Differences in Strain Virulence of the European Stone Fruit Yellows Phytoplasma and Susceptibility of Stone Fruit Trees on Various Rootstocks to this Pathogen

Twenty strains of the European stone fruit yellows (ESFY) phytoplasma showed great di€erences in viru- lence when examined by graft inoculation of trees on peach, peach hybrid GF 677 and P. `Mariannaʹ GF 8/1 rootstocks. The most virulent strains killed all trees on peach rootstocks whereas the mild strains did not cause mortality but induced only mild foliar symptoms and slightly reduced vigour. Virulence often depended on the pathogen±scion combination and was in several cases most severe when the scion consisted of the original host of the pathogen. To examine resistance in stone fruits, trees on a total of 23 rootstocks were inoculated with the ESFY strains. Trees on the Prunus domestica stocks Ackermannʹs, Brompton and P 1275 and on Prunus cerasifera stock Myrabi were little a€ected. Slightly more damage occurred in trees on rootstocks GF 677, GF 8±1, and the P. insititia stocks St Julien A and St Julien GF 655/2. Ishtara, P. cerasifera stockMyrobalan, 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 ¯owering cherry trees on various rootstocks, the least susceptible were those on Gisela 3 and F 12/1 whereas Gisela 1, Weihroot 158 and Gisela 5 were more a€ected. Phytoplasmas were detec- ted 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 susceptible genotypes.