Selective inhibition of the potato scab pathogen by antagonistic bacteria and substrate influence on antibiotic production

Abstract

In a long-term field experiment a soybean cover crop and green manure incorporation prevented the build-up of common scab of potato whereas barley, employed in the same manner, increased disease incidence. When soil from these plots was assayed for organisms antagonistic toS. scabies, a bacterium indentified asBacillus subtilis, was found to be predominant. Laboratory tests showed thatS. scabies was more sensitive to the antibiotic produced by this bacterium than most non-pathogenicStreptomyces spp which were tested. The antibiotic was found to be similar to bacitracin and activity was expressed as units bacitracin. Water extracts of greenhouse-grown soybean and barley were compared, at different concentrations, as a substrate for growth and antibiotic production by this bacterium. It was found that on the soybean extract, antibiotic activity, as measured by the standard filter-paper-disc technique, was 2.5 to 3 times greater per unit of bacterial growth than when barley extract was used as the substrate. Similar results were obtained when extracts of partially decomposed tissue were used. It is suggested that when evaluating antagonistic organisms as a possible factor in the behavior of plant pathogens in soil, the relative sensitivity of the pathogen to the antibiotic activities of the antagonists as well as the substrates available to the antagonists should be considered.