First report of Xanthomonas arboricola on oleander

Nerium oleander L is a long-lasting flowering vegetatively propagated ornamental plant of the Mediterranean region, where it is a major imported flowering pot plant. Only a few bacteria can infect it such as Pseudomonas savastanoi pv. nerii, Agrobacterium tumefaciens and Xylella fastidiosa. Between 2018 and 2022 we collected several infected plant parts in our country. In 2020 we observed atypical symptoms on the leaves and stems, which were not clearly similar to the known bacterial infection of oleander. In our work, we aimed to identify the pathogen. The isolates formed yellow-coloured bacterial colonies on King-B and on YDC agar, were Gram-negative, oxidase negative and induced hypersensitive reaction on tobacco leaves. The biochemical properties were determined by API 20E and API 50CH tests. Brown necrosis was observed on oleander leaves in a pathogenicity test. Multilocus sequence analysis was used for molecular identification of the pathogen. Three housekeeping genes (gyrB, fyuA and rpoD) were amplified. According to symptoms, colony morphology, biochemical features, pathogenicity and molecular methods, the pathogen was identified as Xanthomonas arboricola. This is the first report of the plant pathogenic Xanthomonas spp. on oleander.