Biological control of tobacco black shank and colonization of tobacco roots by a Paenibacillus polymyxa strain C5

Tobacco black shank, caused by a soil-borne pathogen, Phytophthora parasitica var. nicotianae, is one of the most serious diseases which reduce tobacco production worldwide. In this study, P. parasitica var. nicotianae was successfully controlled in vitro and in vivo by a newly isolated strain, Paenibacillus polymyxa C5, which is a plant-growth-promoting rhizobacteria and an effective biocontrol agent. Pot experiments were carried out to evaluate the effect of a novel bio-organic fertilizer (BIO), produced by the solid fermentation of organic fertilizer amended P. polymyxa C5, on the control of tobacco black shank. In comparison with the control, the disease incidence was significantly (P < 0.05) reduced by 50% with the application of BIO. P. polymyxa C5 was tagged with a plasmid-borne gfp gene to investigate its colonization behavior on tobacco roots in a natural soil system. As determined by confocal laser scanning microscopy, P. polymyxa C5 formed a biofilm on the tobacco roots growing in soil, both on the root tip and elongation zone but not inside the roots. It is speculated that the colonization of tobacco roots by P. polymyxa C5 is one of the mechanisms for the protection of tobacco plants from fungal infection.