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