Influence of crop rotation on tobacco bacterial wilt number and pothogenesy

Eggplant, soybean, peanut, sweet potato, late rice, maize, garlic and double cropping rice were respectively planted during autumn after harvest of spring tobacco in pot experiment with soil inoculated drug-resistant Ralstonia solanacearum. The aim of the experiments was to investigate the number dynamics of R. solanacearum in soils under crop rotations. Throughout the growth season of rotation corps, both root and soil samples were collected at each sampling time. R. solanacearum populations were measured using a selective culture medium containing rifampicin. Four weeks after planting, samples with R. solanacearum included roots of eggplant (≈10⁶ cfu·g^-1) and soybean. R. solanacearum was only noted in the roots of late rice and peanut in the second and eighth weeks after planting. About 10⁴~10⁶ cfu·g^-1 bacteria were noted in soils of eggplant, soybean, peanut and sweet potato. The number of R. solanacearum in soils of late rice and maize declined throughout the growth season. R. solanacearum were initially detected, but then dropped to undetectable level in soil of garlic. Monitored rice stubs with R. solanacearum in winter showed that bacterial populations in the stubs and roots of rice and soil peaked at 1.00×10⁵ cfu·g^-1 and 5.17×10⁴ cfu·g^-1, successively. Bacteria population potentially increased during root decay. While it remained at 10⁴ cfu·g^-1 in soils of eggplant, soybean, peanut and sweet potato, it was 10³ cfu·g^-1 in soil of maize in spring of the following year. However, no bacteria were found in soils of garlic and late rice in the spring of the following year. Replanting tobacco in soils after harvesting of the rotation crops showed that different crop rotations significantly influenced disease occurrence time and development in tobacco plants. While the time of disease occurrence depended on soil bacterial population, disease development was influenced by a wider range of factors. The most serious disease was found in tobaccos planted in soils after eggplant, following by soybean, garlic, peanut, sweet potato and then maize. Although disease existed in tobaccos planted in soil after late rice, it was not as serious as in soils after the other crops. Once tobacco was planted in soil after late rice, disease occurred 20 days latter than in soil after eggplant. In fact, the disease index dropped by 83.3%. Bacteria were not observed in roots and soil after 12 months of rice crop rotation. The result suggested that tobacco-rice rotation had a high control effect on tobacco bacterial wilt.