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The purpose of monitoring cereal rust pathogens is to provide a basis for disease control strategies that include breeding for resistance, predicting disease response in commercial cultivars and responding to the dynamics of pathogen change. The means of achieving this vary from regular collection surveys based on assessments of sample collections in greenhouse tests, to monitoring and recording static trap plots. Factors governing the method of approach include the size of the target region, the available research resources and the experience of staff involved. This paper is a brief review of the development of near-isogenic lines as a means of monitoring cereal rust pathogens. Emphasis will be given to wheat stripe/ yellow rust and the development and application of a near-isogenic set of materials based on the spring wheat cultivar Avocet. This parent was selected because of its high degree of susceptibility to the disease, agronomic adaptability (semidwarf, spring habit, moderate vernalisation and day-length requirements), and resistance to stem rust. The relative benefits of using these materials will be discussed in the context of available data.
Approximately nine million ha of wheat (Triticum aestivum and T. durum) were sown annually in the Southern Cone of South America (Argentina, Brazil, Chile, Paraguay and Uruguay) during 2003-2007. Presently, leaf rust (caused by Puccinia triticina) is the most important rust of wheat throughout the region. The pathogen population is extremely dynamic leading to short-lived resistance in commercial cultivars. Leaf rust management relies on the use of resistant cultivars and fungicides. Sources of adult plant resistance conferred by minor additive genes have been increasingly used in breeding programs to obtain cultivars with more durable resistance. Stripe rust (P. striiformis f. sp. tritici) is endemic in central and southern Chile, where fungicides are required to control the disease on susceptible cultivars. Stem rust (P. graminis f. sp. tritici) has not caused widespread epidemics in the last 25 years due to the use of resistant cultivars. Virulence to Sr24 and Sr31, the most important genes conferring resistance to local races, has not been reported in the region. The areas sown with cultivars susceptible to local races in Argentina and Uruguay have increased in recent years. Since most varieties sown in the region are susceptible to Ug99 or derived races, testing and selection for resistance in Kenya, facilitated by the Borlaug Global Rust Initiative, is highly relevant for research aimed at preventing epidemics, which may occur if these races migrate, or are accidentally introduced to our region. The resistances identified in east Africa will also contribute to increasing the levels of resistance to current local races.
The first system describing physiologic specialization in the cereal rust fungi was that by Stakman and Levine (1922) for the wheat stem rust pathogen. Thirty seven biologic forms or “races” were identified using 12 differential wheat lines. Since then, additional variability in physiologic specialization was found and several systems evolved to describe this variation using numbers, letters, or combinations of both. This led to difficulties in comparing races, most often because of differences in the system that is used and the differential lines employed. A system that describes virulence succinctly and allows easilymade comparisons between races is highly desirable. Additionally, differential lines should be monogenic or near-isogenic so that virulence is classified on a genetic basis. Wherever near-isogenic stocks are used, it is vital that the recurrent parent is included. The systems that appear to be best suited to describing virulence with the above parameters are the letter-code and octal nomenclature. Of these, the letter-code system is the most commonly used based on a survey of research scientists working on stem rust. Thus, the letter-code system that uses 20 differential host lines is proposed to describe the nomenclature of Puccinia graminis f. sp. tritici on a worldwide basis. In addition, the source seedstock line for each differential gene is provided.
Race (pathotype) surveys of cereal rust pathogens have been conducted in many parts of the world since the early 1900s. The only way to identify rust pathotypes remains virulence testing in greenhouse tests using genotypes (“differentials”) carrying different resistance genes. Virulence determinations have rarely targeted genes conferring adult plant resistance because of the technical difficulties of working with adult plants under controlled conditions. Where pathotype surveys have been conducted in a robust and relevant way, they have provided both information and pathogen isolates that underpinned rust control efforts, from gene discovery to post-release management of resistance resources. Information generated by pathotype surveys has been used to: devise breeding strategies; indicate the most relevant isolates for use in screening and breeding; define the distribution of virulence and virulence combinations; allow predictions of the effectiveness/ ineffectiveness of resistance genes; and issue advance warning to growers by identifying new pathotypes (both locally evolved and introduced) before they reach levels likely to cause significant economic damage. To be most effective, pathotype surveys should also provide fully characterized isolates (defined pathotypes) for use in identifying new sources of resistance and screening breeding material. Although constrained to some extent by a lack of markers, particularly those not subject to natural selection, surveys have also provided considerable insight into the dynamics of rust pathogen populations, including the evolution and maintenance of virulence, and migration pathways, including periodic long-distance migration events.
The rusts have been ongoing problems for wheat production probably since domestication of the crop about 8,000 years ago. Epidemics vary in size and frequency with host genotype and environment, wet years being ‘rust’ years. Although partial control in modern agriculture was achieved with resistant varieties, conditions favoring epidemics were made worse with the intensification of production and greater resistance gene uniformity in the host. The current Ug99 incident illustrates the situation of very widely adapted successful genotypes grown across huge areas in the presence of an ongoing threat from a recently emerged widely virulent and obviously highly aggressive pathotype of the stem rust pathogen. This paper addresses some of the history of cereal rusts and reviews underlying principles of host pathogen genetics, some of which are being neglected in the period of modern genetics.