Pathogenicity and Growth Characteristics of Rhizopus stolonifer Causing Post-Harvest Soft Rot of Sweet Potato

Abstract

Post-harvest soft rot caused by Rhizopus stolonifer is a major constraint in sweet potato storage and marketing, leading to significant qualitative and quantitative losses. The present study investigates the pathogenicity and growth characteristics of R. stolonifer isolates associated with soft rot of sweet potato tubers. Diseased tubers showing typical watery soft rot, rapid tissue collapse, and white-to-gray mycelial growth were collected from storage facilities and used for pathogen isolation. Pure cultures were obtained on potato dextrose agar and characterized based on colony morphology, mycelial growth rate, sporangiophore development, and sporangial features. The pathogen exhibited rapid radial growth and abundant sporulation under laboratory conditions, indicating strong saprophytic and pathogenic potential. Pathogenicity tests on healthy sweet potato tubers confirmed the ability of R. stolonifer to induce typical soft rot symptoms within a short incubation period, fulfilling Koch’s postulates. Disease progression was characterized by rapid tissue maceration due to enzymatic degradation of host cell walls. Growth studies revealed that the fungus thrives optimally under warm temperatures and high humidity conditions, which favor sporulation and infection spread during storage. Wound sites on tubers significantly enhanced infection severity, highlighting the role of mechanical injury in disease initiation. The study emphasizes the aggressive nature of R. stolonifer and its ability to rapidly colonize stored sweet potatoes under favorable environmental conditions. These findings underscore the importance of proper post-harvest handling, including minimizing injuries and maintaining storage hygiene, to reduce disease incidence. Overall, the results provide valuable insights into the pathogenic behavior and growth dynamics of R. stolonifer, contributing to improved management strategies for post-harvest soft rot in sweet potato storage systems.