Evolution of pathotypes of Puccinia helianthi on sunflower in Australia

Sunflower rust caused by Puccinia helianthi is a major problem in sunflower production in Australia. The disease can be effectively controlled through the use of resistant hybrids but the commercial life of these is often sho rt, due to the evolution of new pathotypes of the fungus. Since 1978, 23 pa thotypes have been recognised, mostly from commercial crops. Possible pathw ays for the evolution of these pathotypes are proposed. Almost all pathotyp es identified since 1986 trace to a common progenitor. Aus4. Avirulence/vir ulence patterns of many pathotypes suggest that sexual recombination may be acting to generate new pathotypes, but the sexual stages of the fungus are rarely seen in the field. It is assumed that bursts of evolution occur in seasons that favour completion of the sexual cycle, and that mutation contr ibutes steadily to the development of new virulence genes in the population of P. helianthi.