Effects of biocontrol agents on suppression of damping-off in Cucumis sativus L. caused by Rhizoctonia solani

Biocontrol agents (Pseudomonas aeruginosa HB-15 and Pantoea sp. HB-22, alone or in combination) were applied on cucumber (Cucumis sativus L.) plants to evaluate their effect on suppressing damping-off caused by the fungal pathogen Rhizoctonia solani. Plant biomass, percent root mortality, phosphorus uptake, and the levels of stress-indicators NH3-NH4+ and proline in the plants were measured following the treatments. Additionally, gene expression induced by the biocontrol agents was compared to the gene expression induced by jasmonic acid and salicylic acid treatments. The biocontrol treatments increased fresh weight and phosphorus uptake, and reduced root mortality of the cucumber plants inoculated with the pathogen. No differences in plant ecophysiological responses were detected among biocontrol agent treatments. Concentrations of NH3-NH4+ and proline in the leaves and roots were higher in the control than in the biocontrol treatments. Salicylic acid- and Pantoea sp. HB-22-treated cucumbers showed increased expression of the acidic peroxidase-encoding gene and the phenylalanine ammonia homologue, while the P. aeruginosa HB-15 treatment did not induce an increase in expression of any of the genes studied.