GENETIC-VARIATION FOR VIRULENCE AND RESISTANCE IN THE WHEAT-MYCOSPHAERELLA GRAMINICOLA PATHOSYSTEM .1. INTERACTIONS BETWEEN PATHOGEN ISOLATES AND HOST CULTIVARS

Genetic variation for virulence in 63 Mycosphaerella graminicola isola tes, originating from 13 countries, was studied in two seedling experi ments. Each experiment was performed according to a partially balanced incomplete block design with four replications over time. The first e xperiment put emphasis on M, graminicola isolates that originated from bread wheat, and comprised 50 isolates that were inoculated on a set of testers containing 19 bread wheat cultivars, four durum wheat culti vars, and one triticale cultivar. In the second experiment more attent ion was paid to M. graminicola isolates that originated from durum whe at, and comprised 15 isolates that were inoculated on a set of testers containing 17 durum wheat cultivars, four bread wheat cultivars, one triticale cultivar, and a Triticum turgidum subsp. dicoccoides accessi on. Two disease parameters, the presence of necrosis (N) and pycnidia (P) estimated as percentages of primary leaves, were employed to measu re disease severity. Genetic variation for virulence in the pathogen i solates and genetic variation for resistance in the host cultivars wer e estimated by analyses of covariance. The significance of cultivar x isolate interactions in both experiments and for each disease paramete r suggested a gene-for-gene interaction between resistance and virulen ce loci in host and pathogen, respectively. An agglomerative hierarchi cal clustering procedure, that used one df component of interaction be tween isolates and cultivars as a proximity measure, was employed to s tudy the similarity between isolates and cultivars. Discrepancies betw een N and P resulted in nonidentical clusters of isolates and cultivar s when considering these parameters separately, which suggested that N and P were under different genetical control. Evidently, isolates of M. graminicola were specialized to either bread wheat or durum wheat. This was particularly evident when considering P. It is proposed, ther efore, to designate two varieties in M. graminicola that refer to the host species specialization in this pathogen.