Biocontrol of charcoal-rot of sorghum by actinomycetes isolated from herbal vermicompost
A total of 137 actinomycetes, isolated from 25 different herbal vermicomposts, were characterized for their antagonistic potential against Macrophomina phaseolina by dual-culture assay. Of them, eight most promising isolates (CAI-17, CAI-21, CAI-26, CAI-68, CAI-78, KAI-26, KAI-27 and MMA-32) were characterized for the production of siderophore, chitinase, protease, hydrocyanic acid (HCN), indole acetic acid (IAA) and further evaluated for their antagonistic potential against M. phaseolina by blotterpaper assay and in greenhouse. All the eight isolates produced HCN and IAA, seven produced siderophore (except CAI-78) and protease (except KAI-27) and four produced chitinase (CAI-26, KAI-26, KAI-27 and MMA-32). In the blotter-paper assay, no charcoal-rot infection was observed in KAI-26 and KAI-27-treated sorghum roots, indicating complete inhibition of the pathogen, while the other isolates showed 47 to 88% lesser charcoal-rot infection compared to the control. In the antifungal activity test against M. phaseolina (in greenhouse on sorghum), all the isolates increased in shoot dry mass by 28 to 53% and root dry mass by 5 to 21%, over the control. In order to confirm the plant growth promoting (PGP) traits of the isolates, the green house experiment was repeated, but in the absence of M. phaseolina. The results further confirmed the PGP traits of the isolates as evidenced by 15 to 34% increase in shoot dry mass on six isolates (except CAI-26 and KAI-27), 14 to 57% increase in root dry mass on five isolates (except CAI-68, KAI-26 and KAI-27), 17 to 60% increase in root length on five isolates (except CAI-17, CAI-21 and CAI-68) and 10 to 64% increase in root volume on six isolates (except CAI-17 and CAI-68). Culture filtrate of three potential actinomycetes (CAI-21, CAI-26 and MMA- 32) at 0.5% inhibited the growth of M. phaseolina, indicating that the metabolites of these actinomycetes were responsible for the inhibition. The sequences of 16S rDNA gene of the isolates matched with Streptomyces but with different species in BLAST analysis. This study indicates that the selected actinomycetes have the potential for PGP and control of charcoal-rot disease in sorghum.
Key words: Antagonistic actinomycetes, biocontrol, charcoal-rot, Macrophomina phaseolina.