Department: Applied Microbiology And Brewing
Alum, E. A.
Umeh, S. O.
Postharvest fungal spoilage is one of the major constraints to sweet potato production. The study investigated the occurrence and biocontrol of the spoilage fungi affecting sweet potato in Ebonyi State. Healthy sweet potato roots and those with rots (100 each in the dry and wet seasons) were collected from Ebonyi State for laboratory analyses. From the sweet potato roots with rots, fungi were isolated on Potato Dextrose Agar using the point inoculation method. Isolated fungi were identified based on morphological charateristics and their identity confirmed through Partial Internal Transcribed Spacer Ribosomal Deoxyribonuceic Acid Sequencing Analysis and a Basic Local Alignment Search Tool Search with the GenBank Sequence Database. Isolates’ pathogenicity was tested on healthy sweet potato and Koch’s postulates established with each isolate through re-isolation. The severity of the pathogens and susceptibility of selected two sweet potato cultivars (‘Tupiochi’ and ‘Oyorima’) grown by Ebonyi sweet potato farmers were evaluated. Impact of fungi rot on the nutritional quality of storage roots of ‘Tupiochi’ and ‘Oyorima’ sweet potato cultivars was evaluated using artificial rot induction and Method of Association of Official Analytical Chemists. Evaluation of antifungal potential of water-reconstituted single and combined methanolic extracts of Garcinia kola, Allium sativum, Zingiber officinale and Moringa oleifera was done using Poisoned Food and Radial Growth Techniques, after which the most potent and broad spectrum extract combinations were evaluated in vivo for efficacy to inhibit postharvest rot on sweet potato using preventive and curative methods. Data were analyzed using Analysis of Variance and differences between means were compared using Duncan’s Multiple Range Test (P=0.05). Three hundred and fifty-two fungi were isolated from 200 sweet potato samples. Five distinct genera (Rhizopus, Aspergillus, Fusarium, Penicillium and Botrydiplodiae) and seven species of fungi were identified and the percentage frequency of isolation varied with the pathogen and season of isolation. The isolated fungi species include Botrydiplodiae theobromae (28.45%), Rhizopus oryzae (22.56%), A. niger (20.17%), A. flavus (13.03%), F. solani (10.99%), P. expansum (4.08%) and A. awamori (1.50%). The percentage frequency of fungi isolation was both qualitatively and quantitatively higher in the dry season (55.11%) than in the wet season (44.89%). All fungi were confirmed positive to pathogenicity test and four categories of rot (dry rot, soft rot, java black rot and blue green mold) were predominantly incited by the fungi. Rhizopus oryzae recorded the highest percentage severity of 76.36% on both sweet potato cultivars followed by A. niger (68.39%), B. theobromae (64.06%) with the least severity (20.39%) exhibited by P. expansum within the test period. None of the two sweet potato cultivars was immune to any of the phytopathogens. Cultivar ‘Tupiaochi’ showed significantly (P= 0.05) higher fungi rot susceptibility than the cultivar ‘Oyorima’. Fungi rot on the roots significantly (P=0.05) depleted carbohydrate, crude fibre and moisture contents while increasing the crude protein, fat and ash of the roots. Methanolic extracts of Z. officinale, A. sativum, M. oleifera and G. kola plants when used individually at a lower concentration (50mg/ml), significantly inhibited the mycelial growth of all the phytopathogens in vitro when compared to water (0%) but gave less than 50% inhibition which ranged from 5.44 - 29.56%. At a higher extract concentration (100mg/ml), the inhibition obtained was significantly (P= 0.05) higher than those of the control (water) and lower concentrations (50mg/ml), with percentage inhibition ranging from 8.84 – 54.04%. On the contrary, the use of two plant extract combinations (100mg/ml concentration each) showed mainly additive and synergistic extract/pathogen interactions that resulted in increased percentage inhibition (ranged from 30.43 – 100%) across most of the pathogens and a few antagonistic interactions. Combinations of Allium sativum and Zingiber officinale and Allium sativum and Moringa oleifera with percentage mycelial inhibition that ranged from 51.25-98.74% and 36.74-100% respectively, proved the best treatment combinations with no antagonism and the broadest spectrum antifungal activity. The result of the in vivo assay showed that combined methanolic extracts of Allium sativum and Zingiber officinale or Allium sativun and Moringa oleifera can be used as potential alternatives for postharvest decay control in sweet potato.