BIOLOGICAL-CONTROL OF POTATO SCAB IN THE FIELD WITH ANTAGONISTIC STREPTOMYCES-SCABIES

Control of potato scab by two suppressive strains of Streptomyces (S. diastatochromogenes strain PonSSII and S. scabies strain PonR) was demonstrated in a 4-year field-pot experiment at Becker, MN, from 1988 to 1992. The suppressive strains were grown on a vermiculite-oatmeal broth base and mixed with scab-conducive soil at 1, 5, and 10% (vol/ vol). Corresponding treatments with 1, 5, and 10% carrier only and nonamended controls were arranged to investigate dilution and/or nutrient effects on scab control. Both strains significantly decreased scab on potato tubers of cv. Norchip compared with the nonamended control treatment. Both suppressive strains at the 1% level were not significantly different in scab control from those at the 5 and 10% levels after the first year. There were no consistent differences in the level of disease control provided by strains PonR and PonSSII. For strain PonSSII, percent disease control after inoculation at the 1% level increased every year, whereas disease control with the 10% inoculation was constant. Effects of vermiculite dilution and/or oatmeal broth nutrients at the 10% level on scab reduction were significant during the first 2 years but were not significant during the last 2 years. The suppressive strains did not affect tuber yield and were reisolated from the tubers grown in inoculated soil in all 4 years of the experiment.