Molecular characterization of ‘Candidatus Phytoplasma asteris’ subgroup I-B associated with sesame phyllody disease and identification of its natural vector and weed reservoir in India

Abstract

An incidence of 12 to 35 % sesame phyllody (SP) disease was observed in sesame fields at Kushinagar and Gorakhpur districts in Uttar Pradesh and Indian Agricultural Research Institute, New Delhi, India during July-October 2013. The association of phytoplasma with sesame phyllody plants at all the three locations and a weed, Sclerocarpus africanus Jacq (SA) showing little leaf and witches’ broom symptoms at Gorakhpur was confirmed by direct and nested PCR amplification of phytoplasma 16S rDNA. Out of four predominant feeding leafhopper species in the symptomatic sesame fields, viz. Empoasca prima (Distant), Exitianus indicus (Distant), Hishimonus phycitis (Distant) (HP) and Cofana unimaculata (Signoret), only H. phycitis was found to be associated with phytoplasma in nested PCR assays with phytoplasma universal primer pair R16F2n/R16R2. Transmission test and population dynamics study further confirmed that H. phycitis was the proven natural and potential vector to transmit the SP phytoplasma from diseased to healthy sesame plants in transmission assays. BLAST analysis of 1.25 kb 16SrDNA partial sequences of nested PCR products obtained from symptomatic SP plants, weed (SA) and the leafhopper (HP) revealed 99 % sequence identities among themselves and 99 % identity with other reported strains of ‘Ca. P. asteris’ (16Sr I group). Phylogenetic analysis also suggested the closest phylogenetic relationship of SP, SA and HP phytoplasmas with those of ‘Ca. P. asteris’ group. RFLP analysis of R16F2n/R16R2 primed 16S rDNA sequences of SP, SA and HP phytoplasma isolates using iPhyClassifier online tool with 17 selected restriction enzymes confirmed the association of ‘Candidatus Phytoplasma asteris’ subgroup B with SP, SA and HP in India. The identity of natural vector and new alternative host of SP phytoplasma in the present study suggested that these may act as potential/natural source for secondary spread of sesame phyllody phytoplasma.