A review on subgroup classification of phytoplasmas enclosed in group 16Sr IV

Abstract

The ribosomal grouping and subgrouping system provides a means for the provisional classification of phytoplasmas. Within this system, the group 16SrIV mainly includes phytoplasma strains associated with lethal yellowing diseases. While subgroups 16SrIV-A, -B, -C and -D are well established and formally recognized in the scientific literature, the same cannot be said for subgroup 16SrIV-E. At least two different groups of sequences have been proposed to designate a 16SrIV-E subgroup, neither of which is included in the database of the widely used online tool for phytoplasma classification known as iPhyClassifier. This confusion lead to difficulties in identifying new strains, subgroup-specific host listing errors, and could result in the omission or duplication of previous work. The designation of a reference sequence for subgroup 16SrIV-E is thereby proposed. In addition, palm-pathogenic phytoplasma diversity in the Americas is discussed, and evidence is provided for new 16SrIV subgroups to be established to accommodate sequences reported from Jamaica and Mexico that cannot be classified into any of the previously reported subgroups.