The effects of ionic strength and fertilization on the growth of Streptomyces scabies were researched in Petri dish experiments and in field experiments in allophanic Andosols in the Abashiri area. The results were as follows : 1) All experiments in Petri dishes were conducted in soil extract solution of allophanic Andosols with pH adjusted to each level. S. scabies was suppressed on agar amended at pH values < 4.8 of low-level ionic strength (μ=0.0008). However, S. scabies was not suppressed at pH values < 4.6 of the high-level ionic strength (μ=0.08). 2) The occurrence of potato scab decreased significantly in the field fertilized with only ammonium sulfate and without phosphate and potassium in the rows. 3) It is considered that aluminum ions are affected severely by ionic strength, and the activity coefficient decreases to a low level when a high-level ionic strength fertilizers are present in soil. Therefore, potato scab can be suppressed at a high soil pH and low level of exchange acidity y_1 when ionic strength is low in the soil in potato tuber growing areas because aluminum ion activity coefficient remains at a high level. Heavy rain and irrigation have the effect of increasing the activity coefficient of aluminum ions. 4) It is estimated that low ionic strength and nitrification of ammonium ions can suppress common potato scab at pH values < 5.1-5.3 in allophanic Andosols, and < 5.7-6.0 in nonallophanic soil.
