Large-scale field survey reveals overall yield loss in Japanese apricot possibly caused by two ampeloviruses

Japanese apricot ( Prunus mume Sieb. et Zucc.) is one of the most popular fruit tree species in Japan and is most densely cultivated in Wakayama Prefecture. Since the 1980s, graft-transmissible symptoms, such as leaf-edge necrosis and incomplete flower development, have been observed in Nanko, the major Japanese apricot cultivar in Wakayama. Two ampeloviruses, plum bark necrosis stem pitting-associated virus (PBNSPaV) and little cherry virus 2 (LChV-2) were detected in Nanko trees showing typical symptoms. However, there is currently insufficient epidemiological data to infer that these viruses cause the symptoms. In this study, we conducted a large-scale survey of symptoms and RT-PCR detection of the two viruses in midwestern districts, the major cultivation areas in Wakayama. Among 208 surveyed Nanko trees, two viral infections were significantly associated with incomplete flower development, low fruit bearing rate, and interveinal chlorosis. LChV-2 infection also affected Nanko fruit and stone sizes. Additionally, we showed that the two viruses were already widely distributed throughout the main cultivation area. These results imply that the two ampeloviruses cause overall yield loss in Japanese apricot in Wakayama Prefecture.