Biocontrol of charcoal rot in sorghum by fluorescent pseudomonads associated with the rhizosphere

Abstract

Charcoal rot of sorghum caused by Macrophomina phaseolina is a disease of economic importance for which a high level of genetic resistance is not available. Therefore, we made an effort to manage this disease with fluorescent pseudomonads associated with field-grown sorghum crops. One hundred and twenty-six fluorescent Pseudomonas spp. from different sorghum growing regions of India were isolated, selected and evaluated for in vitro antifungal activities and three selected strains were further tested for management of charcoal rot under field conditions. Antifungal activities (inhibition of growth, biomass, microsclerotia production, spore germination) of the secondary metabolites and the cell-free culture filtrates of the selected fluorescent pseudomonad strains (SRB129, SRB288 and Pseudomonas chlororaphis SRB127) were studied in detail. SRB127, SRB129 and SRB288 inhibited mycelial growth of M. phaseolina ranging from 30.5 to 76.5% in dual culture assay. The cell-free culture filtrates of these strains at 20% (v/v) concentration significantly reduced the formation and germination of microsclerotia of M. phaseolina. In the field, P. chlororaphis SRB127 emerged as an effective biocontrol agent of charcoal rot of sorghum. The bacterium, when applied as seed treatment, reduced the charcoal rot incidence by >40%, crop-lodging by >20%, and increased grain mass. P. chlororaphis SRB127, when grown in a gnotobiotic sand system, effectively colonized the sorghum root and formed microcolony-like cell-aggregates in some parts of the root. Under glasshouse conditions the bacterium survived in the sorghum rhizosphere without a significant reduction in population. We conclude that the use of a selection of effective strains of bacteria can be a useful component of integrated management of charcoal rot in sorghum.