
of the usual two nuclei and was self-fertile, i.e.
infection on Rhamnus cathartica gave rise to aecia
without spermogonia. This fungus is therefore
homothallic, like the correlated microcyclic
P. mesnieriana (Anikster & Wahl, 1985) and probably
the barley form of P. coronata. In contrast,
P. coronata f. sp. avenae is heterothallic with a
tetrapolar mating system, i.e. two mating type
loci with two alleles each, instead of the usual
bipolar system (Narisawa et al., 1994).
22.4.5 The origin of cereal rusts
It is generally assumed that the cereal rusts
evolved at the centre of origin of the wild grasses
which were the progeny of cultivated cereals.
The argument is strengthened if the alternate
host is native to the same area. This topic has
been discussed extensively by Anikster and Wahl
(1979) and Wahl et al. (1984). Puccinia graminis is
thought to have evolved with Berberis vulgaris,
which is of Asian origin, moving westwards to
an area from Transcaucasia to the Western
Mediterranean, which is the centre of origin of
wheat and rye. Westward migration must have
occurred early in the history of agriculture
because 3300-year-old P. graminis-infected cereal