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Poplars (Populus spp.) of the Family Salicaceae are extensively cultivated worldwide and are susceptible to a variety of bacterial and fungal diseases. In Populus species, leaf rust disease caused by several species of Melampsora leads to considerable damages in plantations. Melampsora larici-populina is the most devastating and widespread fungal pathogen causing leaf rust disease in poplars. In this study, leaves and young stems of rooted cuttings of two poplar clones were treated with L-form bacteria of Bacillus subtilis NCIMB 8054, ATCC 6633 and then challenged with the spores of rust pathogen M. larici-populina. The development of uredinia was evaluated in the laboratory using the leaf disc assay. The L-forms greatly reduced rust severity in inoculated poplar leaves (local effect), while to a lesser extent in non-inoculated leaves obtained from inoculated plants showing a low systemic effect on pustule development. This plant- L-form symbiosis may have contributed significantly to a quantitative resistance to M. larici-populina indicating a promising implication for the use of L-form bacteria of B. subtilis as a biocontrol agent for poplars against the rust pathogen.