Population structure of Puccinia recondita in western Europe during 1995, as assessed by variability in pathogenicity and molecular markers

The population structure of Puccinia recondita f. sp. tritici (Prt) in west ern Europe was examined by assessing variability in pathogenicity and in ra ndomly amplified polymorphic DNA (RAPD) among 61 single uredinial isolates. The isolates were chosen to represent pathotypes detected in a previous su rvey of pathogenic variability in the fungus in western Europe in 1995. Thi rty-five pathotypes were identified by assessing infection types produced b y the 61 isolates on 24 differential lines, each with a single gene for res istance to Prt. In contrast, only 18 RAPD phenotypes were identified by sco ring 19 polymorphic RAPD bands generated with eight RAPD primers. When anal ysed by cluster and bootstrap analyses, the pathogenicity and RAPD results revealed little evidence for robust distinct clusters among the isolates. M ultiple isolates of several pathotypes collected from widely separated loca tions such as Belgium, Germany, France, Italy and Switzerland had the same RAPD phenotype, providing evidence of clonal migration over considerable di stances in western Europe. Some variability tone or two band differences) w as observed in RAPD phenotype within several pathotypes, indicating the pos sible occurrence of genetic changes independent of pathogenicity, and/or th e independent development of pathotypes with different genetic backgrounds. Two groups of isolates identified in the 1995 survey, differentiated by pa thogenicity for genes Lr3a, Lr3bg, Lr3ka and Lr30, were not distinguished b y RAPD phenotype, indicating that the groups probably do not constitute sep arate lineages within the pathogen population. Little correlation was appar ent between the polymorphisms observed in pathogenicity and RAPD phenotypes . The similarity in the genetic backgrounds of the isolates, as assessed by RAPD markers, suggest that the observed differences in pathogenicity may h ave arisen by selection for specific virulences corresponding to genes for resistance in wheat cultivars grown in the region. Three isolates of pathot ype 3, restricted in its distribution to southern France during 1995, were distinct from all other isolates in RAPD phenotype. Circumstantial evidence suggests that this pathotype originated from northern Africa, and that it belongs to a group of leaf rust pathogens specialized to durum wheats.