Canker and Dieback of Alnus rubra Is Caused by Lonsdalea quercina

Understanding the ecology of pathogens is important for disease management. Recently, a devastating canker disease was found on red alder (Alnus rubra) planted as landscape trees. Bacteria were isolated from two groups of symptomatic trees located approximately 1 km apart, and one strain from each group was used to complete Koch's postulates. The results showed that these bacteria can cause disease not only on red alder but also on two other alder species. Unexpectedly, analyses of genome sequences of bacterial strains identified them as Lonsdalea quercina, a pathogenic species previously known to cause dieback of oak species, but not alder. Additionally, a core genome phylogeny clustered bacterial strains isolated from red alder within a subclade of L. quercina strains isolated from symptomatic oak trees. Consistent with the close phylogenetic relationship, there was no obvious evidence for divergence in genome composition of strains isolated from red alder and oak. Altogether, the findings indicate that L. quercina is a potential threat to Alnus species.