Virulence of Canadian isolates ofUstilago tritici: 1964–1998, and the use of the geometric rule in understanding host differential complexity

Abstract

Knowledge of changes in race distribution and composition in a pathogen population is important for developing host breeding strategies and understanding the biology of the pathogen. The virulence of isolates of Ustilago tritici collected during the period of 1964-1998 from across Canada, but mostly from Manitoba and Saskatchewan, was assessed by inoculation onto a differential host series. Individual isolates of the pathogen were inoculated onto 2 (prior to 1978) or 3 (after 1977) differential hosts if they were collected from durum wheats, or 15 differential hosts if they were collected from hexaploid wheats. A total of 609 isolates were assessed for virulence: 261 collected from durum wheats and 348 from hexaploid wheats. Races T32 and T33 were the most commonly identified from durum wheats and races T5 and T10 the most common from hexaploid wheats. The relative frequencies of durum wheat isolates of U. tritici possessing virulence to the durum wheat differentials ranged from 10% on ‘Pentad’ (TD-11) to 95% on ‘Mindum’ (TD-1) and, for the hexaploid wheat isolates, the relative frequencies of virulence to the hexaploid differentials ranged from 0% on 'Sonop' (TD-14) to 95% on ‘Reward’ (TD-7). During the course of this study, some races were detected, which had not previously been reported in Canada or, in some instances, anywhere. The majority of these races did not represent the detection of new virulences, but rather new virulence combinations. A comparison of the reactions of the different races of U. tritici with the differential host series, using Person's geometric rule, indicated that the durum differential hosts likely contain different single genes for resistance, while the hexaploid differential hosts appear to be multigenic.