Symplocos myrtacea Siebold et Zucc.

Family: Symplocaceae
English: Japanese sapphireberry
Japanese: ハイノキ (灰の木) [Hainoki]

In early May the mountain trails are littered with the small white petals. The leaves are also a beautiful tear-drop shape and these plants--which are simialr to S. paniculata, aka sapphireberry--are apparently quite popular in horticulture. Forget orchids, because a pair of plants that surpasses 2m in height can easily sell for more than 100,000 yen. The flowers are followed by small berries that turn navy blue in the autumn, when they are enjoyed by monkeys and birds. When the berries are gone, the monkeys will continue to forage on the leaves throughout the winter; however, I've never seen deer eat the leaves.

References:
新版　屋久島の植物 川原勝征,初島住彦(監修),2001：南方新社
米倉浩司・梶田忠 (2003-)　「BG Plants 和名－学名インデックス」（YList），http://ylist.info（ 2019年8月14日）