Curtobacterium pruni sp. nov. causing black blotch on red apricot fruit in China

The red apricot ( Prunus armeniaca ‘Bei-zhai-hong-xing’) is exclusively cultivated in Beijing and highly popular in the Chinese market due to its nutrient-rich fruits, which contain vitamin C, carotene, calcium, phosphorus, and potassium. In May 2021, distinct black spots or black botches were observed on fruits in many orchards and approximately 50% of fruits were infected, resulting in significant economic losses for farmers. Literature review confirmed no prior identification of the causal pathogen. Early-stage symptoms manifested as small, brown circular lesions that rapidly enlarged and turned black and formed black blotches within several days; no mycelia or spores were observed. Fruits exhibited high susceptibility, whereas leaves remained asymptomatic. Single colonies were isolated from diseased fruits. Representative isolate of Hongxing underwent pathogenicity testing and identification via morphological characterization and 16S rRNA gene phylogenetic analysis. Cells of the pathogen were rod-shaped, Gram-stain-positive, non-spore-forming and of (0.5-3.0) µm × (0.2–0.6) µm in size. The whole-genome sequencing of strain Hongxing showed that the circular chromosome comprised 3,884,414 bp with 70.98% G + C content and encoded 3,657 protein-coding genes. The phylogenomic comparison with other Curtobacterium species demonstrated that the strain Hongxing is distinctive from other known Curtobacterium species. Combined morphological and phylogenetic analyses identified strain Hongxing as a novel Curtobacterium species pathogenic to apricot fruit. We propose the novel species Curtobacterium pruni sp. nov., with type strain Hongxing ^T . To the best of our knowledge, this is the first report of Curtobacterium sp. as a pathogen on red apricot fruits.