Physiological specialization in the western Canadian population ofPhaeosphaeria nodorum

Stagonospora nodorum blotch (SNB) caused by Phaeosphaeria nodorum is an important foliar disease of wheat. Efficient breeding for SNB resistance requires detailed knowledge of the pathosystem so that targeted improvements can be made. The objective of this study was to examine the wheat–P. nodorum pathosystem for evidence of host specificity amongst Canadian isolates. To this end, 49 P. nodorum isolates were inoculated on 16 wheat lines in indoor seedling inoculation experiments. Biologically significant host × pathogen interactions were identified, indicating that host specificity exists in the Canadian isolates tested. This observation is consistent with an inverse gene-for-gene pathosystem based upon host-selective toxins. Unweighted Pair Group Method with Arithmetic Mean (UPGMA) analysis of disease reaction data suggested nine virulence clusters; however, additional sub-groupings could be argued based upon the reaction of specific wheat lines to isolates within virulence clusters. The common wheat lines 86ISMN 2137, RL5407 and W7984 had good resistance to the P. nodorum isolates in seedling tests, which will be useful for improvements in common wheat and possibly durum wheat. The development of a SNB differential set is discussed. P. nodorum isolates characterized in this study will be useful for future studies in the wheat–P. nodorum pathosystem.