‘Candidatus Phytoplasma omanense’, associated with witches’-broom of Cassia italica (Mill.) Spreng. in Oman Nadiya Abubakar Al-Saady, 1 Akhtar Jamal Khan, 1 Alberto Calari, 2 Ali Masoud Al-Subhi 1 and Assunta Bertaccini 2 Correspondence Akhtar Jamal Khan ajkhan@squ.edu.om 1 Department of Crop Sciences, College of Agricultural & Marine Sciences, Sultan Qaboos University, PO Box 34, Al-Khod 123, Oman 2 Department of Agroenvironmental Sciences (DiSTA), Plant Pathology, Alma Mater Studiorum University of Bologna, I-40127 Bologna, Italy Samples from plants of Cassia italica exhibiting typical witches’-broom symptoms (Cassia witches’-broom; CWB) were examined for the presence of plant pathogenic phytoplasmas by PCR amplification using universal phytoplasma primers. All affected plants yielded positive results. RFLP analyses of rRNA gene products indicated that the phytoplasmas detected were different from those described previously. Phylogenetic analysis of 16S rRNA gene sequences confirmed that CWB represents a distinct lineage and shares a common ancestor with ‘Candidatus Phytoplasma phoenicium’. Molecular comparison revealed that the 16S rRNA gene sequences of the four CWB strains (IM-1, IM-2, IM-3 and IM-4) identified in symptomatic C. italica samples were nearly identical (99.6–100 % similarity). The closest relatives were members of the pigeon pea witches’-broom phytoplasma ribosomal group (16SrIX; 95–97 % sequence similarity). On the basis of unique 16S rRNA gene sequences and biological properties, the phytoplasma associated with witches’-broom of C. italica in Oman represents a coherent but discrete novel phytoplasma, ‘Candidatus Phytoplasma omanense’, with GenBank/DDBJ/EMBL accession number EF666051 representing the reference strain. Phytoplasmas are plant-pathogenic, unculturable, wall-less bacteria that are associated with a wide variety of economically important plants (McCoy et al., 1989; Seemu ¨ller et al., 2002). In nature, phytoplasmas are transmitted by homopterous insect vectors (Chiykowski & Sinha, 1989). Traditionally, phytoplasma diseases have been determined on the basis of the host range and characteristic symptoms induced in the plant hosts. Molecular analyses are now used increasingly to identify and differentiate phytoplasmas. Phylogenetic analysis of 16S rRNA gene sequence has revealed that phytoplasmas constitute a coherent genus of the Mollicutes (Lee et al., 1998; Seemu ¨ller et al., 2002). Currently, they are divided into 18 groups and more than 40 subgroups based upon RFLP analysis of 16S rRNA gene sequences, and a total of 24 members of ‘Candidatus Phytoplasma’ have been proposed based on 16S rRNA gene phylogeny (IRPCM Phytoplasma/Spiroplasma Working Team – Phytoplasma Taxonomy Group, 2004; Arocha et al., 2005; Valiunas et al., 2006; Lee et al., 2006). Italian senna [Cassia italica (Mill.) Spreng.], previously known as Cassia obovata (Collad.) or Cassia aschrek (Forsk.), commonly found throughout the Arabian Peninsula, is a wild plant species belonging to the family Fabaceae. It is native to Africa (Sudan) and produces yellow flowers in bunches that become black when opened completely. The seeds are poisonous to animals and plant extracts are used for medical and cosmetic purposes. In this study, we describe a phytoplasma disease of a new host-plant species, C. italica, in Oman. Evidence is also presented to demonstrate that the causal phytoplasmas of C. italica witches’-broom constitute a novel candidate species, for which the name ‘Candidatus Phytoplasma omanense’ is proposed following the guidelines defined by the International Committee on Systematic Bacteriology (now the International Committee on Systematics of Prokaryotes) for provisional classification of incompletely described micro-organisms (Murray & Stackebrandt, 1995). Abbreviation: CWB, Cassia witches’-broom. The GenBank/EMBL/DDBJ accession numbers for the sequences of the 16S rRNA gene and the 16S–23S rRNA gene spacer region of ‘Candidatus Phytoplasma omanense’ strains IM-1, IM-2, IM-3 and IM-4 are EF666051–EF666054, respectively. Details of reference phytoplasmas used in RFLP analysis are available as supplementary material with the online version of this paper. International Journal of Systematic and Evolutionary Microbiology (2008), 58, 461–466 DOI 10.1099/ijs.0.65425-0 65425 G 2008 IUMS Printed in Great Britain 461