Biological control of damping-off of tomato caused by Pythium aphanidermatum by using native antagonistic rhizobacteria isolated from Omani soil

The potential of antagonistic bacteria isolated from tomato rhizosphere soils of Oman in the control of damping-off disease of tomato was investigated. A total of 27 bacterial isolates were isolated from 18 soil samples collected from the rhizosphere of tomato from Al-Batinah South, Al-Sharqia North and Muscat Governorate. These bacterial isolates were tested in vitro for their antagonistic activity against Pythium aphanidermatum using a dual culture technique. Of the 27 bacterial isolates tested, four isolates designated D1/3, D1/8, D1/17 and D1/18 were effective in inhibiting the mycelial growth of P. aphanidermatum, by inducing an inhibition zone of 32.3, 10.3, 6.3 and 9.9mm, respectively. Compatibility tests using a cross-streak assay on nutrient agar medium indicated that these four bacterial isolates were compatible with one another. The bacterial isolates were identified as Klebsiella oxytoca (D1/3), Exiguobacterium indicum (D1/8) and Bacillus cereus (D1/17 and D1/18), on the basis of the rRNA gene sequences. Among the isolates tested for in vitro plant growth promoting activity, D1/8 induced the maximum shoot length and seedling vigor. The potential of bacterial antagonists either individually or in combination in the control of damping-off disease of tomato was tested under greenhouse conditions. Among the biocontrol treatments, the combined application of D1/8 and D1/17 was the most effective, where damping-off incidence was reduced by 27% relative to the infected control. These bacterial antagonists appear to be potential candidates to be developed as bio-inoculants for the ecofriendly management of damping-off of tomato under desert farming ecosystem.