Structure and pathogenic variability in Ascochyta rabiei populations on chickpea in the Canadian prairies

A population study of Ascochyta rabiei from the Canadian prairies was conducted to assess pathogenicity among isolates with the objectives to investigate the existence of a race or pathotype structure and to evaluate whether there had been a shift to higher aggressiveness between 1998 and 2002. Ninety-nine isolates collected in 1998, 2001 and 2002 were inoculated onto seven differential chickpea genotypes. Significant isolate x differential interactions occurred, but accounted for a small proportion of the total variability. It was found that very few interaction effects between all combinations of differentials and isolates were significant and frequency distributions of disease severity of isolates tested on the differentials revealed continuous distributions. These results suggest that no genotype-specific relationship existed between A. rabiei and its host and that Canadian populations of the pathogen cannot be objectively classified into races or pathotypes. Isolates from 2001 and 2002 caused significantly more disease than isolates from 1998, suggesting that disease epidemics encountered since 1999 were in part caused by a shift in the population to higher aggressiveness. © 2008 The Authors.