Virulence changes in Puccinia graminis f. sp. tritici in Canada

Abstract

Virulence differences on “standard” Puccinia graminis tritici differentials are not chronologically related and do not form evolutionary series. Chronologically related virulence differences between strains of "standard" race 15B-1L on "single-gene" differentials form an evolutionary series of single-step virulence changes, while virulence differences between strains of "standard" race groups 11-32, 17-29, and 56-125 do not. Most virulence changes were from avirulence to virulence, or vice versa, but on varieties with resistance genes Sr7a and Sr11 intermediate virulence occurred, which suggests that there was more than one kind of genetic change. There was no indication of an overall increase or decrease in the number of virulence genes present in all races, although virulence on genes Sr7a and Sr5 has increased. Random virulence changes on single host resistance genes, probably the result of mutations, seem to occur frequently and may be the most common kind of change in North America under present conditions. They permit the rust to overcome single-gene resistance, but they are not very effective in overcoming complex resistance. The stability of complex resistance in North America suggests that other processes capable of producing a wider range of variability occur infrequently in nature.