| Summary: | 碩士 === 國立屏東科技大學 === 熱帶農業暨國際合作系所 === 100 === The emergency of several environmental and human threats, brought from the abuse of pesticides in agriculture, has strengthened the search for sustainable strategies to control plant diseases. Suppression of soil-borne diseases observed after soil amendments with Brassica crops, attributed to bioactive compounds from glucosinolate (GSLs) hydrolysis and modifications on soil microbial communities, is currently one of the main sources for developing alternative control methods. In this study, the potential use of three Brassica species, including B. rapa, B. napus and B. juncea, as soil amendments to control Rhizoctonia solani damping-off disease was investigated in vitro and in greenhouse. In vitro, assays included the use of a nutrient-rich and a nutrient-poor medium to evaluate the effect of toxic compound and the nutritional content added with Brassica tissues on the suppression of R. solani hyphal growth (fungi-toxicity). In greenhouse, the incidence of damping-off disease was evaluated in cabbage (Brassica oleracea L. var. capitata) seedlings growing in three soil sources amended with dry Brassica tissues. The impact of Brassica amendments on soil reaction, seedling growth and microbial dynamics were also evaluated in order to elucidate possible mechanisms involved in disease reduction. Fungi-toxicity tested in the nutrient-rich medium demonstrated that tissues from all three Brassica species released fungi-toxic compound against R. solani hyphal growth. However, results in the nutrient-poor medium with water soluble extracts from Brassica tissues, indicated that nutritional content in the tissues can reduce fungi-toxicity, which may negatively influence the efficiency of bio-fumigation after field application. Moreover, greenhouse bioassays demonstrated that Brassica amendments strongly impact on microbial population, while disease suppression was not consistent with fungi-toxicity observed in vitro. Volatile compounds released by B. juncea demonstrated to be highly toxic to the growth of R. solani (98.2-100 % hyphal inhibition) in vitro; however, disease suppression by this specie observed in greenhouse varied with soils sources. On the other hand, B. rapa and B. napus revealed steady reduction of disease incidence along three different soils (< 13.3 % incidence, similar to non-diseased soil), which could partially be explained by the higher fungi-toxicity of compounds in their water soluble extractions, than those of B. juncea. Nevertheless, disease incidence on Brassica amended soils were strongly related with the ratio between actinomycetes and fungi populations, showing that at ratios higher than 350, disease incidence was not significant. Populations of actinomycetes higher than 12.2 x 105 CFU x g-1, and fungi lower than 9.1 x 103 CFU x g-1, were also related with disease suppression in some soils, but in a moderated way. The high correlation found between soil microbial community and disease control strongly supported the hypothesis that microbial modification after Brassica amendments is the main mechanism involved in disease suppression in the long term. In addition, soil reaction was similarly affected by amendments regardless Brassica specie, suggesting that released products from Brassica tissues may act as a buffer in soil determining to a specific soil reaction. However, resulting soil reaction didn’t show relationship with disease suppression or the dynamics of soil microorganisms. Similarly, seedling growth was slightly reduced followed by amendments regardless Brassica specie, which suggested that GSL hydrolysis products could possess phytotoxic activity, or that Brassica amendment could have promoted the population of phytotoxic bacteria. Stable results in disease control and microbial community modification suggest significant potential of using B. rapa (first option) and B. napus (second option) for controlling R. solani damping-off disease in field. However, the optimization of Brassica amendment in field for different target crops must be further evaluated to ensure its efficacy in different agrosystems.
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