FUNGICIDAL POTENTIAL OF THREE PLANT EXTRACTS IN THE MANAGEMENT OF ROOT ROT DISEASE OF SWEET POTATO [Ipomoea batatas (L.) Lam] CAUSED BY FUNGAL PATHOGENS

Abstract

This study sought to find eco-friendly alternative management of root rot disease of sweet potato in storage as often used synthetic pesticides have detrimental effects not only on the consumer but the environment. This study therefore investigated the use of three plant extracts with potential in managing sweet potato root tuber rots in Northern Ghana. Phytochemical analysis was conducted on both aqueous and ethanol extracts of leaves of bitter leaf (Vernonia amygdalina), lemon grass (Cymbopogon citratus) and holy basil (Ocimum sanctum). The antifungal activities of the aqueous and ethanol plant extracts were tested in vitro at 50 and 100% extract concentrations against the mycelia growth of isolated sweet potato root tuber rot fungal pathogens while 10% basic aqueous concentration of each of the plant extract was tested in vivo for weight loss of root tubers, severity of rot, lesion diameter and sprout count. The phytochemical analysis revealed the presence of alkaloids, flavonoids, saponins, and phytosterols but varied among the plant extracts tested. Seven fungal species namely; Aspergillus niger, Fusarium oxysporum, Lasiodiplodia theobromae, Aspergillus flavus, Aspergillus clavatus, Trichoderma harzianum and Rhizopus stolonifer were isolated from both white-flesh and orange-flesh sweet potato varieties. However, five isolates namely; A, niger, F. oxysporum, L. theobromae, T. harzianum and R. stolonifer were pathogenic on fresh and healthy root tubers after pathogenicity. Aspergillus niger had the highest frequency of occurrence and the highest necrotic lesion on both white-flesh and orange-flesh varieties. The mycelia growth was significantly (P < 0.05) reduced by each of the plant extracts at both 50% and 100% concentration levels in both aqueous and ethanol solvents. Vernonia amygdalina and Cymbopogon citratus were highly efficacious on Fusarium oxysporum than any of the isolated fungal pathogens. The plant extracts reduced the sweet potato root tuber weight loss significantly (P < 0.05) from week one to week six in storage. Also, root tuber rot and sprout were suppressed by Vernonia amygdalina and Cymbopogon citratus respectively. Vernonia amygdalina managed sweet potato root tuber rots to the barest minimum followed by Cymbopogon citratus and then Ocimum sanctum. This study showed that aqueous and ethanol extracts of leaves of Vernonia amygdalina, Cymbopogon citratus, and Ocimum sanctum have fungicidal properties against sweet potato root rot fungal pathogens (A. niger, F. oxysporum, L. theobromae, T. harzianum and R. stolonifer); hence can be exploited in the management of sweet potato root tuber rot in storage.