Rosmarinus officinalis L. is an evergreen shrub widely cultivated as an ornamental plant and for its bioactive compounds. However, the increasing intensification of rosemary cultivation has led to the emergence of phytosanitary problems caused by fungal and bacterial pathogens. In 2018, rosemary plants showing branch knot symptoms and severe desiccation were collected from an open-field cultivation in Southern Italy. Bacterial isolation from symptomatic tissues yielded several colonies with pseudomonad-like features. In this study, an accurate diagnosis of the observed disease was performed, together with the molecular, biochemical, and physiological characterization of the isolates and the assessment of their pathogenicity and interactions. Bacterial identification based on LOPAT tests, BIOLOG GEN III microplates, and multi-locus sequence analysis revealed the presence of Pseudomonas viridiflava , P. asturiensis , and P. alliivorans . All three species were pathogenic to rosemary, although with different levels of virulence, with P. alliivorans causing the most severe symptoms. Two- and three-species co-inoculation assays on rosemary and tomato plants indicated that competitive interactions predominated among the strains, with no evidence of synergistic effects on disease development. These results were supported by in vitro assays, which revealed antagonistic behaviors and mutual inhibition among the species. Overall, this study provides new insights into the complex interactions among Pseudomonas species involved in rosemary disease and highlights the role of interspecific competition in shaping disease outcomes.
Semenzato et al. (2026) studied this question.