The soilborne fungi Sclerotinia sclerotiorum , Rhizoctonia solani and the oomycete Pythium ultimum are among the most destructive pathogens for lettuce production. The application of the biocontrol agent Paenibacillus alvei K165 to the transplant soil plug of lettuce resulted in reduced S. sclerotiorum , R. solani and P. ultimum foliar symptoms and incidence compared to untreated controls, despite the suppressive effect of the pathogens on the rhizosphere population of K165. In vitro , K165 inhibited the growth of S. sclerotiorum and R. solani but not P. ultimum . Furthermore, the expression of the pathogenesis‐related ( PR ) gene PR 1 , a marker gene of salicylic acid ( SA )‐dependent plant defence, and of the Lipoxygenase ( LOX ) and Ethylene response factor 1 ( ERF 1 ) genes, markers of ethylene/jasmonate ( ET / JA )‐dependent plant defence was recorded. K165‐treated plants challenged with P. ultimum showed up‐regulation of PR 1 , whereas challenge with R. solani resulted in up‐regulation of LOX and ERF 1 , and challenge with S. sclerotiorum resulted in up‐regulation of PR 1 , LOX and ERF 1 . This suggests that K165 triggers the SA ‐ and the ET / JA ‐mediated induced systemic resistance against P. ultimum and R. solani , respectively, while the simultaneous activation of the SA and ET / JA signalling pathways is proposed for S. sclerotiorum .