Evidence and characterisation of Xanthomonas arboricola pv. juglandis causing bacterial blight of walnut in Montenegro

This study represents the first evidence of the bacterial blight caused by Xanthomonas arboricola pv. juglandis (Xaj) on walnut trees in Podgorica (Montenegro). Disease symptoms appeared on leaves in the form of dark, angular leaf spots surrounded by yellow-green haloes and lesions spread across the whole leaf. Isolated bacteria were preliminarily identified using PCR with pathovar-specific primer pair XajF/XajR and further characterised based on multi-locus sequence analysis with nine housekeeping genes (fusA, gapA, gltA, gyrB1, lepA, rpoD, dnaK, fyuA, and gyrB2). Montenegrin walnut isolates were homogeneous among themselves and the most closely related to different X. arboricola strains originating from Juglans regia isolated elsewhere. The pathogenicity of isolates was confirmed on walnut leaves, fruits, and branches. All inoculations resulted in the formation of necrotic lesions that initially developed at the site of bacteria entry, with later developing chlorotic areas on leaves along the leaf veins. This finding of Xaj causing leaf blight symptoms on walnuts in Montenegro highlights its expanding distribution across Europe and indicates a potential threat to walnut plantations in Montenegro.