Emerging bacterial diversity associated with oak decline: First report of several new taxa involved, including Pseudomonas quercina sp. nov. and Pseudomonas dehesae sp. nov., isolated from holm oak in Spain

Acute oak decline (AOD) is a complex decline syndrome associated with a polymicrobial bacterial assemblage, together with abiotic and other biotic factors. Although AOD and its associated bacteria are increasingly reported across Europe, holm oak and the Iberian Peninsula remain understudied. We characterized the culturable fraction of the bacterial community associated with declining Quercus ilex showing symptoms including bleeding stem lesions at one site in northwestern Spain over four consecutive years, combining isolation from wood and bark tissues, molecular identification, detached-branch inoculation assays, and strain re-isolation from necrotic tissues. We confirmed the presence of the core AOD bacteria in Q. ilex , including Gibbsiella quercinecans , previously reported from this host in Spain, together with the first record of Brenneria goodwinii on Q. ilex in Spain and the first record of Rahnella victoriana in Spain. Beyond the recognized pathobiome, several taxa not previously linked to oak decline showed necrogenic potential, including Klebsiella oxytoca, Pantoea phytostimulans, Enterobacter hormaechei subsp. hoffmannii, and two previously undescribed Pseudomonas species, for which the names Pseudomonas quercina sp. nov. and Pseudomonas dehesae sp. nov. are proposed. These findings expand the known diversity of culturable bacteria associated with symptomatic oaks, reveal previously unrecognized necrogenic enterobacteria and pseudomonads (including novel species), identify candidate bacteria for further testing in intact plants and natural disease systems and reveal Q. ilex in the Iberian Peninsula as host to a diverse and partly novel decline-associated microbiota.