Evaluation of antagonistic mechanisms of bacterial species recognized for the first time for their biocontrol activity against fire blight (Erwinia amylovora)

The results of our previous studies showed that 8 examined bacterial strains originating from the apple phyllosphere or soil environment showed both antagonistic activity towards E. amylovora and the protective ability of apple flowers and terminal shoots against fire blight. Five of the strains represented bacterial species in which this activity was observed for the first time (L16 Pseudomonas vancouverensis, 3M Pseudomonas chlororaphis subsp. aureofaciens, 43M Enterobacter ludwigii, 59M Pseudomonas protegens and 35M Pseudomonas congelans) (Mikiciński et al. 2020). We now present an attempt to explain the potential mechanisms related to the biocontrol capacity of the first four of the above mentioned strains. The studied strains were very effective in protecting pear fruitlet slices against fire blight. The disease severity after preventive treatment of the slices with the tested strains ranged from 0.0 to 0.6, while that of the untreated slices was 4.0 (rating scale: 0.0–4.0). Among the mechanisms studied, the L16 strain, characterized by the highest antagonistic activity, showed the ability to produce siderophores, biosurfactant, hydrogen cyanide (HCN), salicylic acid (SA) and indole-3-acetic acid (IAA). The L16 strain also degraded nicotinic acid. The 43M strain showed the lowest activity, producing only IAA and degrading nicotinic acid. A study of the detection of genes encoding antibiotics characteristic of pseudomonads showed the presence of prnD and gacA in the 3M strain and phlD, pltB, pltC and gacA in 59M. However, none of the genes sought were detected in the L16 strain.