GROWTH PROMOTIONAL EFFECT OF Azotobacter chroococcum ON Abelmoschus esculentus (OKRA) AND ITS ANTAGONISTIC ACTIVITIES AGAINST SOME SELECTED PHYTOPATHOGENS

Abstract

Azotobacter chroococcum is an aerobic, free-living, ubiquitous, heterotrophic, non- symbiotic biological nitrogen fixer present in soils. A. chroococcum is plant growth promoting rhizobacteria (PGPR) that aggressively colonize the root zone and promote plant growth, protect the plants from phytopathogens and also act as biocontrol agents. A. chroococcum synthesizes and secretes considerable amount of biologically active substances such as vitamins (riboflavin), amino acids (thiamine), plant growth hormones (nicotine, indole acetic acid, and gibberellins), antifungal compounds (HCN), and siderophores. The aim of study is to isolate and identify A. chroococcum from rhizospheric soil of Eastern Nepal to observe its effect on growth of okra plant and its antagonistic activity against some phytopathogens. The rhizospheric soil was collected from five places of Eastern Nepal, in sterile plastic bag and cultured in mannitol N-free agar media by serial dilution and incubated at 25°C for 48 hrs. Morphological, cultural, biochemical and physiological characterization was done and bacteria were identified. Isolated A. chroococcum were cultured in mannitol N-free broth and inoculated on okra plants by seed inoculation technique. It was found that okra plant inoculated with A. chroococcum was improved in growth parameter in height, root length, fresh and dry compared to inoculated plant. Increasement in height of plants was found to be around 21.33% comparing to that of uninoculated plants. No significant difference was found on number of leaves in between treatments and control plants. In vitro effects on selected phytopathogens were observed on modified mannitol N-free agar by dual culture method. Similarly, different isolates were found to suppress plant pathogens being the AD most potent in suppressing R. solani and F. oxysporum by 38.5% and 24.9% and AB being most potent in suppressing S. rolfsii by 35.5%. while other isolates were found to be moderate potent.