ABSTRACT
A major disease of yam is rot caused by a plethora of microbial species most notably from the fungi kingdom. Efforts to reduce crop losses which involve the use of pesticides/insecticides over the years have been found to be deleterious to humans in the long-run. The use biological control method is seen as an alternative for management of plant diseases. Consequently, the purpose of this study is to determine the effects of leaf extracts against the fungi thought to be responsible for yam rots.
Neem leaves were collected fresh from the market, identified, allowed to dry before extraction was carried out using aqueous and ethanol solvent. Preliminary phytochemical analysis was carried out on the extracts. Spoilt/diseased yam tubers were obtained from three open markets (Uselu, New Benin and Oluku markets) and were analysed using standard methods for total fungal counts, and subsequent identification of the isolates using cultural, microscopic and molecular techniques. Pathogenicity tests on apparently healthy yam tubers were carried out using a 10 mm cork borer with a 3-day old fungal culture, which was monitored for 5 days with measurement of the necrotic lesion for each day. Poisoned food technique was used for evaluating the in vitro antifungal effect of the neem leaf extracts on the pathogens at different concentrations of 200 mg/ml, 100 mg/ml, 50 mg/ml and 25 mg/ml the mycelia growth inhibition was measured and the minimum inhibitory concentration was evaluated using a linear regressions model. Data obtained were subjected to descriptive and inferential statistics.
Results from the experiment showed that, the total fungal counts obtained from diseased yam tubers which ranged from log10 4.75 cfu/g to log10 4.80 cfu/g. The isolated species of fungi responsible for yam spoilage were Aspergillus niger, Aspergillus flavus and Penicillium italicum as these isolates were able to infect the apparently healthy yam upon inoculation in vitro. Aspergillus niger (36.24%) and Penicillium italicum (34.62%) were the isolates with the highest disease severity index in the study. Aqueous and ethanolic portions of the leaf-extract were found to contain certain phytochemical constituents such as flavonoids, tannins and alkaloids. The leave extract showed some level of bioactivity on the test isolates by inhibiting mycelia growth extension in vitro with A. niger having 37.56% inhibition for ethanolic extract of neem leaves, while 59.32% was observed for aqueous extracts. There was also a 43.47% and 52.94 % mycelia inhibition for aqueous and ethanolic extracts of P. italicum respectively. When ethanol and aqueous neem leaf extract were tested against fungal pathogens, it was discovered that the ethanolic neem leaf extract had minimum inhibitory concentrations (MIC) of 68 mg/ml, 69 mg/ml, and 71 mg/ml for A. niger, P. italicum, and A. flavus, respectively. Also was observed for aqueous extract that, the MIC were 70 mg/ml for both A. niger and P. italicum with A. flavus having an MIC of 72 mg/ml. The study shows that the neem leaves extracts contain some biological active compounds and molecules that could reduce or manage plant disease of yam condition thereby contributing to food security.
