Studies on oak wilt caused by Raffaelea species in the Far East
As reported earlier, extensive oak tree mortality has been observed in Japan since the late 1980s (see EPPO RS 99/027, 2003/067, 2007/070), mainly on Quercus crispula but also on Q. serrata. This Japanese oak wilt (formerly on the EPPO Alert List) is caused by a recently described fungus, Raffaelea quercivora, which is carried by an ambrosia beetle, Platypus quercivorus (Coleoptera: Platypodidae). It is noted that similar massive mortalities of deciduous and evergreen oaks had taken place in the 1930s in the Prefectures of Yamagata and Kyushu, but at that time the causal agent was not identified. In addition to Japan, oak tree mortality has been reported from the Russian Far East on Quercus mongolica, first in 1979 on isolated trees but covering an area of 100 ha in 2002. However, the possible causes of oak tree mortality have not been identified in the Russian Far East.
In the Republic of Korea, massive oak tree mortality was first observed in 2004, on Q. mongolica, in the northern part of the country. It was estimated that 680 and 1220 oak trees were killed in 2004 and 2005, respectively. The pathogen involved in this disease is thought to be an undescribed species of Raffaelea (apparently different from R. quercivora), which is transported by another ambrosia beetle, Platypus koryoensis. In Korea, symptoms were similar to those observed in Japan. Shortly after trees are attacked by P. koryoensis, necrosis spreads within the sapwood. Wilting and subsequent tree death is thought to be caused by the blockage of the vascular system resulting from the infection by this Raffaelea sp. P. koryoensis is reported to occur in the Korean Peninsula, Taiwan and the Russian Far East.
In Taiwan, both P. koryoensis and P. quercivorus are reported to occur but no massive oak tree mortality has been observed. Investigations showed that another Platypus species, P. taiheizanensis, was able to reproduce in fallen trees belonging to the genus Castanopsis. In the sapwood, discoloration similar to the necrosis caused by R. quercivora was observed in all cases. However, this necrosis did not develop across the whole trunk section.
Finally, investigations were carried out in Northern China (Liaoning and Jilin provinces) but did not detect the presence of P. koroyensis (although it was highly suspected that it occurred there) or of extensive oak tree mortality.
Although several hypothesis have been proposed (Raffaelea spp. might be introduced exotic species, climate change could have favoured insect vector development and spread or caused additional tree stress, changes in the forest age structure or poor management might have also weakened oak trees), the reasons for the emergence of these massive oak tree mortalities observed in Japan or the Republic of Korea remain largely unknown.
Sources
Hong KJ, Kwon YD, Park SW, Lyu DP (2006) Platypus koryoensis (Murayama) (Platypodidae: Coleoptera), the vector of oak wilt disease. Korean Journal of Applied Entomology 45(2), 113-117 (abst.).
Joon Hwan Shin (2007) Forest damage history and future directions for forest landscape restoration in Korea. IUFRO Conference on Forest Landscape Restoration (Seoul, KR, 2007-05-14/19). http://www.iufro.org/download/file/1898/75/10100-et-al-seoul07-stanturf.pdf
Kamata N, Goto H, Komura R, Kubo M, Mikage M, Tsuyuki S, Muramoto KI (2005) Field research in China and Korea by the EMEA group and some implications for Japanese oak wilt. Proceedings of the International Symposium on Environmental Monitoring in East Asia ‘Utilization of remote sensing for monitoring of vegetation change’ (Kanazawa, JP, 2005-11-28/29), 127-133. http://dspace.lib.kanazawa-u.ac.jp/dspace/bitstream/2297/6340/1/C0000003306-127.pdf