Biological control of root rot of groundnut with antagonistic Pseudomonas fluorescens strains

The potential of Pseudomonas fluorescens for the management of groundnut (Arachis hypogaea) root rot caused by Macrophomina phaseolina was evaluated under greenhouse and field conditions. Various strains of P. fluorescens isolated from the rhizosphere of groundnut were screened for their ability to inhibit mycelial growth of M. phaseolina. Among the various strains tested in vitro, five strains were found to inhibit the mycelial growth of M. phaseolina. Talc-based powder formulations of the effective strains of P. fluorescens were developed and evaluated for their efficacy in the management of groundnut root rot under greenhouse conditions. Seed treatment or soil application of powder formulation of P. fluorescens strain Pf1 effectively reduced groundnut root rot when compared to other strains. Seed treatment with Pf1 powder formulation exhibited a similar level of control to the soil application with P. fluorescens or soil drenching with carbendazim. Seed treatment with P. fluorescens reduced the root rot incidence from 88.8% (with non-bacterized seeds) to 33.3%. Four field trials were conducted over a 2-year period to determine the efficacy of seed treatment with powder formulation of Pf1 in checking groundnut root rot. Seed treatment with powder formulation of Pf1 resulted in significant reduction in root rot incidence and increase in pod yield in all field trials. Ten strains of P. fluorescens were tested for their ability to produce HCN, salicylic acid, siderophore and lytic enzymes. The strain Pf1 that showed maximum in vitro antagonism produced more HCN, salicylic acid, siderophore and β-1,3-glucanase in vitro when compared to other strains. The results suggest that more than one mechanism might be involved in suppression of M. phaseolina by P. fluorescens.