Rhizosphere actinobacteria for combating Phytophthora capsici and Sclerotium rolfsii, the major soil borne pathogens of black pepper (Piper nigrum L.)

Abstract

The experiment was conducted with an objective to survey potential black pepper growing tracts of Kerala and Karnataka for the isolation and characterization of rhizosphere actinobacterial strains for exploiting its antagonistic potential against major pathogens of black pepper as well as for growth promotion. Accordingly fifty actinobacterial strains were isolated and were morphologically characterized and studied for its antagonism against major soil borne pathogens of black pepper viz., Phytophthora capsici and Sclerotium rolfsii. Three isolates (IISRBPAct1, IISRBPAct25 and IISRBPAct42) showed more than 90% inhibition against the targeted pathogens. The isolate IISRBPAct1 showed 91–94% inhibition to both of the pathogens followed by IISRBPAct42 (68–94%) and IISRBPAct25 (86–90%). The potential isolates were characterized morphologically using light microscopy and Scanning Electron Microscopy (SEM). Molecular characterization was done by 16S-rDNA sequencing using two sets of actinomycetes specific primers viz., 1) S-C-Act-235-S-20 and S-C-Act-878-A-19 and 2) 27f and1525r were identified as belonging to Streptomyces sp. The isolates exhibited production of different hydrolytic enzymes such as amylases, proteases, lipases, and cellulases. Further the isolates were evaluated for their Plant Growth Promoting (PGP) traits and biocontrol traits such as production of Indole Acetic Acid (IAA) and siderophores. IISRBPAct1 showed production of both IAA and siderophore while IISRBPAct25 and IISRBPAct42 produced only siderophore. In planta experiment was conducted to evaluate the growth promotion activity as well as pathogen suppression. Out of the three potential Streptomyces spp IISRBPAct1 showed maximum growth promotion in terms of shoot biomass, shoot height and number of laterals where as maximum root biomass was observed with IISRBPAct42. Highest reduction of disease incidence was observed on treatment with IISRBPAct1 (98.10%) against Sclerotium rolfsii while IISRBPAct25 showed highest reduction of foot rot incidence (80.73%). The results of the study clearly revealed the biocontrol and PGPR properties of three Streptomyces sp. which can be developed as potential candidates for the biological control of major black pepper pathogens.