Control of Rhizoctonia solani damping-off disease after soil amendment with dry tissues of Brassica results from increase in Actinomycetes population

• We evaluated Brassica soil amendments to control R. solani damping-off disease.
• Dynamics of fungi, bacteria and Actinomycetes population in soils were also evaluated.
• Amendment with B. napus and B. rapa promoted population of Actinomycetes.
• The ratio Actinomycetes:fungi showed strong correlation to disease suppression.
• Brassica amendments suppress damping-off disease by means of microbial modification.

This study investigated the potential use of leaf residues from Brassica rapa, Brassica napus and Brassica juncea to control Rhizoctonia solani damping-off disease. Fungitoxicity against R. solani of volatiles and water-soluble extracts from tested tissues were evaluated in vitro. To identify the mechanisms of disease control, the dynamics of fungi, bacteria and Actinomycetes populations in soils, soil pH and seedling growth were evaluated in a greenhouse. In three soils tested, amendments with B. rapa achieved the strongest overall suppression of R. solani (96.7–100%), followed by B. napus (86.7–100%) and B. juncea (40–100%). Brassica amendments also significantly impacted soil microbial populations. Disease incidence was strongly correlated with the ratio of Actinomycetes/fungi population in soils. Average disease incidence in soils treated with B. rapa and B. napus amendment was less than 4% when the ratio of Actinomycetes/fungi was higher than 505.6 ± 178.2. In addition, disease suppression in greenhouse conditions was not significantly correlated with the in vitro fungitoxicity of the three Brassica species. The amendments with B. rapa and B. napus showed moderate fungitoxicity in vitro, but achieved significant suppression of R. solani in soil. This indicates that compounds in these two species of Brassica might regulate microbial populations to achieve long-term disease control. Results indicate that alternation of microbial populations by B. rapa and B. napus is the main mechanism involved. The considerable phytotoxicity of these two species suggests that to maximize performance in controlling R. solani, further investigation into their selective phytotoxicity is needed.

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