Phosphate Solubilization Potentials of Rhizosphere Fungi Isolated from Insecticide Treated Soil

Insecticides are used widely to control a variety of pests and often residues of these insecticides are left in soil which may have impact on the phosphate solubilization potentials of rhizosphere fungi. Rhizosphere soils were collected from carefully uprooted cowpea seedlings with hand trowel on 50, 70 and 90^th days of germination containing the lambda-cyhalothrin and dimethoate insecticides. Fungi associated with the above samples were identified by standard microbiological techniques. Screening for phosphate solubilization potential of the isolates was done by spot inoculation on Pikovskaya agar by measuring the clear zones around the colonies supplemented with tricalcium phosphates [Ca₃(PO₄)₂]. The phosphatase produced by the fungal species was optimized using parameters such as incubation time, pH, temperature, carbon source and nitrogen source in submerged fermentation. The isolated rhizosphere fungi were identified as Rhizopus stolonifer, Aspergillus niger, Aspergillus fumigatus, Aspergillus terreus, Trichoderma viride, Arthroderma fulvum and Fusarium oxysporum. Among the isolates, Trichoderma viride showed the best ability to solubilize phosphate with solubilization index of 2.82 with dimethoate of 12.5ml/L of water. The optimization study for the enzyme production showed that the best incubation time for phosphatase production was 72 hours by Trichoderma viride. It was observed that the optimum pH for production of phosphatase by Trichoderma viride was 6.5. This study suggests that all the isolated fungi especially Trichoderma viride can adapt to these insecticidal treatments, which make them useful as biofertilizers to increase uptake of phosphorous in plants.