Inoculum density and population dynamics of suppressive and pathogenic Streptomyces strains and their relationship to biological control of potato scab

Several Streptomyces strains are capable of suppressing potato scab caused by Streptomyces scabies. Although these strains have been successful in the biocontrol of potato scab in the field, little is known about how populations of pathogenic Streptomyces in the potato rhizosphere are influenced by inoculation of the suppressive strains. The effects of inoculum densities of pathogenic and suppressive Streptomyces strains on their respective populations on roots and in rhizosphere soil were examined during the growing season. The relationships between inoculum density or rhizosphere population densities and disease severity were also investigated. Populations of suppressive Streptomyces strain 93 increased significantly on roots with increasing inoculum dose. At its highest inoculum dose, the suppressive strain reached a population density greater than 10⁶ CFU/g root 14 weeks after planting. The ability of the suppressive strain to increase its populations with increasing inoculum density was hindered at high inoculum doses of the pathogen, suggesting that density-dependent competitive interactions may be occurring between the two antagonists. Strain 93 was most effective at preventing scab early in the growing season (8 weeks after planting), when tubers were most susceptible to the scab disease. Population densities of the suppressive strain in soil were more highly negatively correlated with scab severity than were populations on roots, suggesting that rhizosphere soil rather than potato roots may be the primary source of inoculum of the suppressive strain for tubers.