First detection of Diplodia bulgarica, a new pathogen causing black canker of apple trees in Poland

The cultivated apple (Malus × domestica Borkh.) is the most economically important crop in Poland, the largest producer of apple fruits in Europe. During the growing season of 2024, typical symptoms of branch cankers and scaling-off of the bark were observed on the 24-year-old and over 60-year-old apple trees in the historical garden located in Nieborów, Lodzkie voivodeship. Fungi with greyish mycelium were repeatedly isolated from the interior of symptomatic branches. Based on cultural and morphological characteristics and DNA sequence data comparisons for ITS rDNA and translation elongation factor 1-alpha (EF1-α) regions, the causing fungus was identified as Diplodia bulgarica. Pathogenicity tests were performed on apple trees cv. 'Najdared', by inoculating them with mycelial plugs. All examined strains caused necrotic lesions on the leader, leading to the death of the whole tree, confirming the pathogenicity of the studied fungal isolates toward the apple tree.