Federal Biological Research Centre for Agriculture and Forestry, Institute for Plant Protection in Fruit Crops, D-69221 Dossenheim, Germany.
Root and shoot samples from 24 symptomatic or nonsymptomatic apple trees infected with ‘Candidatus Phytoplasma mali’ were collected at different locations in Germany and France and used to inoculate rootstock M11 top grafted with cv. Golden Delicious. Inoculated trees were monitored over a 12-year period for apple proliferation (AP) symptoms and categorized as not or slightly, moderately, or severely affected. Based on symptomatology, the phytoplasma strains were defined as being avirulent to mildly, moderately, or highly virulent. Determination of phytoplasma titers by quantitative polymerase chain reaction (PCR) with DNA from roots revealed similar phytoplasma concentrations in all virulence groups. Molecular characterization of the strains by differential PCR amplification with five sets of primers resulted in 13 profiles. Six strains that were maintained in periwinkle and tobacco were molecularly characterized in more detail. The genome sizes of these strains as determined by pulsed-field gel electrophoresis using yeast chromosomes as size references ranged between 640 and 680 kb. Cleavage of the chromosome with the rare cutting restriction enzymes ApaI, BamHI, BssHII, MluI, and SmaI resulted in macro fragment patterns distinctly different in all strains. Similar results were obtained by Southern blot hybridization with three probes derived from strain AT. Differential PCR amplification at an annealing temperature of 52°C using eight primer pairs derived from strain AT revealed heterogeneity of target sequences among all strains. Based on these results, there is considerable variability in virulence and genomic traits in ‘Ca. P. mali’. However, correlations between molecular markers and virulence or phytoplasma titer could not be identified.