Recent Expansion of Japanese Oak Wilt in Japan: Damage Increase in Urban Areas

Yukiko Takahashi (Department of mushroom science and forest microbiology)

Research Field: Forest health

Main subject: Forest pathology, Mechanism of disease developments and outbreak

Japanese Oak Wilt

Japanese oak wilt (JOW), a forest damage caused by the cooperation of a tree pathogen Raffaelea quercivora and its vector ambrosia beetle Platypus quercivorus, has been a profound threat for forests in Japan since the 1980s.

• In the past, the damage mainly occurred in the Sea of Japan region and western Japan, but in recent years, it has expanded to unpopulated areas in the northern Tohoku and Kanto regions.

• In particular, damage in the Kanto region has occurred in parks and green spaces in urban areas, and different pest management methods are needed.

Photo. A, a JOW-infested dead oak tree; B, trunk base of A with pinholes made by Platypus quercivorus (blue circle) and P. calamus (yellow circle); C, a JOW-infested almost dead oak tree (half of the main trunk was dead); D, the trunk base of C with pinholes made by P. quercivorus; E, a transverse section of the trunk of A; F, pinholes and mycelial mat (bordered by the white dashed line) under E’s bark; G, a living oak with pinholes made by Platypus spp.; H, mycelial mat under the bark of a living oak attacked by Platypus ssp.-; I, a living oak attacked by Platypus spp with decayed Armillaria tabescence fruit bodies near the base of the trunk (enclosed by the white dashed circles).

Interesting Aspect of this Theme

JOW is one of the most important forest pests caused by insect outbreaks and are involved in related fungi. Platypus quercivorus, a causal agent of JOW, is an ambrosia beetle that reproduces in their galleries in the host tree and cultivates yeasts for their diet. They also carry a pathogenic fungus, Raffaelea quercivora, which it does not use as food.

Ambrosia beetles usually uses debilitated or dead trees, so it is said to be unusual for it to kill trees like JOW.

Bark beetles such as the mountain pine beetle have caused mass mortality of trees in North America and elsewhere. The damage caused by JOW and bark beetle outbreaks is similar in that they are both caused by insect outbreaks.

Why do secondary bark beetles, which use dead trees, turn into primary bark beetles, which use living trees? What is the trigger? There may be a common denominator in the outbreaks of forest pests.

The interesting part of this theme is the cooperation between the insect and the fungus. The virulence of the fungus is weak, but a mass attack by the insects will kill a tree. Perhaps, the insect also resists the host's defensive response due to the presence of R. quercivora.

One of my research findings is a study that looked at the dynamics of the fungus in the tree and the response of the host. This is not the whole story as it depends on the condition of the host, but I found that the pathogen is not strong enough to invade the host and needs to cooperate with the insect to spread enough to kill it.

Future Perspective

There are concerns about the effects of climate change; global warming is expected to expand the area and scale of pest damage. To prepare for possible damage in the future, we need to conduct risk assessment and take measures to mitigate that.

I would like to clarify under what circumstances damage is likely to occur. This will give us hints on how to avoid it. We would like to clarify how we can manage the situation to prevent major outbreaks.

It is difficult to reduce the damage to zero, but I believe it is possible to make it smaller.