Discovery of Thecaphora schwarzmaniana on Rheum ribes in Iran and Turkey: implications for the diversity and phylogeny of leaf smuts on rhubarbs

A leaf smut discovered on wild rhubarb Rheum ribes in Iran and Turkey is evaluated to identify the species. Based on morphological analyses using light and scanning electron microscopy and molecular phylogenetic analyses using ITS and LSU ribosomal DNA sequences, it is shown that the smut fungus belongs to Thecaphora schwarzmaniana, a rare species previously known on Rheum cordatum, R. lobatum, R. macrocarpum, and R. turkestanicum in some central Asian countries. This is the first documented case of Thecaphora schwarzmaniana on Rheum ribes and significantly expands the range of the species on the Asian continent. The ITS and LSU sequences of the Iranian and Turkish material differed from sequences of Thecaphora schwarzmaniana from Rheum palmatum from China. The re-study of the morphology of the respective specimen revealed morphological differences allowing the conclusion that Thecaphora on Rheum palmatum belongs to a distinct species. The new species is not described here due to the paucity of available material. Molecular phylogenetic analyses, based on ITS and LSU sequences, recovered Thecaphora on Rheum spp. in a clade including species on host plants from diverse unrelated families, indicating that multiple inter-family host jumps took place in the course of evolution of Thecaphora species. Vast similarity of ITS and LSU sequences of Thecaphora spp. on Rheum spp. point to their close relation suggesting that co-evolution could shape the diversification of Thecaphora species on Polygonaceae following an initial host jump from another host plant family.