Measures Against Oak Withering

In many neglected satoyama (traditional woodland landscapes), a phenomenon known as oak withering has been spreading. This causes oak trees to wither and die, with their leaves turning red even before autumn arrives. Like other Japanese forests, our Natural Water Sanctuary Kyoto Nishiyama is vulnerable to this disease.

Oak withering is caused by an insect known as the oak ambrosia beetle. When the beetle bores holes in trees to lay its eggs, it introduces a fungus known as the oak wilt fungus (Raffaelea quercivora), which ultimately kills the tree.

At the base of trees affected by oak withering, you can often see sawdust expelled by the beetles as they bore into the wood.

However, the beetle alone cannot be blamed. In the past, these beetles played an important role by causing mature konara oaks to die and helping to rejuvenate the forest.

However, due to the adoption of modern energy sources like gas and electricity, people stopped managing Japan’s traditional konara oak forests, which were originally used for firewood and charcoal. This resulted in an overabundance of large, mature konara oak forest stands that have now become ideal feeding grounds for the ambrosia beetle.

Tree infected with oak wilt fungus

As an early-stage countermeasure, we wrap infected trees with special adhesive sheets with the sticky side facing inward. These sheets trap newly emerged adult beetles exiting from their holes, helping to control the spread of the infection.

Our ongoing surveys suggest that the damage from oak withering has affected about 30–40% of the konara oak population. So at this point, we don’t expect all konara oaks in the sanctuary to die off. We will continue monitoring and implementing pest control measures as needed.

Tree wrapped with adhesive sheets

The photo below shows a fungus called poison fire coral. It often appears near the base of withered konara or Japanese Oak trees. As oak withering spreads, sightings of this mushroom have become more common. It is an extremely poisonous mushroom, dangerous even to touch—so please be cautious if you ever encounter it.

Highly toxic mushroom: poison fire coral

Learn more about pest and disease control