Phytophthora megasperma and P. nicotianae causing root and crown rot in sesame fields in Iran and information on cultivar resistance

Phytophthora root and crown rot is one of the most destructive diseases that affecting on the growth and yield of sesame in tropical and subtropical regions. The aim of this investigation was to identify Phytophthora species causing root and crown rot in the main sesame-producing regions (Fars, Khuzestan, Bushehr and Kohgiluyeh & Boyer-Ahmad) of Iran. In total, 55 Phytophthora isolates were obtained and identified as P. nicotianae (50 isolates) and P. megasperma (5 isolates). To confirm morphological identification of the Phytophthora species, phylogenetic analysis was performed for 23 isolates by sequencing of beta-tubulin and 28S rDNA genes. The phylogenetic results confirmed 18 isolates from clade 1 of Phytophthora grouped with P. nicotianae, and five isolates from clade 6 of Phytophthora making a monophyletic group with P. megasperma. The pathogenicity of six isolates of P. nicotianae and two isolates of P. megasperma were evaluated on seven sesame cultivars using a greenhouse trial during two years (2020-2021). Based on the results, P. nicotianae was the most aggressive species, causing severe root and crown rot and mortality of sesame seedlings 60 days after inoculation. In addition, in almost all Phytophthora positive plants, the aerial symptoms and especially the general condition (mortality percentage) of sesame seedlings corresponded with the severity of damage to the root system. In the present study, we found cultivars with high resistance and suitable for cultivation in areas infested with P. nicotianae and P. megasperma. To the best of our knowledge, this is a new report of P. megasperma on sesame worldwide.