Quantitative Detection and Risk Analysis of Ralstonia solanacearum Species Complex in Tobacco-planting Soils from Yunnan Province

The purposes of this research were to investigate the relationship between the pre-planting soil abundance of Ralstonia solanacearum species complex (RSSC) and the occurrence of tobacco bacterial wilt, as well as to analyze the distribution patterns of RSSC in the soils of tobacco planting areas from Yunnan Province. We used a previously established qPCR detection system to quantitatively detect RSSC in the soil before transplanting flue-cured tobacco and examined the incidence of tobacco bacterial wilt during the rapid growth stage to the maturity stage of tobacco. The results showed that: (1) The population threshold of the pathogens causing tobacco bacterial wilt was 10³ to 10⁴ genomes per gram of soil. (2) The population of RSSC showed a falling trend with soil depth, mainly concentrated in the soil layer within 40 cm. (3) Analysis of 716 soil samples revealed that samples with RSSC contents in the range of 10⁴ to 10⁵ genomes per gram of soil accounted for the most significant proportion (37.43%). Among these, Baoshan, Kunming, and Lincang had relatively high proportions (58.33%, 51.43%, and 45.25%, respectively). Five cities, including Dali, Zhaotong, Wenshan, Chuxiong, and Yuxi, had the highest proportion of samples with less than 10³ genomes per gram, which were 62.50%, 61.54%, 56.04%, 48.48%, and 41.67% respectively. (4) The abundance of RSSC in soil samples from gramineous crops in spring rotation were significantly lower than those from continuously cropped fields. Compared with winter fallow, the contents of RSSC in soils planted with crops from the families of Solanaceae, Polygonaceae, and Cruciferaceae were higher. Conversely, the contents of RSSC in soils cultivated with crops from the families of Leguminosae and Asteraceae were lower, and only former plants significantly reduced the RSSC in soils. According to the incidence threshold of 10³~10⁴ RSSC genomes/g in the soil, the risk of bacterial wilt in Kunming, Lincang, Pu’er, Qujing, Honghe, and Baoshan of Yunnan Province is relatively high, and attention should be paid to prevention and control.