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 dem onstrated in a 4-year field-pot experiment at Becker, MN, from 1988 to 1992. The suppressive strains were grown on a vermiculite-oatmeal bro th base and mixed with scab-conducive soil at 1, 5, and 10% (vol/ vol) . Corresponding treatments with 1, 5, and 10% carrier only and nonamen ded controls were arranged to investigate dilution and/or nutrient eff ects on scab control. Both strains significantly decreased scab on pot ato tubers of cv. Norchip compared with the nonamended control treatme nt. Both suppressive strains at the 1% level were not significantly di fferent in scab control from those at the 5 and 10% levels after the f irst year. There were no consistent differences in the level of diseas e control provided by strains PonR and PonSSII. For strain PonSSII, pe rcent disease control after inoculation at the 1% level increased ever y 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 str ains did not affect tuber yield and were reisolated from the tubers gr own in inoculated soil in all 4 years of the experiment.