Suppression of aggressive strains of 'Candidatus phytoplasma mali' by mild strains in Catharanthus roseus and Nicotiana occidentalis and indication of similar action in apple trees.

To study antagonistic interactions of 'Candidatus Phytoplasma mali' strains, graft inoculation of Catharanthus roseus and Nicotiana occidentalis was performed with mild strains 1/93Vin and 1/93Tab as suppressors and three aggressive strains as challengers. Inoculation of the suppressors was carried out in either the cross-protection modus prior to grafting of the challengers or by co-inoculating suppressors and challengers. Monitoring using multiplex real-time polymerase chain reaction assays revealed that, in long-term cross-protection trials with C. roseus, suppressor 1/93Vin was present in all root and randomly collected stem samples over the entire observation period. In contrast, the challengers were never detected in such stem samples and rarely in the roots. Following simultaneous inoculation, the suppressor successively colonized all stem and root regions whereas detection of challenger AT steadily decreased. However, this strain remained detectable in up to 13 and 27% of stem and root samples, respectively. The cross-protection trials with N. occidentalis yielded results similar to that of the cross-protection experiments with C. roseus. Comparison of the symptomatology of infected apple trees with the presence of putatively suppressive strains indicated that suppression of severe strains also occurs in apple. Phylogenetic analysis using a variable fragment of AAA+ ATPase gene AP460 of 'Ca. P. mali' revealed that suppressors 1/93Vin and 1/93Tab, together with several other mild strains maintained in apple, cluster distantly from obviously nonsuppressive strains that were predominantly highly virulent.