‘Totoro’s forests’ threatened by oak wilt disease

The Yomiuri Shimbun
Trees with brown leaves that have died from oak wilt are seen in Tokorozawa, Saitama Prefecture, on Aug. 1.

A contagious disease known as oak wilt is spreading in oak forests on the Sayama Hills that straddle Saitama Prefecture and Tokyo.

The forests have been dubbed “Totoro no Mori,” or Totoro’s forests, after the popular animated film “My Neighbor Totoro” by a foundation working for their preservation.

Oak wilt is caused by a fungus carried by the oak ambrosia beetle. A tree infected with the disease becomes unable to absorb water, and its leaves wilt and turn red or yellow, resembling autumn leaves out of season. Eventually, the tree dies.

The disease used to be seen in the Tohoku region on the Sea of Japan side. In recent years, however, it has spread across the country’s Honshu main island, causing serious damage.

According to the Totoro no Furusato Foundation in Tokorozawa, Saitama Prefecture, which bought the forests in order to conserve them, 61 oak trees were infected with the disease in Totoro’s forests in 2020.

That number rose to 263 trees in 2021, of which 64 died. The number of infected trees is increasing further this year.

Damage is spreading particularly among aged trees. Today there are more old trees in forests that contain various types of trees — in the past, trees aged 15 years or older were cut for firewood and charcoal. This is believed to be one of the reasons for the spread of oak wilt.

The highly poisonous mushroom kaentake, or poison fire coral, also tends to grow in forests where oak wilt has occurred. The foundation is taking various measures against the disease, such as setting traps for the beetles and felling dead trees.