Relationships between Asian isolates of citrus greening bacterium
Citrus greening bacterium (EPPO A1 quarantine pest) occurs in many parts of Asia and Africa. The Asian and African organisms are transmitted by different vectors (respectively Diaphorina citri and Trioza erytreae, both EPPO A1 quarantine pests), and show different climatic preferences. The Asian organism produces severe symptoms under cool (22 to 24°C) and warm temperatures (27 to 30 °C), whereas the African organism induces severe symptoms only under cool conditions. The names Candidatus Liberobacter asiaticum and Candidatus Liberobacter africanum have recently been proposed to designate these pathogens. Relationships between several citrus greening isolates from Asia (Japan, Philippines, Indonesia and Thailand) have been studied by comparing sequences of 16S rDNA and of 16S/23S intergenic region. A comparison with known sequences of isolates from other parts of Asia (China, India, Nepal) and Africa was also included in this study. Results showed that sequences of 16SrDNA were identical among all isolates from Japan, Philippines, Indonesia and Thailand, and very similar (98.8 % to 100% similarity) to published sequences of Thai, Nepalese and Indian strains, but less similar to an African isolate (97.5 %). Comparison of sequences of 16S/23S intergenic region gave similar results: all studied isolates presented identical sequences, which were also very similar to published sequences of Indian and Chinese strains (99.2 %), but less similar to the African isolate (85.2%). It is concluded that several isolates from Japan, Philippines, Indonesia, Thailand and Nepal constitute one strain of L. asiaticum, which is similar to Indian and Chinese strains but distinct from the African isolate (L. africanum).
Note: The studied isolates from Japan originated from Iriomote and Okinawa islands (Ryukyu Archipelago). In Japan, the disease was thought to be present only in the southern part of the small island of Iriomote. But this study has showed that it is present at least in 4 locations on Okinawa island. The authors noted that this may cause a problem, because D. citri occurs there and Okinawa island is one of the major citrus-producing area of Japan.
Sources
Subandiyah, S.; Iwanami, T.; Tsuyumu, S.; Ieki, H. (2000) Comparison of 16S rDNA and 16S/23S intergenic region sequences among citrus greening organisms in Asia.
Plant Disease, 84(1), 15-18.