Background

Ralstonia solanacearum is a soil-borne bacterial phytopathogen causing bacterial wilt disease in more than 200 crop species representing 50 different families (Seleim et al., 2014), the majority from the family Solanaceae (Thera et al., 2010). It can thrive up to several years in the soil retaining the capability to infect upon arrival of a host (Coutinho and Wingfield, 2017). Its natural route of entry is through the roots and finally, it ends up colonizing in the xylem vessels causing blockage of water conduit due to secreted exopolysaccharides with a corresponding wilting symptom observed in the leaves (Genin and Denny, 2012; Peeters et al., 2013 ; Seleim et al., 2014). Despite having usual practices of disease control such as the use of resistant plant varieties, industrial pesticides (Yuliar et al., 2015), the use of biocontrol agents is a much-preferred choice in the current scenario due to environment friendly nature of the biocontrol agents. Bacteria and bacteriophages have been reported as biocontrol agents against R. solanacearum, of which bacteria being the predominant (Yuliar et al., 2015). The assay for the potential to antagonize R. solanacearum under in vitro conditions is quite easy as standard methods like agar well diffusion (Balouiri et al., 2016), disc diffusion (Gupta et al., 2015) and spot-on lawn assay (Vijayakumar and Muriana, 2015) are available. To be considered as a biocontrol agent of practical use it must primarily have the ability to control the pathogen under in planta condition and it should not have any negative effect on other living beings. The ability of self-sustainability and neutral effect on other living beings is a subject of later consideration, the first and foremost being its ability to antagonize the pathogen under in planta condition (Wang et al., 2019). Conventional methods of bacterial biocontrol agent screening against R. solanacearum under in planta conditions such as seed bacterization (Siddiqui and Meon, 2009) and soil drenching (Van Elsas and Heijnen, 1990) have several limitations as they require viz. 1) large amount of inoculum, 2) prolonged maintenance of plants and 3) large space. In general, these demerits sum up these methods as time consuming and costly. To further advance the research on biocontrol bacteria against R. solanacearum, a quick screening method escaping the limitations of above-mentioned methods is required. Considering the entry of this pathogen through the roots, it was hypothesized that the application of biocontrol bacteria in the root would have the maximum chance to antagonize R. solanacearum thereby attenuating or eliminating its disease-causing capacity. The method described here is easy to perform and economically viable as it requires very less resource and time. The method is useful to screen biocontrol efficacy of several bacterial isolates simultaneously against R. solanacearum in a very limited space.