Efficacy of native antagonistic rhizobacteria in the biological control of Pythium aphanidermatum-induced damping-off of cucumber in Oman

The efficacy of bacterial isolates obtained from the rhizosphere soil samples collected from cucumber fields of Muscat Governorate of Oman was assessed against Pythium aphanidermatum, the causal agent of damping-off of cucumber. Of the 58 isolates screened in a dual culture assay, only four isolates (B11, B9, AT3 and 4A) inhibited the growth of P. aphanidermatum and produced inhibition zones of above 5 mm. Among the four isolates, B11 was the most effective followed by B9, AT3 and 4A, which recorded inhibition zones of 14 mm, 13 mm, 11 mm and 6.8 mm, respectively. Scanning electron micrographs of hyphae of P. aphanidermatum grown in the presence of antagonists showed morphological abnormalities. These antagonistic bacteria were identified based on sequence analysis of 16S rDNA gene. The potential of the promising antagonists, Pseudomonas resinovorans (B11) and Pseudomonas aeruginosa (AT3) was evaluated either individually or in combination with a commercial formulation of Trichoderma viride against damping-off of cucumber under greenhouse conditions. Among the biocontrol treatments, soil application of P. resinovorans (B11) was the most effective, which resulted in a significantly lower damping-off incidence (24 to 30%) compared with Pythium-infected control (82 to 84%). To our knowledge, this is the first report on biological control of P. aphanidermatum with P. resinovorans.