EVALUATION OF ANTAGONISTIC BACTERIA FOR SUPPRESSION OF BACTERIAL RINGROT OF POTATO

Bacterial strains with potential for biological control of bacterial r ing rot of potato caused by Clavibacter michiganensis subsp. sepedonic us were isolated from the surface of potato tubers. Eighty-eight poten tial biocontrol candidates, selected on the basis of in vitro antibios is to C. m. sepedonicus, produced inhibition zones with radii ranging from 0.5 to 16 mm on test plates. All antagonistic isolates were scree ned in the greenhouse for biocontrol activity on micropropagated potat o plantlets root-inoculated with C. m. sepedonicus. Eight strains cons istently prevented infection of plantlets but there was no significant correlation between the width of the Inhibition zone in the in vitro assay and ring rot suppression in the plant bioassay. Three strains th at showed a high Il:vel of biological control potential were identifie d as a saprophytic enteric bacterium (strain 7G), an Arthrobacter sp. (strain 16C), and a soil coryneform bacterium (strain 18A). These were tested in a field plot by co-inoculating cut seed potato tubers with C. m. sepedonicus and antagonists. Strains 7G and 18A significantly in creased plant stand whereas 16C decreased disease incidence. The relat ive number of ostensibly ring rot-free progeny tubers was generally gr eater when antagonists were present.