Brenneria roseae subsp. roseae, a new pathogen of walnut (Juglans regia L.) in Europe

Abstract Walnut production in Hungary has grown dynamically in the 2000s, with the area planted almost quadrupling. The production is mainly based on local varieties, using almost exclusively Hungarian nurseries produced graftings as propagating material. In the summer of 2023, numerous symptoms with bark cracking and brown oozing spots of varying size appeared in a mature walnut orchard, in the central part of the country. In order to identify the causative pathogen, samples of necrotic tissue of sapwood were taken from two symptomatic trees. Bacteria of round-shape, light cream colour, convex colonies could be obtained from both samples. Based on the 16S rRNA gene sequence, the two newly isolated bacterial strains (III/9 and IV/11) were 100% identical to each other and showed 99.7% similarity to the type strain of Brenneria roseae subsp. roseae . The phylogenetic analysis based on four housekeeping protein coding gene ( atpD , gyrB , infB and rpoB ) sequences of the newly isolated strain III/9 confirmed the taxonomic position. The Hungarian stain III/9 clustered together Brenneria roseae subsp. roseae , including strain Br190, which was isolated from walnut in Iran (Allahverdipour et al., 2021). Pathogenicity test on two-years old walnut graftings cv. ‘Milotai intenzív’ and ‘Bonifác’ demonstrated that Brenneria roseae subsp. roseae strain III/9 can cause tissue necrosis and oozing on woody parts of the walnut trees. Based on these results, Brenneria roseae subsp. roseae should be added to the list of pathogens of walnut with a proven European occurrence.