Update on the situation of huanglongbing and Diaphorina citri in Japan
In a recent review, Iwanami (2022) presents the history of huanglongbing and its management in Japan. In Japan, the prevalent pathogen and vector are ‘Candidatus Liberibacter asiaticus’ and Diaphorina citri (both EPPO A1 List), respectively. The vector, D. citri, is widespread in the subtropical islands that are located between Taiwan and Kyushu main island. On these subtropical islands, Murraya paniculata, a preferred host of D. citri, is widely planted as a hedge in residential areas.
Huanglongbing was not detected in Japan until the 1980s. Initial surveys conducted in citrus commercial orchards did not immediately detect the disease. In 1988, the disease was first found in 2 Citrus depressa trees (1 in a small orchard and 1 in a private garden) on Iriomote island (Okinawa prefecture, Ryūkyū archipelago) and these trees were subsequently destroyed. It is considered that if surveys had targeted private gardens, it is likely that the disease would have been detected earlier. It is even supposed [MM2]that huanglongbing was endemic in some of the subtropical islands of Japan before 1988. Later surveys detected huanglongbing on Iriomote (1993), Okinawa (1994) and 5 other islands in Okinawa prefecture. In Kagoshima prefecture, the disease was found in Yoron island (2003), and other islands located to the south of Oshima island.
In Okinawa prefecture, it was considered that the disease was too widespread to attempt eradication, but an eradication programme was launched in Kagoshima prefecture, as the disease was restricted to limited areas (southern subtropical islands). In 2012, the disease was successfully eradicated from the Kikaijima island. At present, the northern limit of huanglongbing in Japan is Tokunoshima island (Kagoshima prefecture). In Okinawa, the application of phytosanitary measures (e.g. control of D. citri, destruction of infected trees, restrictions on the movement of citrus planting material, surveys) allowed the establishment of a disease-free zone. In addition, an early warning system has been put into place in the coastal areas of Kyushu main island to protect citrus-growing areas.
Sources
Iwanami T (2022) Occurrence and control of citrus greening (huanglongbing) in Japan. Japan Agricultural Research Quarterly: JARQ 56(2), 105-120. https://doi.org/10.6090/jarq.56.105