Organic manuring for potato scab control and its relation to soil manganese

The suggestion that organic amendment of soil controls scab by reducing insoluble soil manganese to soluble forms toxic to Streptomyces scabies was investigated in pot and field experiments by assessing numbers of manganese-reducing micro-organisms in soil. Dried grass meal (2 tons/acre) gave significant control of scab but increases in soluble manganese were insufficient to account for this control. Populations of micro-organisms in soil increased after adding grass meal but decreased again by the time tubers were initiated and scab infection began. The results suggest that manganese reduction could affect scab if soil contained easily reducible manganese, because dried grass meal increased the numbers of manganese-reducing micro-organisms.