The Diaporthe sojae species complex: Phylogenetic re-assessment of pathogens associated with soybean, cucurbits and other field crops

• The Diaporthe species associated with soybean, cucurbits, and other field crops were re-evaluated.
• The limits of Diaporthe sojae species complex were determined within the genus.
• Phylogenetic placement of the soybean pathogens, D. longicolla and D. sojae, and D. phaseolorum were clarified.
• Molecular data and morphological characters of several significant pathogenic Diaporthe species are provided.

Phytopathogenic species of Diaporthe are associated with a number of soybean diseases including seed decay, pod and stem blight and stem canker and lead to considerable crop production losses worldwide. Accurate morphological identification of the species that cause these diseases has been difficult. In this study, we determined the phylogenetic relationships and species boundaries of Diaporthe longicolla, Diaporthe phaseolorum, Diaporthe sojae and closely related taxa. Species boundaries for this complex were determined based on combined phylogenetic analysis of five gene regions: partial sequences of calmodulin (CAL), beta-tubulin (TUB), histone-3 (HIS), translation elongation factor 1-α (EF1-α), and the nuclear ribosomal internal transcribed spacers (ITS). Phylogenetic analyses revealed that this large complex of taxa is comprised of soybean pathogens as well as species associated with herbaceous field crops and weeds. Diaporthe arctii, Diaporthe batatas, D. phaseolorum and D. sojae are epitypified. The seed decay pathogen D. longicolla was determined to be distinct from D. sojae. D. phaseolorum, originally associated with stem and leaf blight of Lima bean, was not found to be associated with soybean. A new species, Diaporthe ueckerae on Cucumis melo, is introduced with description and illustrations.