'Candidatus Phytoplasma cynodontis', the phytoplasma associated with Bermuda grass white leaf disease

Bermuda grass white leaf (BGWL) is a destructive phytoplasmal disease affecting Cynodon dactylon, first reported in Taiwan and now found across Asia, Sudan, and Italy. The disease causes chlorosis, shoot proliferation, stunting, and plant death. This study examined BGWL isolates from multiple geographic regions to determine if they represent a distinct species. Phylogenetic analysis of 16S rDNA sequences showed BGWL strains were identical or nearly identical, with the Australian cynodon white leaf (CWL) strain differing by only one to three positions. Analysis of the 16S–23S rDNA spacer region, serological comparisons, vector transmission specificity, and host-range testing all supported separation of the BGWL agent as a distinct taxonomic entity. The BGWL phytoplasma possessed a unique 21-nucleotide signature sequence and displayed no serological cross-reactivity with the related sugarcane white leaf phytoplasma. RFLP analysis consistently distinguished BGWL from more distantly related phytoplasmas. Based on these molecular and biological markers, the authors propose designating the BGWL pathogen as a novel species: 'Candidatus Phytoplasma cynodontis,' with strain BGWL-C1 selected as the reference strain.

Key findings

• BGWL isolates from Italy, Thailand, Sudan, and Indonesia showed identical or nearly identical 16S rDNA sequences, with Australian CWL strain differing by only 1-3 nucleotide positions, demonstrating high strain homogeneity
• The BGWL phytoplasma possesses a unique 21-nucleotide signature sequence (5'-AATTAGAAGGCATCTTTTAAT-3') not found in any other organism, providing molecular differentiation
• Multiple molecular markers beyond 16S rDNA—including 16S–23S rDNA spacer sequences, serological comparisons, vector transmission specificity, and host-range testing—supported designation as a distinct species despite <2.5% divergence threshold
• The phytoplasma causes a 530 kilobase pair chromosome, representing the smallest known mollicute genome and smallest self-replicating organism genome reported to date
• Related phytoplasmas from brachiaria white leaf, carpet grass white leaf, and date palm diseases showed nearly identical sequences but insufficient taxonomic data for formal classification