Effect of a soil amendment on the survival of Ralstonia solanacearum in different soils.
The effect of soil amendments (SA) composed of urea (200 kg N/ha) and CaO (5000 kg/ha) on the survival of R. solanacearum in 4 Philippine soils was investigated in a series of laboratory experiments. Within 3 weeks of application, the SA either caused an initial decrease, a final decline, or no change in the pathogen population, depending on the particular soil type. An initial decrease occurred in soil with a basic pH and resulted in a significantly lower pathogen population immediately and at 1 week after amending the soil. This decrease was probably due to the high pH in the soil during urea hydrolysis. A final decline in the R. solanacearum population after 3 weeks occurred in 2 soils in which nitrite accumulated after 1 week. In these soils, no decline in bacterial levels occurred when nitrite formation was inhibited by 2-chloro-6-trichloromethyl pyridine. In soil with low pH, no nitrite accumulated, and the R. solanacearum population did not decline. The suppressive effects of pH and nitrite on R. solanacearum growth were confirmed by in vitro experiments. Ammonium reduced the growth of R. solanacearum, but was not suppressive. Interactions of pH with ammonium and nitrite also occurred, whereby ammonium reduced growth of R. solanacearum only at pH 9 and nitrite was suppressive only at pH 5. Nitrite had no effect on R. solanacearum growth in vitro.