Studies on in vitro production of antimicrobial substances by Azotobacter chroococcum isolates/mutants

An attempt was made to study the in-vitro production of antimicrobial/antifungal substances by various strains of Azotobacter chroococcum against various plant pathogens. In addition, nature and role of these substances were studied on the germination and reduction in disease (index) by Rhizoctonia solani in mustard crop. About 37 % of Azotobacter chroococcum isolates/mutants showed a zone of inhibition against Rhizoctonia solani and 25 % against Xanthomonas campestris. Approximately 16 % of isolates/mutants of A. chroococcum were siderophore positive (sid + ), The antimicrobial substances in Azotobacter were found to be extracellular, proteinaceous in nature and seem to be associated with some complex material also. Seed germination of mustard Brassica juncea (L.) cv. RH 30' increased when treated with A. chroococcum isolate/mutants (29, 18 and mala 27) alone and in combination with plant pathogens, i. e., Rhizoctonia solani and Xanthomonas campestris. A soil pouch experiment conducted on B. juncea (L.) under pot house conditions showed that root rot caused by R. solani was significantly reduced by all the selected Azotobacter isolates/mutants. However, percent disease index was minimum with A. chroococcum 29, which was sid - also indicating that siderophore production alone might not have a role in decreasing plant pathogenicity.