Antifungal effects of soluble silica on several postharvest pathogens of papaya in-vitro

Abstract

Papaya (Carica papaya L.) is a popular fruit grown worldwide in tropical and sub- tropical climates. The shelf life of papaya is comparatively short due to its climacteric nature and postharvest fungal diseases. This study aimed investigating the effects of a GRAS (Generally-Regarded-As-Safe) compound, soluble silica (potassium silicate; K₂SiO₃), on the in vitro growth of several postharvest fungal pathogenic species from papaya fruit. Their pathogenicity was confirmed through Koch’s postulates. Through colony morphology and micromorphology, the fungal pathogens were identified as two Aspergillus spp. (sp. 1, and sp. 2), Cladosporium sp., Fusarium sp., Lasiodiplodia sp., Penicillium sp., Pestalotia sp. And Rhizopus sp. Effects of soluble silica on fungal growth were tested by assessing radial growth on 7500 ppm silica- amended potato dextrose agar (PDA), mycelial dry mass in potato dextrose broth (PDB) and spore germination percentage in 7500 ppm silica- amended sterilized distilled water (SDW). Silica-free PDA, PDB and SDW served as the controls in those experiments, respectively. Silica effect on the dimensions of fungal structures was measured using light microscope. Silica treatment significantly (P < 0.05) inhibited the radial growth of Aspergillus sp. 1 and 2, Fusarium sp., Lasiodiplodia sp., Pestalotia sp. And Rhizopus sp. On PDA, as well as the mycelial dry weights of Aspergillus sp. 2, Cladosporium sp., Fusarium sp., Lasiodiplodia sp., Penicillium sp., Pestalotia sp. And Rhizopus sp. In PDB. Spore germination percentages of Fusarium sp., Lasiodiplodia sp., Pestalotia sp., and Rhizopus sp. Were also significantly inhibited by silica treatment. A reduction of the mycelial width, conidial length and width of all fungal spp. Was observed upon silica treatment. This study indicates a direct inhibitory effect of soluble silica on papaya postharvest fungal pathogens in vitro and warrants further investigation of its applicability on fruit, as a safer alternative to conventional fungicides in managing postharvest fungal diseases.