Occurrence of Postharvest Fungal Rots of Sweet Potato (Ipomoea batatas (L) Lam.) in Southwest Nigeria and their Control with Sawdust Extracts

A study was conducted in three agroecological zones (AEZs) of Southwest, Nigeria to evaluate the incidence and pathogenicity of postharvest fungal rots of sweetpotato and their control with extracts of sawdust from some tropical trees. Survey of rotted tubers was conducted in 18 markets across the three AEZs: humid rainforest (HF), derived savannah (DS), and southern guinea savannah (SGS). Fungi associated with rotted tubers were isolated, identified and their pathogenicity determined. In vitro fungitoxicity of Anogeissus leiocarpus, Gmelina arborea and Cola nitida sawdust extracts were assessed in an experiment laid out in a Completely Randomized Design (CRD) with 3 replicates. Six fungi species found to be associated with rot on tubers were Botryodiplodia theobromae, Rhizopus stolonifer, Aspergillus niger, Trichoderma viride, Penicillium oxalicum and Fusarium oxysporum. Highest (35%) rot incidence was observed in HF zone with R. stolonifer as the most prevalent. Botryodiplodia theobromae was most prevalent (68.75%, 54.54%) in SGS and DS zones respectively. All the six isolated fungi were pathogenic to sweetpotato but induced varying levels of rot severity. Botryodiplodia theobromae, R. stolonifer or A. niger induced complete (100%) rot of inoculated tubers. Sawdust extracts reduced mycelial growth of test pathogens at three sawdust concentrations (50 g/L, 75 g/L and 100 g/L) tested. Inhibition of fungal growth increased with extract concentration. Anogeissus leiocarpus sawdust extract at 100 g/L exhibited highest range of mycelial growth inhibition (8.80 - 73.0%) across tested pathogens. Gmelina arborea sawdust extract at 100 g/L significantly inhibited (p<0.05) mycelial growth of B. theobromae, P. oxalicum and T. viride while C. nitida exhibited strong fungitoxicity to F. oxysporum at 100 g/L. Application of the sawdust extracts at 50 g/L, 75 g/L and 100 g/L concentrations has the potential to minimize postharvest fungal rot of sweetpotato.