Oak and other trees have died in at least 23 prefectures across Japan due to a mold carried by an insect, according to the government-backed Forestry and Forest Products Research Institute.
The number of prefectures is up from a total of eight in the 1980s, according to a study by the institute made available Monday.
The institute, based in Tsukuba, Ibaraki Prefecture, said it has started developing measures to predict sites of potential outbreaks and to remove the insect, Platypus quercivorus, commonly known as the oak ambrosia beetle, blamed for the disease.
According to the study, large numbers of the 5-mm insect, called Kashinonaga-kikui-mushi in Japanese, bore into oak trees in summer for breeding.
A mold brought by the insect kills tree cells and halts the flow of sap, withering the trees in a week or two.
Some experts say global warming has expanded the range of the insect.
After damage was reported in Miyazaki and Kagoshima prefectures in the 1930s, similar cases were confirmed sporadically in eight prefectures such as Niigata, Fukui and Hyogo by the 1980s.
The 23 prefectures include Akita, Nagano and Kyoto.