Differences in strain virulence of the European stone fruit yellows phytoplasma and susceptibility of stone fruit trees on various rootstocks to thispathogen

Twenty strains of the European stone fruit yellows (ESFY) phytoplasma showe d great differences in virulence when examined by graft inoculation of tree s on peach, peach hybrid GF 677 and P. 'Marianna' GF 8/1 rootstocks. The mo st virulent strains killed all trees on peach rootstocks whereas the mild s trains did not cause mortality but induced only mild foliar symptoms and sl ightly reduced vigour. Virulence often depended on the pathogen-scion combi nation and was in several cases most severe when the scion consisted of the original host of the pathogen. To examine resistance in stone fruits, tree s on a total of 23 rootstocks were inoculated with the ESFY strains. Trees on the Prunus domestica stocks Ackermann's, Brompton and P 1275 and on Prun us cerasifera stock Myrabi were little affected. Slightly more damage occur red in trees on rootstocks GF 677, GF 8-1, and the P. insititia stocks St J ulien A and St Julien GF 655/2. Ishtara, P. cerasifera stock Myrobalan, and peach rootstocks Higama and GF 305 were shown to be moderately susceptible and a high susceptibility was found in trees on peach rootstocks Montclar, peach seedling, Rutgers Red Leaf, and Rubira, on apricot seedlings and St Julien 2. Of flowering cherry trees on various rootstocks, the least suscep tible were those on Gisela 3 and F 12/1 whereas Gisela 1, Weihroot 158 and Gisela 5 were more affected. Phytoplasmas were detected by either DAPI (4 ' -6-diamidino-2-phenylindole) staining or polymerase chain reaction in all rootstocks and scions tested. However, detection frequency and phytoplasma concentrations were usually lower in the more tolerant hosts than in suscep tible genotypes.