Previously unreported symptom of bull’s-eye rot (Kigusare-byo) on apple fruits caused by Phlyctema vagabunda Desm.

Abstract

The incidence of small (mostly <5 mm in diameter), dark-colored lesions (DCSs: dark-colored spots) on apple fruits (e.g., cvs. Fuji, Gunma Meigetsu, Toki, Akita 19), mostly appearing from 3 weeks before harvest to 7 weeks after harvest has increased in Akita Prefecture and Aomori Prefecture in Japan since 2011. A fungus was frequently isolated from these lesions. When nonwounded apples on trees were inoculated with the fungus, spots very similar to DCSs developed, and the fungus was reisolated from the spots. The fungus was identified as Phlyctema vagabunda Desm. [synonyms Neofabraea alba (E.J. Guthrie) Verkley, N. vagabunda (Desm.), G. album Osterw.] based on its morphological and molecular characteristics. These results clearly indicate that P. vagabunda can cause DCSs and suggest that P. vagabunda is responsible for the increased incidence of DCSs on apple fruits. Overseas, the fungus is known to cause bull’s-eye rot of apples and pears. To the best of our knowledge, there are no reports describing the DCS symptom on Fuji apples caused by P. vagabunda. The most frequently observed symptoms of bull’s-eye rot of apples are very similar to those of Kigusare-byo reported in Japan. P. vagabunda was also isolated from the lesions of Kigusare-byo during storage in this study and caused typical symptoms of Kigusare-byo on apple fruits. Therefore, Kigusare-byo is, at least partially, caused by P. vagabunda, and the DCSs can be considered as a symptom of Kigusare-byo, i.e., bull’s-eye rot in apples. Based on these results and observations, we recommend “bull’s-eye rot” as the English name for Kigusare-byo. Although Kigusare-byo or bull’s-eye rot has been considered primarily a storage disease characterized by large rot lesions, we found that it may occur on fruits before harvest.