Sesame phyllody disease-a potential emerging threat to sesamecultivation in the north east region of India

Abstract

Sesame phyllody disease associated with the presence of phytoplasmas is becoming a serious problem to many sesame growing countries of the world including India. However no systematic study on the disease has been done to assess the disease incidence, characterization of the associated phytoplasma strains and effect on the yield in north eastern states of India. Therefore an extensive survey was conducted from 2015 to 2017 in seven north eastern states of India covering 52 districts to record the incidence of the disease and to perform phytoplasma associated molecular characterization. The infected plants showed symptoms of virescence, phyllody, floral proliferation, flattened stem, splitting of capsules and witches’ broom. The disease incidence varied from 2% to 29% in different locations. The highest disease incidence (29%) was observed in the Sonitpur district of Assam in the variety ST-1683. Sesame phyllody infection was found to reduce the number of pod and seed yield per plant from 72.82%-100% and 57.20%-100%, respectively. A total of 96 symptomatic sesame samples were collected and analysed on extracted DNA with polymerase chain reaction using phytoplasma specific primers, amplifying 16S rRNA and secA genes. PCR and sequencing analysis confirmed the presence of ‘Candidatus Phytoplasma asteris’-related strains in all the symptomatic samples. Pairwise sequence comparison and phylogenetic analyses of 21 strains on 16S rRNA gene and 9 strains on secA gene sequences confirmed the phytoplasma classification in the aster yellows groups. Among the 7 leafhopper species collected only Hishimonus phycitis tested positive for sesame phyllody phytoplasma presence. Seven species of weeds and wavy leaf basket grass (Oplismenus undulatifolius) and flax (Linum usitatissimum) in and around the infected sesame fields were also positive for phytoplasma presence indicating them as potential reservoir hosts of the phytoplasma associated with sesame phyllody disease in the north east region of India.