Oak mortality in Japan is caused by Raffaelea quercivora and its insect vector Platypus quercivorus
In Japan, wilting and mass mortality of Japanese oak species (Quercus serrata and Q. crispula) has been observed since the second half of the 1980s. The disease has been observed mainly along the west coast of Honshu (Sea of Japan) but also in the Kii Peninsula (south-east coast of Honshu) and the southern part of Kyushu. Recent studies have shown that the cause of oak tree mortality is the blockage of the ascent of xylem sap induced by a fungus Raffaelea quercivora (EPPO Alert List) which is vectored by an ambrosia beetle, Platypus quercivorus (Coleoptera: Platypodidae). Images can be viewed on the Internet at:
http://cse.ffpri.affrc.go.jp/keiko/hp/oak.html
http://cse.ffpri.affrc.go.jp/keiko/hp/oakwilting-overview.html
In Japan, massive attacks of oak trees by P. quercivorus are characteristic before the appearance of wilting symptoms. Numerous tunnels of P. quercivorus were observed in all dead and dying trees and R. quercivora was isolated predominantly from discoloured sapwood, beetle galleries, body surface and mycangia of P. quercivorus. On wilting oaks, a dark-colouration of the xylem is observed especially on the lower part of the trunks where beetle galleries are mainly found. A blockage of sap ascent in the trunk has been related to the rapid and significant spread of the fungus within the beetle galleries. Several inoculation studies have demonstrated that P. quercivorus is a vector of R. quercivora. Investigations in damaged forests indicated that damage occurs just after trees are felled or blown down by the wind, especially in stands with numerous large trees. It is noted that many of these diseased forests had previously been used for charcoal production but were no longer managed appropriately. Host plants of the insect vector are all Fagaceae. Concerning the fungus, several studies have demonstrated the pathogenicity of R. quercivora on Quercus crispula and Q. serrata. Further inoculation studies of the fungus on different oak species also confirmed that sapwood discoloration and non-conduction of water were more important in Q. crispula and Q. serrata than in other tested species (Fagus crenata, Quercus glauca, Castanopsis cuspidata var. sieboldii, Pasania edulis). Finally concerning geographical distribution, the insect vector (P. quercivorus) occurs in several Asian countries (it is known to occur in Japan, India, Indonesia, Taiwan, and Papua New Guinea), but the pathogen (R. quercivora) has only been reported from Japan.
Sources
Kinuura H, Kobayashi M (2006) Death of Quercus crispula by inoculation with adult Platypus quercivorus (Coleoptera: Platypodidae). Applied Entomology and Zoology 41(1), 123-128. http://www.jstage.jst.go.jp/article/aez/41/1/123/_pdf
Kobayashi M, Ueda A (2005) [Wilt disease of Fagaceae trees caused by Platypus quercivorus (Murayama) (Coleoptera: Platypodidae) and the associated fungus: aim is to clarify the damage factor.] Journal of the Japanese Forest Society 87(5), 435-450. (abst.)
Kuroda K (2001) Responses of Quercus sapwood to infection with the pathogenic fungus of a new wilt disease vectored by the ambrosia beetle Platypus quercivorus. Journal of Wood Science 47, 425-429.
Murata M, Yamada T, Matsuda Y, Ito S (2007) Discoloured and non-conductive sapwood among six Fagaceae species inoculated with Raffaelea quercivora. Forest Pathology 37(2), 73-79.
INTERNET (last retrieved on 2007-04).
APHIS-USDA website
Davis ED, French S, Venette RC (2005) Mini risk assessment. Platypus quercivorus Murayama [Coleoptera: Platypodidae]. http://www.aphis.usda.gov/plant_health/plant_pest_info/pest_detection/downloads/pra/pquercivoruspra.pdf
Tokyo University Forests website
Defence responses of oak sapwood in relation to wilt of oak trees in Japan by Yamada T ; Ichihara Y. http://www.uf.a.u-tokyo.ac.jp/research/yamada/nz.pdf