Biocontrol potential of endophytic bacteria against a collection of Leptosphaeria maculans isolates causing blackleg in oilseed rape

BACKGROUNDBlackleg, caused by Leptosphaeria maculans, is a major disease limiting oilseed rape production worldwide, and its management increasingly requires sustainable alternatives to chemical fungicides. In this study, we evaluated the antagonistic activity and plant growth-promoting potential of three endophytic bacteria, Bacillus velezensis Bro5, Bacillus subtilis Bro11, and Pantoea agglomerans Bru13, against a geographically diverse collection of 139 L. maculans isolates from five oilseed rape-producing regions of Argentina. RESULTSDual culture assays revealed strong inhibitory activity by Bro5 and Bro11, with mean inhibition rates of [~]80% across isolates, while Bru13 showed variable inhibition (<75% for most isolates). Greenhouse and growth chamber assays confirmed the protective potential of these strains. At the cotyledon stage, Bro11 and Bro5 reduced lesion size by 47% and 28%, respectively, while their combination achieved a 51% reduction. In greenhouse trials, combined application of Bro5 and Bro11 reduced stem base necrosis by 45% and increased the proportion of plants with [&le;]50% damage to 98%, compared to only 70% in controls. Key disease metrics, including disease index, incidence, and severity, decreased by 60%, 23%, and 26%, respectively. Beyond pathogen suppression, inoculation with the Bro5-Bro11 consortium enhanced plant growth, increasing shoot biomass by 89% at early stages, and improving stem dry weight and diameter by 10% and 35%, respectively, at maturity. CONCLUSIONThese findings highlight the robustness of Bacillus endophytes as biocontrol agents, their capacity to suppress diverse pathogen isolates, and their dual role in plant growth promotion, supporting their potential integration into sustainable blackleg management programs.