Biocontrol effect of Trichoderma harzianum T4 on brassica clubroot and analysis of rhizosphere microbial communities based on T-RFLP

Clubroot is a serious threat to cruciferous crops in Shanghai, China, and Trichoderma harzianum T4 (T4) has shown good efficiency for its control. In this study, T4 was applied alone or in combination with the fungicide cyazofamid or Bacillus subtilis XF-1, respectively, in a pot experiment to determine its efficiency of controlling clubroot. The control efficiency of T4 (79.3%) was significantly higher than that of XF-1 (63.1%, p < .05), and was similar to that of cyazofamid (93.2%, p > .05). Furthermore, T4 could promote the growth of the Chinese cabbage Brassica, particularly with respect to its fresh weight. These results indicated that T4 could be used as a green biofungicide not only to control the incidence of clubroot but also to increase the production of vegetables. The structure of microbial communities changed substantially when T4 was applied to the soil, as revealed by a non-metric multidimensional scaling test. T4 could not only reduce the average abundance of Bacillus sp. and Acidiphilium sp. (terminal restriction fragment 231 (TRF231), the most dominant fragment) but could also increase the average abundance of Streptococcus sp. (TRF307, the second most dominant fragment). Therefore, the high control efficiency of T4 against clubroot results not only from its ability to directly inhibit spore germination of Plasmodiophora brassicae but also from its ability to regulate the microbial communities in the rhizosphere soil of B. chinensis, which could then inhibit the growth of P. brassicae in soil indirectly.