Effects of intercropping and soil amendment with urea and calcium oxide on the incidence of bacterial wilt of tomato and survival of soil-borne Pseudomonas solanacearum in Taiwan

Intercropping and soil amendment experiments were conducted to determine if they reduced populations of Pseudomonas solanacearum and bacterial wilt of tomato at the Asian Vegetable Research and Development Center (AVRDC) and at three other locations in Taiwan. At AVRDC, intercropping tomato with cowpea planted within the row significantly reduced bacterial wilt ( P  < 0.05) compared to when tomato was cropped alone. The P. solanacearum population in soil was not affected by intercropping with cowpea, soybean, or Welsh onion. At the same site, however, a preplanting soil amendment consisting of urea (200 kg ha^?1 N) and CaO (5000 kg ha^?1) significantly reduced the pathogen population and tomato bacterial wilt ( P  < 0.001). The effect of the soil amendment was not consistent when applied to soil from three other sites in Taiwan; in soil from two sites no reduction of the pathogen population occurred. At these sites, tomato bacterial wilt in the field was not reduced significantly after amending. In comparison with a non-amended control, the addition of only CaO reduced the P. solanacearum population in AVRDC soil significantly ( P  < 0.05), but the reduction was significantly greater when the complete soil amendment was added. In contrast, urea alone did not affect the survival of P. solanacearum in the soil. In a greenhouse experiment with AVRDC soil, P. solanacearum was undetectable 2 weeks after soil amendment, but in the same treatment tomato yield was significantly reduced by 48% ( P  < 0.05) compared with non-amended treatments. The suppressive effect of the soil amendment on the P. solanacearum population was probably due to the generation of one or several toxic substances during the transformation of urea in the presence of CaO.