The avirulence of Erysiphe graminis f.sp. agropyri, Ak-1, on Triticum aestivum 'Norin 4' and 'Norin 10' and T. compactum 'No.44' is conditioned by four genes; three operate singly against each cultivar and one operates against all three cultivars. If the forma specialis – genus specificity follows the gene-for-gene relationship, four major genes should be involved in the resistance of the three cultivars to Ak-1, one carried only by 'Norin 4', one carried only by 'No.44', one carried only by 'Norin 10', and one carried by all three cultivars. The first and second genes were considered to be the previously reported genes Pm10 and Pm11, respectively. The third and fourth genes were successfully detected using F1 hybrid cultures between Ak-1 and E. graminis f.sp. tritici, Tk-1. They were located on chromosomes 6B and 7D and designated Pm14 and Pm15, respectively. These results strongly support the assumption that the forma specialis – genus specificity follows the gene-for-gene relationship. It is, therefore, concluded that this type of specificity belongs to cultivar specificity rather than plant-species specificity and that the resistance to inappropriate formae speciales is essentially cultivar resistance and not nonhost resistance.Key words: powdery mildew, Erysiphe graminis, wheat, wheatgrass, resistance.
Host range and forma specialis of cocksfoot powdery mildew fungus (Erysiphe graminis DC.) found in Japan.
