Leaf spot disease of Malus micromalus caused by Alternaria alternata and A. ovoidea recently documented in China

Malus micromalus is a famous ornamental tree species in China. Its fruits are also edible and have medicinal value. M. micromalus leaf spot disease was observed in October in Nanjing, which is located in Jiangsu Province, China. This illness induces the formation of nearly spherical lesions on the leaves, characterized by dark brown coloration and accompanied by black specks on the affected area and prominent concentric rings at the boundary between the infected and healthy tissue. In the advanced stage, the leaves undergo yellowing and subsequent abscission. The plant's growth is significantly impaired by the illness. Koch's postulates confirmed the identity of the fungal isolates (MS1-1 to MS1-9) were pathogenic to M. micromalus. The morphological features of the pathogens were consistent with those of Alternaria spp. Therefore, because of their morphological traits, the nine isolates were categorized as Alternaria species. Phylogenetic analyses were performed through a combined assessment of internal transcribed spacer region (ITS), glyceraldehyde-3-phosphate dehydrogenase (GAPDH), Alternaria main allergen gene (Alt a 1), the RNA polymerase II second largest subunit (RPB2), and portions of translation elongation factor 1-alpha (TEF1-α), and the nine isolates were identified as A. alternata and A. ovoidea. Leaf spot disease in M. micromalus caused by Alternaria has never been reported before, either in China or worldwide.