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Abstract

Cucumber (Cucumis sativus L.) damping-off, which is caused by the soilborne oomycete Pythium aphanidermatum, is an extremely damaging disease. An environmentally friendly and safe method of controlling damping-off in cucumber and other vegetable crops is biological control, which employs naturally existing antagonistic microorganisms. In this investigation, bacteria from root tissues (endophytic bacteria) and rhizosphere soil (rhizosphere bacteria) were isolated from Tamarix nilotica (Tamaricaceae), commonly known as salt cedar, and their potential to inhibit P. aphanidermatum and their efficacy in managing damping-off in cucumber under controlled glasshouse conditions were assessed. Ten rhizospheric and six endophytic bacteria with different morphological traits were isolated using nutrient agar medium. Of them, one endophytic bacterium (TaniEB3) and one rhizosphere bacterium (TaniRB10) showed strong inhibitory activity against P. aphanidermatum in in vitro tests and caused morphological defects in the pathogen's hyphae, as seen by scanning electron microscopy. These bacterial isolates were determined to be Sphingopyxis soli (TaniEB3) and Bacillus cereus (TaniRB10) based on MALDI-TOF-Biotyper analysis. In vitro compatibility analysis employing cross-streak assay revealed that S. soli TaniEB3 and B. cereus TaniRB10 were incompatible. B. cereus TaniRB10 and S. soli TaniEB3 exhibited NaCl tolerance up to 5% and 7.5%, respectively. In the two-sealed-base-plates tests, S. soli TaniEB3 was the only isolate to produce antimicrobial volatile chemicals that inhibited P. aphanidermatum's growth. Soil treatment with B. cereus TaniRB10 and S. soli TaniEB3 reduced the damping-off incidence in cucumber by 37.5% and 12.5%, respectively, compared to the P. aphanidermatum-infected control. As a biocontrol agent for managing P. aphanidermatum damping-off of cucumber, B. cereus TaniRB10 merits further investigation.

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