Etiology and Koch's Postulates Confirmation of Diaporthe phaseolorum Causing Pod Blight on Common Bean

Abstract

Pod blight symptoms observed on common bean (Phaseolus vulgaris L.) were investigated to determine the etiology and confirm the pathogenic role of Diaporthe phaseolorum. Field surveys in bean-growing regions revealed characteristic disease symptoms including brown to black lesions on pods, stem necrosis, seed discoloration, and premature pod drying under warm and humid conditions. Fungal isolates obtained from infected pod and stem tissues produced fast-growing colonies with white to gray mycelium that later became dark with the formation of pycnidia on potato dextrose agar. Microscopic examination revealed hyaline, ellipsoid, and aseptate conidia consistent with Diaporthe species morphology. Pathogenicity assays conducted on healthy bean plants under controlled greenhouse conditions successfully reproduced pod blight symptoms, thereby fulfilling Koch’s postulates and confirming D. phaseolorum as the causal agent. Molecular identification using ITS rDNA and translation elongation factor 1-alpha (TEF1-α) gene sequencing confirmed the isolates as D. phaseolorum, showing high similarity with reference sequences in global databases. Phylogenetic analysis placed all isolates within a well-supported D. phaseolorum clade, indicating relatively conserved genetic structure with minor intraspecific variation among regional populations. Disease development was strongly associated with high humidity and prolonged pod wetness, which favored infection and fungal colonization. The study highlights the increasing importance of D. phaseolorum as a pod-infecting pathogen in common bean production systems and emphasizes the need for accurate molecular diagnostics for early detection. Integrated disease management strategies, including crop rotation, use of healthy seed material, and timely fungicide application, are recommended to reduce pod blight incidence and improve bean yield and quality.