Characterization of Pathogenic Bacteria Associated with Wetwood Disease in Populus deltoides

Populus species are highly susceptible to wetwood formation, which adversely affects tree growth, timber quality, and wood processing. In this study, 28 aerobic and 7 anaerobic bacterial strains were isolated and purified from I-69 poplar trees infected with wetwood using tissue-based pathogen isolation techniques. Preliminary screening identified three highly pathogenic isolates, including two aerobic strains (AB4 and AB14) and one anaerobic strain (ANAB1), all of which induced wetwood symptoms in 100% of inoculated seedlings with pronounced severity. Through comprehensive characterization, including morphological analysis, physiological–biochemical profiling, and 16S rRNA gene sequencing, these strains were taxonomically classified as Pantoea agglomerans (AB4), Escherichia fergusonii (AB14), and Enterobacter hormaechei (ANAB1). These 35 strains were subsequently inoculated into one-year-old healthy poplar seedlings through three distinct methods, including stem injection, root infection, and leaf infection. Experimental results demonstrated that only stem injection successfully induced wetwood symptoms, while root and leaf infection failed to produce pathological manifestations. For stem-inoculated specimens, pathogenicity was evaluated based on three diagnostic parameters, including heartwood discoloration length, pigmentation intensity, and affected tissue area ratio. Significant variability in symptom severity was observed among different bacterial strains. These findings expand the known diversity of bacterial species associated with wetwood development and provide valuable insights for understanding its etiology and for guiding future disease management strategies.