Seoul is no safe haven from pine wilt disease — 15 trees infected in Seocho-gu alone

As pine wilt disease spreads rapidly across southern South Korea, infected trees are turning up in Seoul as well. Particularly alarming is Seocho-gu, which had recorded no infected trees at all this year until 15 pines were confirmed to have contracted the disease. Forestry officials warn that the chuseok holiday — a period of heavy travel and cooler weather — could accelerate the spread if infected timber is moved or used as firewood.

As of Monday, a total of 26 pine trees in Seoul had been confirmed infected with pine wilt disease. The infections were found across eight districts, with more than half — 15 trees — concentrated in Seocho-gu.

A Seoul Metropolitan Government official said the cluster in Seocho-gu was traced to Inreungsan, a mountain near the border with Seongnam in Gyeonggi Province. "There is a military base on the mountain that restricts civilian access, which meant we were unable to conduct surveillance there," the official said. "We believe that is why the disease was able to spread undetected."

All of this year's Seoul infections have occurred in the three Gangnam-area districts. Gangnam-gu recorded three infected trees and Songpa-gu one. Beyond those districts, Nowon-gu had three infected trees, while Seongdong-gu, Seongbuk-gu, Dongjak-gu and Gangdong-gu each reported one.

The first confirmed case this year appeared in July in Gangil-dong, Gangdong-gu. Most recently, an infected tree was found last month in Sadang-dong, Dongjak-gu. Notably, infections have now appeared in Seongdong-gu, Seongbuk-gu and Dongjak-gu — districts that do not border other cities or provinces. The Dongjak-gu case is considered especially urgent because it was found in Samil Park, which sits adjacent to a residential neighborhood.

Pine wilt disease was first detected in Seoul in 2007 in Nowon-gu, followed by additional outbreaks in Yongsan-gu in 2015 and Jungnang-gu in 2016. The city was later declared disease-free before a new outbreak emerged in Seocho-gu in 2023. The most recent prior case was recorded in Nowon-gu in 2024.

Pine wilt disease is spread primarily by the Japanese pine sawyer beetle, which targets pine trees, and the white-spotted sawyer, which targets Korean white pine. The nematode — a thread-like parasite roughly 1 millimeter long — lives inside the beetle and enters a tree through wounds made when the beetle feeds on young shoots.

"The nematode itself cannot move on its own, but the sawyer beetles fly around and spread it from tree to tree," said Park Il-kwon, a professor at Seoul National University's Department of Forest Sciences. "It seems like pine wilt disease has been spreading particularly widely this year, likely because control efforts have not been carried out effectively for various reasons."

The Seoul Metropolitan Government said it is mobilizing all available resources to prevent further spread. The city is managing the disease through a three-pronged response framework centered on strengthening preventive surveillance, containing spread from confirmed outbreak sites and preventing human-caused transmission.

"We have removed all infected trees and are carrying out pine wilt control treatments on every pine tree within a 5-kilometer radius of each confirmed case," a city official said. "Starting in November, we plan to administer preventive nematicide injections to prepare for next year."

The nationwide spread of pine wilt disease has reached a serious level. About 1.77 million pine trees across the country have been damaged by the disease, with particularly severe outbreaks expanding in North Gyeongsang Province — including Pohang and Gyeongju — and South Chungcheong Province, including Taean and Boryeong.

Forestry authorities are running a pine wilt prevention campaign ahead of and during the chuseok holiday, urging the public to refrain from transporting or using infected timber. Officials are particularly concerned about two scenarios: infected trees being hauled for landscaping purposes, and farming households illegally collecting infected wood for use as firewood as temperatures drop.