Host-parasite interactions in tan spot [Pyrenophora tritici-repentis] of wheat

Tan spot of wheat, caused by the ascomycete Pyrenophora tritici-repentis, is an economically important disease in all the major wheat growing areas worldwide. Even though the pathogen was known to occur on grasses and episodically on wheat for more than eight decades, large-scale epidemics of tan spot were first recorded in the early 1970s. The increased incidence was associated with stubble retention, a practice implemented in the context of soil conservation. The present review highlights some of the recent developments that have occurred in studies of the wheat - P. tritici-repentis interaction and discusses the implications for our understanding of host-parasite relations in general. Races of P. tritici-repentis produce at least three host-specific toxins, effective on particular host lines or cultivars. Single dominant and independently inherited genes control host reaction to these toxins, with one gene for each toxin. Eight races of the pathogen have now been identified from collections made in several parts of the world, accounting for all virulence patterns expected from three toxins matching three "susceptibility" genes in the host. Genetic analyses of host and pathogen suggested that a one-to-one relationship existed in the wheat - P. tritici-repentis interaction. The model described for tan spot of wheat appears to be a mirror image of the classical gene-for-gene in that it is based on compatibility. Thus, it is proposed that the gene-for-gene model can be extended, as previously predicted by others, to pathosystems involving multiple host-specific toxins. The relationship between toxin production and the evolution of new races is also discussed.