Ralstonia solanacearum is the causa

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 < 0.05) the percentage of Ralstonia solanacearum that
infected plants by 72.04%. The isolates E13 and S25 have also been demonstrated to
improve plant growth by increase of plant height to 44.63% and 44.84%, fresh weight
to 68.75% and 75.85% and dry weight to 86.17% and 115.42%, respectively compared
with non-treated control. Morphological and cultural characterization of these three
active isolates showed that they belong to the genus Streptomyces. The antagonism of
these isolates against Ralstonia solanacearum according to in vitro and in vivo tests
results, along with their high efficiency as regards the improvement of plant
development, suggests that these three actinomycete strains E7, E13 and S25 could be
useful for biocontrol of potato bacterial wilt.
Key words: Potato, Ralstonia, Actinomycetes, Pseudomonas, Biocontrol