This drone picture taken on Sept. 16 shows pine trees in Miryang, South Gyeongsang Province that turned brown after getting infected with pine wilt disease. Courtesy of Green Korea United

By Lee Hae-rin

An estimated two million pine trees are turning brown after becoming infected with pine wilt disease, according to a local environmental group. The group says that the current damage is close to the levels of 2013 and 2014 when over two million evergreen trees were infected ― if not worse.

Environmental group Green Korea United announced Monday that the group had conducted on-site and drone observations across the country since June this year, during which, it discovered over two million pine trees infected with pine wilt disease.

The country's southeastern region, which includes the cities of Busan, Pohang and Gyeongju, is the hardest hit by the spread of the disease, the group said. Some of the damaged forests are designated as world heritage and cultural heritage protection areas. The infection is heading northwards, quickly spreading to areas near Seoul and in Gangwon Province.

Seo Jae-chul, a senior ecological activist of the organization who conducted on-site visits, said the damage from pine wilt disease has never been as visible before. This year is the first time that infected trees have been detected near roads and railroads since the disease was first discovered in the country in 1988, the activist said.

Pine wilt disease is caused by 1-millimeter-long worms called pinewood nematodes. These tiny worms are parasites of long-horned beetles and infest host tree branches during their spring feeding season. After they arrive, the nematodes reproduce rapidly and clog the tree's vascular system, disrupting the tree's water and nutrient flow and eventually leading to the death of the host tree. The withering rate of infested trees is 100 percent, as there is no cure for the lethal disease once a tree has become infected.

Pinewood nematodes visible via microscope / Courtesy of the National Forest Service

The sudden spread of the lethal disease is caused by climate change, according to experts.

Seo said that the average spring temperature has gone up due to global warming and activated the reproduction and spread of insects that carry the nematodes. Kim Dong-soon, a professor of plant resources and the environment at Jeju National University, also said that the recent drought and typhoons might have weakened the trees, disrupting their resin secretion and facilitating the movement of the pine nematodes from beetles to trees.

Meanwhile, the group's estimate of two million infected trees is far greater than the latest statistics from the National Forest Service (NFS). The NFS conducts surveys of pine wilt disease damage with 135 local governments every year, and as of this April, the government had reported that only 380,000 trees were infected with the disease.

Seo pointed out that the government's statistics are incomplete and far from realistic. Despite the rapid spread of the disease since spring this year, several local governments have halted monitoring or failed to provide accurate reports on local damage, the activist said.

The NFS told The Korea Times Tuesday that it will conduct a study to grasp the extent of the additional damage by November of this year with the Korea Forestry Promotion Institute's pest control center. Last year, the government body monitored 520,000 hectares of pine forests nationwide with a drone, covering about 34 percent of the total pine forest area.

Also, the NFS plans to conduct intensive pest control from this October to April of next year, in which infected trees are cut down, turned into wood chips and burned to prevent the disease from spreading to other plants and trees. Non-infected trees surrounding the damaged trees will be given a type of inoculation so as to stop the spread of the parasitic infection.

If left unmanaged, the disease could quickly spread to infect over 40 to 50 percent of the country's pine trees, the group warned. In a worst-case scenario, the dead trees' roots could weaken the stability of mountain slopes and cause landslides in future torrential rains, Seo said.