Overview on a century progress in research on sesame phyllody disease

Abstract

Sesame is the most ancient oil seed plant cultivated in the tropics of Asia and South Africa. Among the major constraints, phyllody associated with phytoplasma presence is the major limiting factor affecting its cultivation firstly described more than 100 years ago. Almost all the sesame varieties under cultivation are susceptible to the disease that continues to cause severe economic losses. Different symptomatology is present: the most common symptoms are phyllody, flower virescence and witches’ broom affecting growth and yield. Several detection methods like Diene's stains and TEM techniques were developed for the detection of phytoplasmas in phloem sieve tissues, however the PCR based assays were established for the identification of the phytoplasmas associated with the disease. After rRNA sequencing and restriction analyses, a wide genetic diversity was identified among sesame phyllody associated phytoplasmas: four ribosomal groups 16SrI, 16SrII, 16SrVI and 16SrIX were detected. The disease spreads in nature by different leafhopper species, among them Orosius orientalis was reported as the major vector. Moreover many weed species are reported hosts of the sesame phyllody-associated phytoplasmas. No effective control measures of the disease were developed, exceptresistance, management of insect vectors andaltering the dates of sowing to avoid peaks of insect vector population. So far no resistant genotype of sesame is available. However, attempts have been made to confer resistance against the phyllody disease in sesame through intra-and interspecific crosses of two wild species Sesamum mulayanum and S. alatum. The review reports uptodate information about history, geographic distribution, symptomatology, diagnosis, genetic diversity, epidemiology and management of sesame phyllody disease.