Antagonistic potential of selected rhizospheric Streptomyces from potatoes against Ralstonia solanacearum species complex

This work aimed to control Ralstonia solanacearum species complex, the causal agent of potatoes bacterial wilt by selecting potatoes rhizospheric Actinomycetes as biocontrol agents. The research approach focused on the isolation of Actinomycete strains from rhizospheric soils of potatoes using serial dilution technique on Starch Casein Agar medium, in vitro antagonistic activity of isolated strains with cross streak and agar cylinder disk methods, evaluation of antagonistic mechanisms of active isolates by different assays, phenotypic characterization and identification of potent Actinomycetes. According to the results obtained, 71 Actinomycetes were isolated from 17 soil samples of Manandona and Androakavato in Vakinankaratra region, Madagascar. Seven (7) isolates (9.85%) showed antagonistic activity against Ralstonia solanacearum species complex (phylotypes RI, RII and RIII) with an inhibition zone values ranging from 3 to 16 mm (isolate A28 against the phylotype RII and the isolate A5 on the phylotype RIII, respectively). The isolates were active on RI and RIII than RII. Three (3) selected isolates (A10, A28 and A40) used as antagonistic mechanisms, the production of indole acetic acid (0.11 to 0.14 µg/ml), ammonia, hydrogen cyanide, extracellular enzymes (protease, cellulose and urease), phosphate solubilization and antibiosis. Phenotypic characterization demonstrated that the three selected Actinomycete isolates were identified as Streptomyces. From these results, it would be suggested that the use of Actinomycetes as bioinocula for plant growth promoting, nutrient mobilization or biocontrol agent in biopesticide - biofertilizers against Ralstonia solanacearum species complex is promising. It is a simple technology that can be applied as part of sustainable agriculture.