Bioactivity of Zingiber officinale and Piper nigrum plant extracts in controlling post-harvest white yam (Dioscorea rotundata) tuber rot fungi

Rotten white yam (Dioscorea rotundata) tuber samples were collected from farms in three communities. Nine fungal organisms were isolated by direct tissue plating on potato dextrose agar medium. These were: Fusarium oxysporum, Aspergillus flavus, Aspergillus niger, Penicillium spp., Lasiodiplodia theobromae, Trichoderma viride, Rhizopus spp., Pestalotia guepini and Alternaria solani. Each of these isolates caused rot lesions when inoculated into healthy yam tubers. These organisms were re-isolated, identified and compared with the initial isolates to confirm their pathogenicity. L. theobromae, A. niger, Rhizopus sp. and A. solani were the most frequently isolated fungi species from the rotten yam tuber samples, with frequency of isolation of 30.07, 16.08, 16.08 and 12.59%, respectively. Zingiber officinale rhizome and Piper nigrum seeds were tested against L. theobromae and F. oxysporum in vitro. The botanical extracts were prepared by cold water extraction method at a concentration of 60% w/v. The two extracts showed significance in inhibiting the growth of the two fungi when data was statistically analysed (p = 0.05) using Genstat 9.2 package. This implies that they have some anti-fungal properties which need to be further investigated in vivo to establish their suitability in protecting yam tubers from storage rot. However, P. nigrum showed to be much stronger than ginger in terms of bioactivity.