Aims Verticillium wilt is difficult to suppress, and causes severe yield losses in a broad range of crops. Five Serratia and five Paenibacillus endophytic isolates showing antagonistic properties against fungal pathogens were compared for their plant growth-promoting (PGP) potential under different plant growth conditions with the objective of evaluating the PGP of endophytic strains in different ad planta systems. Methods Preselected isolates were applied to the surfacesterilized seeds of oilseed rape and cauliflower using biopriming. The isolates' PGP effect and root colonization capacities were compared under gnotobiotic conditions. One strain from each genus was selected and tested for its PGP qualities in sterile and non-sterile soil. Results Serratia treatment resulted in different levels of PGP, while Paenibacillus strains damaged roots under gnotobiotic conditions. P. polymyxa Sb3-1 did not have a significant effect on plant growth in non-sterile soil; however it did promote plant growth in the sterile soil. S. plymuthica 3RP8 and P. polymyxa Sb3-1 were selected for further testing of their biocontrol effect under field conditions. Conclusions The choice of growth environments in the investigation of plant-bacterium interaction is crucial. Non-sterile soil is suggested as the ideal medium for use in studying the PGP effect.

Biological control of plant diseases using soil amendments such as animal manure and composted materials can minimize organic waste and has been proposed as an effective strategy in crop protection. In this study, 35 organic amendments (OAs) and 16 compost mixtures were evaluated against Verticillium dahliae by assessing both the antagonistic effect on the mycelial growth of two representative isolates of V. dahliae and the effect on the reduction of microsclerotia viability of the pathogen in naturally infested soil. Eleven OAs and five compost mixtures showed a consistent inhibition effect in in vitro sensitivity tests, with solid olive‐oil waste compost one of the most effective. Therefore, a bioassay with olive plants was conducted to evaluate the suppressive effect against V. dahliae of these selected OAs and compost mixtures. Significant reduction in the severity of the symptoms of V. dahliae indicates the potential use of grape marc compost (100% disease severity reduction) and solid olive‐oil waste, combined with other OAs. Microorganism mixtures and dairy waste OAs had a potential suppressive effect when they were combined with compost, showing a 73% and 63% disease severity reduction, respectively. A mixture of agro‐industrial waste with other biological control agents is a promising strategy against verticillium wilt of olive. To the authors' knowledge, this is the first report on the effectiveness of compost extracts (compost teas) on the inhibition of natural microsclerotia of V. dahliae, and also on verticillium wilt suppression in olive with solid olive‐oil waste.