Increasing Incidence of Apple Valsa Canker and Predominance of Cytospora mali in Gyeongsangbuk-do, South Korea

From 2015 to 2023, a survey was conducted to determine the occurrence of apple Valsa canker disease in major apple-producing regions in Korea. Infected branches were collected for the isolation and identification of the pathogens. During the survey period, a total of 38 fungal strains were isolated from trees infected with apple Valsa canker disease. A phylogenetic analysis using a combined dataset of the internal transcribed spacer (ITS) region and large subunit (LSU), actin (act1), RNA polymerase II (rpb2), and translation elongation factor 1-α (tef1-α) gene sequences was performed, identifying all of the isolates as Cytospora mali. According to the survey, the average annual incidence rate of apple Valsa canker disease was 2.8%. The infection rate was 2.2% in 2015, and it showed a decreasing trend until 2017. However, in 2018, the incidence rate began to gradually increase, reaching 4.2% in 2022 and sharply rising to 6.8% in 2023. The increasing incidence of apple Valsa canker disease is causing significant economic damage to apple producers, highlighting the need for effective control measures. Although a total of 21 pathogen species causing apple Valsa canker disease have been reported in East Asia, this study confirmed that C. mali is the dominant species causing the disease in Korea.