SELECTION OF RHIZOBACTERIA ANTAGONISTIC TO Ralstonia solanacearum E.F. Smith CAUSING BACTERIAL WILT IN TOMATO AND THEIR BIOCONTROL MECHANISMS
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2000
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University of Agricultural Sciences, Dharwad
Abstract
"As many as 431 native rhizobacterial isolates of tomato were screened in vitro on KB medium for the inhibition of Ralstonia solanaceamm causing bacterial wilt disease. Of them, 38 strains were found to be potent antagonists with the zone of inhibition of the pathogen varying from 8-20 mm dia., and 23 of them were fluorescent pseudomonads. When assessed for mechanism of biocontrol, all the potent antagonists produced at least one antibacterial metabolite: antibiotics, siderophores, or HCN. Twenty-five strains were found to be antibiotic producers, 23 (only fluorescent pseudomonads) were siderophore producers, 12 were HCN producers, 13 were both antibiotic and siderophore producers, nine were both antibiotic and HCN producers, 12 were both siderophore and HCN producers and nine strains produced all the three metabolites. In the greenhouse experiment, Sid and Sid'' mutants obtained by the Tn5 mutagenesis of fluorescent Pseuodurnonas strains RBL 101 and RSI 125, failed to control the disease as much as their wild type did, thus confirming fluorescent siderophore production as the mechanism of biocontrol in these strains. The greenhouse experiments revealed that 27 antagonistic strains were found to be plant growth promoting (PGPR). The PGPR strains produced copious amounts of plant growth promoting substances viz., lAA and GA. Thirteen rhizobactcrial strains were found to be deleterious (DRB), producing unidentified gaseous metabolilcs. 'I'he unusual poor growth and biomass of tomato in the UIC plants was due to the predominance of twice deleterious histosphere bacteria (DHB). However, the PGPR strain fluorescent Pseuodurnonas RDV 108 inhibited DHB and nullified its inhibitory effect. The fluorescent Pseuodurnonas strain RDV 107 was found to be the best bio control agent with a percent disease control of 77.30 besides exhibiting good root colonization ability, plant growth promotion and siderophore and antibiotic production. "
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No . of references 56