Biocontrol of potato wilt by selective rhizospheric and endophytic bacteria associated with potato plant

Abstract

Ralstonia solanacearum is the causative agent of wilt disease in plants, which constitutes a severe problem to agricultural crops, particularly for potato production in Madagascar. The present study focuses on the isolation, in vitro and in vivo assays of potential rhizospheric and endophytic bacteria associated with healthy potato plant, capable to inhibit the growth of Ralstonia solanacearum for controlling potato bacterial wilt. A total of 77 bacteria strains were isolated from six soil rhizospheric samples and six vegetal material samples of healthy potatoes in the district of Antsirabe II. Forty of them were telluric actinomycetes, 25 were endophytic actinomycetes and 12 were fluorescent Pseudomonas spp. An additional 30 phytopathogenic isolates were obtained from six rhizopsheric soil samples of diseased potatoes. Morphological, cultural, biochemical characterization and molecular identification with the Ralstonia solanacearum specific primers 759/760 revealed that 24 of the pathogenic isolates belong to the Ralstonia solanacearum species, biovar two; the causal agent of potato bacterial wilt. Isolates from healthy plants were, then, examined in vitro and in vivo for their antagonistic activity against Ralstonia solanacearum strain for their potential to improve potato plant growth. In vitro antagonism of actinomycete and Pseudomonas isolates against Ralstonia solanacearum development was performed using agar diffusion technique, while in vivo tests were conducted under greenhouse conditions. Ten antagonistic strains including two Pseudomonas, four telluric actinomycetes, and four endophytic actinomycetes inhibited the tested Ralstonia strain. Four strains, E7, E13 (endophytic actinomycete from root potatoes), S25 (telluric actinomycetes) and P7 (fluorescent Pseudomonas), showed high antagonistic activity against the pathogen with zones of inhibition from 23 to 40 mm. Of the fours strains tested in greenhouse, E7 significantly reduced (p