Abstract
Cassava is a key food and energy crop in the tropical and subtropical regions. In South America, oversprouting disease (OVSPD) and cassava frogskin disease (CFSD) occur frequently, however, their etiologies remain unknown. OVSPD is characterized by excessive shoot proliferation, chlorosis, and dwarfism, whereas CFSD largely affects roots, causing cork-like tissues with honeycomb patterns and reduced starch content. In Brazil, phytoplasma organisms were found to be associated with both diseases, with sequences of the 16SrIII-B subgroup phytoplasmas linked to OVSPD and the 16SrIII-L and 16SrIII-A subgroups linked to CFSD. To clarify the identity and taxonomic position of the putative phytoplasma in the diseases, high-throughput sequencing (HTS) of total plant DNA from cassava plants with OVSPD and others with CFSD symptoms was performed, and the complete genome of a novel phytoplasma was assembled from OVSPD plants. In contrast, HTS from CFSD affected plants revealed short sequence fragments of diverse phytoplasmas, however, the assembly of larger scaffolds failed. Genome analyses of two OVSPD phytoplasma assemblies confirmed a novel phytoplasma species within the 16SrIII group. It has an average nucleotide identity (ANI), below the 95% species demarcation threshold relative to the genomes of other phytoplasma species and we propose the name '
Candidatus
Phytoplasma manihotii' sp. nov. A broader screening showed that this phytoplasma occurs only in OVSPD plants, suggesting that the genome fragments detected in CFSD plants may have resulted from other sources, such as genomic integration or horizontal gene transfer.