'Candidatus Phytoplasma spartii', 'Candidatus Phytoplasma rhamni' and 'Candidatus Phytoplasma allocasuarinae', respectively associated with spartium witches'-broom, buckthorn witches'-broom and allocasuarina yellows diseases

This study describes three novel phytoplasma species associated with witches'-broom and yellows diseases in woody shrubs. Spartium witches'-broom (SpaWB) affects Spanish broom in Italy and Spain, buckthorn witches'-broom (BWB) affects buckthorn in Germany, and allocasuarina yellows (AlloY) affects Slaty she-oak in Australia. The researchers analyzed 16S rDNA sequences and restriction fragment length polymorphisms (RFLP) to characterize these phytoplasmas—non-culturable, wall-less prokaryotes related to other plant-pathogenic mollicutes. The three phytoplasmas share less than 97.5% 16S rDNA sequence similarity with each other and known phytoplasmas, meeting criteria for novel species designation. Though most closely related to apple proliferation group phytoplasmas, they show distinct differences in restriction sites, spacer region sequences, and host specificity. Based on these unique molecular properties and different natural plant hosts, the authors formally propose designating these agents as three new 'Candidatus' species: 'Candidatus Phytoplasma spartii,' 'Candidatus Phytoplasma rhamni,' and 'Candidatus Phytoplasma allocasuarinae.'

Key findings

• Three novel phytoplasma species were identified and formally designated as 'Candidatus' species based on less than 97.5% 16S rDNA sequence similarity to other known phytoplasmas
• Each phytoplasma has a unique natural host plant and distinct geographical distribution: SpaWB in Spain/Italy, BWB in Germany, and AlloY in Australia
• Molecular characterization using 16S rDNA sequencing and RFLP analysis revealed unique restriction sites and spacer region differences distinguishing the three novel species from each other and from related apple proliferation group phytoplasmas
• Host range testing indicates each phytoplasma is specifically associated with its respective plant host with no cross-infection detected