INOCULUM DENSITY AND POPULATION-DYNAMICS OF SUPPRESSIVE AND PATHOGENIC STREPTOMYCES STRAINS AND THEIR RELATIONSHIP TO BIOLOGICAL-CONTROL OFPOTATO SCAB

Several Streptomyces strains are capable of suppressing potato scab ca used by Streptomyces scabies. Although these strains have been success ful in the biocontrol of potato scab in the field, little is known abo ut how populations of pathogenic Streptomyces in the potato rhizospher e are influenced by inoculation of the suppressive strains. The effect s of inoculum densities of pathogenic and suppressive Streptomyces str ains on their respective populations on roots and in rhizosphere soil were examined during the growing season. The relationships between ino culum density or rhizosphere population densities and disease severity were also investigated. Populations of suppressive Streptomyces strai n 93 increased significantly on roots with increasing inoculum dose. A t its highest inoculum dose, the suppressive strain reached a populati on density greater than 10(6) CFU/g root 14 weeks after planting. The ability of the suppressive strain to increase its populations with inc reasing inoculum density was hindered at high inoculum doses of the pa thogen, suggesting that density-dependent competitive interactions may be occurring between the two antagonists. Strain 93 was most effectiv e at preventing scab early in the growing season (8 weeks after planti ng), when tubers were most susceptible to the scab disease. Population densities of the suppressive strain in soil were more highly negative ly correlated with scab severity than were populations on roots, sugge sting that rhizosphere soil rather than potato roots may be the primar y source of inoculum of the suppressive strain for tubers. (C) 1997 Ac ademic Press.