Bacterial Communities Associated With Acute Oak Decline of Sessile Oak (Quercus petraea) in Southern Sweden.

Acute oak decline (AOD) is a rapidly progressing disease affecting various oak species (Quercus spp.). Recent studies have shown that AOD is associated with a consortium of Gram-negative, facultatively anaerobic bacteria (e.g., in Enterobacterales) in the United Kingdom and continental Europe. However, there is limited information on the bacterial contributions and key genera associated with oak diseases and broadleaf forest ecosystems in Nordic countries. The primary objective of this brief study was to collect the first data on the bark microbiomes of symptomatic, declining sessile oaks (Q. petraea) in Sweden. Pairs of healthy and diseased bark samples were collected from symptomatic trees near Ankarsrum (Kalmar County), Sweden. After total DNA extraction, the bacterial 16S rDNA region was amplified, and Oxford Nanopore Technologies was used for long-read high-throughput DNA metabarcoding of the bacterial microbiome. We found a dominance of enterobacterial phytopathogens, including two of the typical genera associated with AOD, Brenneria and Rahnella, exclusively in the diseased bark samples. Our findings extend the known distribution of AOD-associated bacteria to Sweden and Scandinavia and show that diseased oaks in this region host a microbiome similar to those found in other parts of Europe.