Phylogenetic and Morphological Identification of the Novel Pathogen of Rheum palmatumLeaf Spot in Gansu, China

AbstractA new leaf spot disease was observed on leaves of Rheum palmatum(Chinese rhubarb) in Northwest China (Gansu Province) starting in 2005. A Septoria-like fungus was isolated and completion of Koch’s postulates confirmed that the fungus was the casual agent of the leaf spot disease. Morphology and molecular methods were combined to identify the pathogen. The fungus produced conidiomata pycnidia and the conidia were 2~5 septate, 61.2~134.1 μm in length and 3.53~5.3 μm in width, which is much larger than the known Spetoriaspecies that infects Polygonaceae species. Phylogenic analysis of the internal transcribed spacer region confirmed that this Septoria-like fungus is within the Septoriagenus but distinct from known Septoriaspecies. Together, these morphological and phylogenetic data support that the R. palmatuminfecting Septoriastrain is a newlydescribed plant pathogenic species.