Japanese oak wilt as a newly emerged forest pest in Japan: why does a symbiotic ambrosia fungus kill host trees?

Abstract

Japanese oak wilt (JOW) has been known since the 1930s, but in the last 15 years epidemics have intensified and spread to the island's western coastal areas. The symbiotic ambrosia fungus Raffaelea quercivora is the causal agent of oak dieback, and is vectored by Platypus quercivorus (Murayama). This is the first example of an ambrosia beetle fungus that kills vigorous trees. Mortality of Quercus crispula Blume was approximately 40 percent but much lower for associated species of Fagaceae, even though each species had a similar number of beetle attacks. It is likely that other oaks resistant to the fungus evolved under a stable relationship between the tree, fungus and beetle during a long evolutionary process. Quercus crispula was probably not part of this coevolution. This hypothesis was supported by the fact that P. quercivorus showed the least preference for Q. crispula, yet exhibited highest reproductive success in this species (The index of an increasing rate = ca. 4). On contrary, on the other oaks the index was almost one that guarantees a stable population dynamics for P. quercivorus. Therefore, P. quercivorus could spread more rapidly in stands with a high composition of Q. crispula. Each of individual trees other than Q. crispula can be utilized by P. quercivorus for several years. On contrary, P. quercivorus can reproduce only one year on each Q. crispula tree because necrosis of sapwood tissues spread widely after the first-year attack. The relationship among Q. crispula - R. quercivora - P. quercivorus seems evolutionary unstable. JOW seems to be an invasive pest of Q. crispula. Concentric patterns of JOW spread also support this hypothesis.

Parent Publication

• Proceedings, 17th U.S. Department of Agriculture interagency research forum on gypsy moth and other invasive species 2006

Citation

Kamata, Naoto; Esaki, Koujiro; Kato, Kenryu; Oana, Hisahito; Igeta, Yutaka; Komura, Ryotaro. 2007. Japanese oak wilt as a newly emerged forest pest in Japan: why does a symbiotic ambrosia fungus kill host trees?. In: Gottschalk, Kurt W., ed. Proceedings, 17th U.S. Department of Agriculture interagency research forum on gypsy moth and other invasive species 2006; Gen. Tech. Rep. NRS-P-10. Newtown Square, PA: U.S. Department of Agriculture, Forest Service, Northern Research Station: 1-3.