Molecular Plant Pathology - Pathogen Profiles
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Stem rust of small grains and grasses caused by Puccinia
graminis
Kurt J. Leonard and Les J. Szabo
Department of Plant Pathology, University of Minnesota and United States
Department of Agriculture, Agricultural Research Service, Cereal Disease
Laboratory, St. Paul, MN 55108, USA
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| Summary
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Stem rust has been a serious disease of wheat, barley, oat and rye, as
well as various important grasses including timothy, tall fescue and
perennial ryegrass. The stem rust fungus, Puccinia graminis, is
functionally an obligate biotroph. Although the fungus can be cultured
with difficulty on artificial media, cultures grow slowly and upon
subculturing they develop abnormal ploidy levels and lose their ability to
infect host plants [Bushnell and Bosacker (1982) Can. J. Bot.
60, 18271836]. P. graminis is a typical heteroecious
rust fungus with the full complement of five distinct spore stages that
occur during asexual reproduction on its gramineous hosts and sexual
reproduction that begins in the resting spore stage and culminates on the
alternate host, barberry (Berberis spp.). There appears to be
little polymorphism for resistance/susceptibility in Berberis
species, but complex polymorphisms of resistance/susceptibility and
matching virulence/avirulence exist in gene-for-gene relationships between
small grain species and the forms of P. graminis that infect
them.
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Taxonomy:
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Puccinia graminis is a rust fungus in the phylum Basidiomycota,
class Urediniomycetes, order Uredinales, and family Pucciniaceae, which
contains 17 genera and approximately 4121 species, of which the majority
are in the genus Puccinia[Kirk et al. (2001) Ainsworth
and Bisby's Dictionary of the Fungi. Wallingford, UK: CAB
International]. Various subdivisions of P. graminis into
subspecies, varieties and formae speciales have been proposed based on
spore size and host range. Crossing studies and DNA sequence comparisons
support the separation of at least two subspecies, but not the proposed
separation based on spore size.
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| Host range:
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The host range of P. graminis is very broad compared with that of
most Puccinia spp.; it includes at least 365 species of cereals and
grasses in 54 genera [Anikster (1984) The Cereal Rusts. Orlando,
FL: Academic Press, pp. 115130]. Wheat stem rust, P. graminis
f. sp. tritici, was shown to infect 74 species in 34 genera in
artificial inoculations of seedlings, but only 28 of those species belonging to
eight genera were known to be natural hosts of the fungus. Other formae
speciales of P. graminis have narrower host ranges than P. graminis
f. sp. tritici.
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| Disease Symptoms:
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Infections in cereals or grasses occur mainly on stems and leaf sheaths,
but occasionally they may be found on leaf blades and glumes as well. The
first macroscopic symptom is usually a small chlorotic fleck, which
appears a few days after infection. About 810 days after infection, a
pustule several millimetres long and a few millimetres wide is formed by
rupture of the host epidermis from pressure of a mass of brick-red
urediniospores produced in the infection. These uredinial pustules are
generally linear or diamond shaped and may enlarge up to 10 mm long.
The powdery masses of urediniospores appear similar to rust spots on a
weathered iron surface. With age, the infection ceases production of
brick-red urediniospores and produces a layer of black teliospores in
their place, causing the stems of heavily infected plants to appear
blackened late in the season.
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Life cycle of Puccinia graminis: The asexual uredinial stage is
repeated on the grass host with a new generation of dikaryotic
urediniospores every 14-20 days under favourable conditions.
Teliospores, which begin the sexual stage of the life cycle, typically
form as the grass host matures in late summer or autumn. Karyogamy occurs
in maturing teliospores, and meiosis of the resulting diploid nucleus
begins before the teliospore enters dormancy. In spring, meiosis is
completed and teliospores germinate to produce four haploid basidiospores,
two of each mating type (+ and ). Basidiospores infect barberry, the
alternative host, on which the fungus produces haploid pycnia.
Fertilization occurs with the fusion of a pycniopspore with a flexuous
hypha of opposite mating type. Following fertilization, a dikaryotic
aecium forms and begins producing dikaryotic aeciospores, which complete
the life cycle by infecting the grass host. In regions with mild winters
and adequate summer moisture, P. graminis may persist in the
uredinial (asexual) stage on autumn-sown cereals in the winter and on
volunteer cereal plants or susceptible wild grasses.
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