Abstract

Six concentrations of benomyl (g/ml): 0.00 (control), 0.05, 0.10, 0.20, 0.40 and 0.80 were assayed by using 1.5 kg of top soil collected from nursery of Federal University of Technology, Akure, Ondo State, Nigeria. Cowpea (IT - 93K - 452 - 1) seeds were planted in the soil sample in different polyethylene bags. Harvesting of cowpea seedlings was done on 5, 10, 15, 20, 25 and 30 days of germination. Nodule and soil bacterial counts were done; nodulating as well as rhizosphere and non-rhizosphere bacteria were cultured and identified. The activity of the isolated bacteria was evaluated by rock phosphate solubilisation. Cowpea seedlings were healthy without much loss of root nodules and beneficial rhizosphere bacteria at 0.10 and 0.20 g/ml of benomyl. Nodule loss of 94.3% occurred at 0.80 g/ml of benomyl when compared to control. Bradyrhizobium japonicum was responsible for nodulation of cowpea. Bacillus subtilis, Staphylococcus aureus and Micrococcus luteuswere identified from rhizosphere while Arthrobacter simplex, Azotobacter chroococum,Pseudomonas aeruginosa and B. subtilis also were identified from non-rhizosphere of cowpea. Bacillus subtilis significantly (p < 0.05) increased solubilisation of rock phosphates in the presence of benomyl. As benomyl concentration increased from 0.00 to 0.80 g/ml, there was a decrease in bacterial population in the rhizosphere. The optimum concentration of benomyl on beneficial soil bacteria was 0.10 to 0.20 g/ml. Bacillus subtilis could be inoculated to rhizosphere soil of cowpea to increase the amount of soluble phosphate available for the growth of cowpea seedlings in benomyl treated soil.

Key words: Benomyl, rock phosphate solubilisation, rhizosphere and non-rhizosphere bacteria.