First Report of Pine Wilt Disease on Pinus koraiensis in Korea.

Abstract

Pine wilt disease is one of the most important forest tree diseases, especially in the East Asian countries of Japan, China, and Korea. The causal agent, Bursaphelenchus xylophilus (3), is transmitted by the insect vectors, pine sawyer beetles (Monochamus spp.). The vectors mainly attack coniferous trees and the infected trees die within several weeks. In Korea, pine wilt disease was first reported in Busan City in 1988, and now, the damaged area covers 7,820 ha and more than 60 cities in Korea. The main host trees are Pinus densiflora and P. thunbergii, which are the most common pines distributed in Korea. In 2006, however, we found pine wilt disease in a forest of P. koraiensis located in Gwangju City in Gyeonggi Province. Symptoms were systemic and leaves turned brown or yellowish. Wood samples were collected from the affected trees, and nematodes were extracted by a modified Baermann's funnel method. After 24 h, nematodes were collected from wood chips, and morphology was observed with an inverted light microscope (Leica DE/DMI 3000B, Wetzlar, Germany). Morphology was characterized by a typical Aphelenchoid-type esophagus, head constriction, female vulva flap, female tail, and a male spicule shape similar to B. xylophilus. For more accurate identification, DNA was extracted from individual nematodes with a DNeasy Kit (Qiagen, Valencia, CA). The internal transcribed spacers (ITS) 1 and 2 and 5.8S regions were amplified by PCR (1). Sequenced nucleotide information was compared with the sequences of B. xylophilus already reported in GenBank (Accession Nos. AB294736, AB277208, AM 157747, AY 347913, and BXU92464). ITS DNA sequences of the nematode from Korean pine was >99% identical to B. xylophilus in GenBank. The B. xylophilus from Korean pine tree was also successfully cultured in Botrytis cinerea medium and pathogenicity was tested from June to October 2007. More than 95% mortality was observed with the inoculation of 20 replicate 15-year-old trees of P. koraiensis and P. densiflora with 15,000 nematodes per tree. Therefore, we confirmed that pine wilt disease occurred in P. koraiensis in Korea. P. koraiensis is an endemic species in Korea and distribution is limited to the northern Korean Peninsula and some locations in Russia. It has been shown that P. koraiensis is susceptible to the pine wood nematode by an inoculation test in Japan (2); however, to our knowledge, this is the first report of pine wilt disease on P. koraiensis under field conditions in Korea.