EFFECT OF A SOIL AMENDMENT ON THE SURVIVAL OF RALSTONIA-SOLANACEARUM IN DIFFERENT SOILS

The effect of a soil amendment (SA) composed of urea (200 kg of N per ha) and CaO (5,000 kg/ha) on the survival of Ralstonia solanacearum in four Philippine soils was investigated in a series of laboratory expe riments. Within 3 weeks after application, the SA either caused an ini tial decrease, a final decline, or no change in the pathogen populatio n, depending on the particular soil type. An initial decrease occurred in a soil with a basic pH and resulted in a significantly (P < 0.001) lower pathogen population immediately and at 1 week after amending th e soil. This decrease was probably due to the high pH in the soil duri ng urea hydrolysis. A final decline in the R solanacearum population a fter 3 weeks occurred in two soils in which nitrite accumulated after 1 week. In these soils, no decline in bacterial levels occurred when n itrite formation was inhibited by 2-chloro-6-trichloromethylpyridine. in the soil with low pH, no nitrite accumulated and the R. solanacearu m population did not decline. The suppressive effects-of pH and nitrit e on R. solanacearum growth were confirmed by in vitro experiments. Am monium reduced the growth of R. solanacearum, but was not suppressive. Interactions of pH with ammonium and nitrite also occurred, whereby a mmonium reduced growth of R. solanacearum only at pH 9 and nitrite was suppressive only at pH 5. Nitrate had no effect on R. solanacearum gr owth in vitro.