Isolation and Identification of Fungal Species Associated with Postharvest Spoilage of Watermelon (Citrullus lanatus) Fruits in Sokoto Metropolis, Nigeria

Abstract

Postharvest fungal spoilage represents a major constraint to watermelon (Citrullus lanatus) production, marketing, and food safety in sub-Saharan Africa, yet the fungal pathogens responsible for its spoilage in Sokoto metropolis, northwestern Nigeria, remain poorly characterized. This study aimed to isolate and identify fungal species associated with watermelon spoilage, determine their frequency of occurrence across nine market locations, and evaluate their pathogenicity on healthy fruits. Spoiled watermelon fruits were collected from nine sampling points in Sokoto metropolis, and fungal isolates were recovered on Sabouraud Dextrose Agar (SDA), purified by repeated sub-culturing, and identified using macroscopic colony morphology and microscopic examination with lactophenol cotton blue staining; pathogenicity was subsequently assessed following Koch's postulates. Five fungal species were isolated and identified: Aspergillus flavus, Aspergillus niger, Fusarium oxysporum, Rhizopus oligosporus, and Mucor hiemalis. Aspergillus flavus recorded the highest frequency of occurrence (28.57%), followed by F. oxysporum (23.81%), R. oligosporus (19.05%), A. niger (14.29%), and M. hiemalis (14.29%), and all isolates induced characteristic spoilage symptoms in inoculated healthy fruits, thereby fulfilling Koch's postulates. These findings demonstrate that fungal contamination, particularly by the aflatoxigenic A. flavus, constitutes a significant food safety concern for watermelon marketed in Sokoto, and improved postharvest handling, proper sanitation, and adequate cold storage are urgently recommended.