Effect of Soil Amendment with Urea and Calcium Oxide on Survival of Ralstonia solanacearum, the Causal Agent of Bacterial Wilt or Rhizome Rot of Ginger

Abstract

The effect of soil amendment with urea and calcium oxide on survival of Ralstonia solanacearum, the causal agent of bacterial wilt or rhizome rot of ginger, was studied by mixing urea and calcium oxide at the rate of 80 : 800 kg/rai in the artificial infested soil with 2.83 × 10⁷ cfu/ml of bacteria. The treated soil was left one week before transplanting the two months old gingers. The soil was sampled during week 0 - 4 to evaluate the population of R. solanacearum and to compare with the control by serial dilution method and spread plate on SM-1 medium. The experiment showed that the population of R. solanacearum in the soil amendment with urea and calcium oxide decreased from 0.88 × 10⁷ cfu/ml to 0.15 × 10⁵ cfu/ml in week
one, 0.1 × 10⁴ cfu/ml in week two and 0 cfu/ml in week three. The control treatment still contained high population level of 0.26 × 10⁷ cfu/ml in week one, 0.13 × 10⁶ cfu/ml in week two and three and 0.11× 10⁶ cfu/ml in week four. This population level could cause typical wilt of the tested gingers. The tested gingers transplanted in the treated soil showed no symptom. The decrease of bacteria in the treated soil was due to the toxicity of ammonium, ammonia and nitrate degraded from urea in high pH soil condition (average 7.0-7.2). Therefore the soil amendment with urea and calcium oxide at the rate of 80 : 800 kg/rai is recommended to decrease population of R. solanacearum in the infested fields for bacterial wilt control. The treated soil should be left at least 3 weeks before planting a new crop to avoid toxicating to ginger seedlings.