Molecular Plant Pathology – Pathogen Profiles

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

SummaryStem 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. Bot60, 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.

Taxonomy:  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.

Host range:  The host range of P. graminis is very broad compared with that of most Pucciniaspp.; 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.

Disease Symptoms: 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.

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.