Previous work (within HDC Studentship CP54) has shown that rhizobacteria that modify crop hormone status (Variovorax paradoxus 5C-2, Bacillus subtilis IR-15) can stimulate lettuce head weight of crops grown under both well-watered and drying soil conditions in both pot experiments and small-scale (6 m2 cropping area per treatment) polytunnel experiments. More recent work (2012 season) conducted on grower’s holdings showed that V. paradoxus 3C-1 increased iceberg lettuce head weight by 20%. The current proposal aims to determine suitable propagation, storage and inoculation techniques for large-scale inoculum production using both commercially available (Bacillus subtilis QST713, marketed as Serenade by AgraQuest) and experimental inoculants (available at Lancaster University). Inoculants will be produced (or purchased) and after different durations of storage, will be supplied (by either a soil drench or foliar spraying) at different concentrations to lettuce seedlings in pot trials. Rhizobacterial colonisation of the root system and plant growth will be measured. Having determined the optimum concentrations and method of application, 2 different inocula will be added to transplants in the nursery and then seedlings planted out to the field in covered (“managed drought”) and uncovered experiments. Plants will be grown at two different levels of soil water availability (by automatic irrigation according to different soil moisture thresholds) until harvest. Head weight, disease incidence and rhizobacterial colonisation of the root system will be determined at harvest.