Inoculation Experiments of a Nematode, Bursaphelenchus sp., onto Pine Trees

KIYOHARA Tomoya, TOKUSHIGE Yozan

doi:10.11519/jjfs1953.53.7_210

Epidemic occurrence of a virulent blight of pine trees throughout the southwestern part of Japan, especially in Kyushu district, has long been noticed among Japanese foresters. A large number of pine trees in many stands declined and finally died off; for instance, about 39-million trees or 380-thousand cubic metres in volume were damaged in 1968. The cause of this damage has been attributed to the attack of bark beetles up to the present time. In the previous paper (TOKUSHIGE & KIYOHARA¹⁾) the authors reported preliminarily that a species of nematode, Bursaphelenchus sp., was commonly detected in the wood of dead pine trees collected from many localities in Kyushu. To determine the effect of this nematode onto healthy pine tree, several series of inoculation test were conducted in certain pine stands (Table 1). From these experiments the authors came to the conclusions that Bursaphelenchus sp. detected from the dead pine tree is considered as one of the causal agents for the death of pine trees and it plays an important role in the deterioration of pine forests throughout the southwestern part of Japan. Results obtained are summarized as follows: (1) When about 600 nematodes were injected into the heartwood at the base of stem, amounts of oleoresin exudation from the punched wound gradually reduced but the inoculated trees did not die after all; whereas almost all of pine trees inoculated with the nematodes at the rate of more than 30, 000 per tree were finally killed (Table 2). Therefore, pine trees were usually inoculated with 30, 000 nematodes per tree in the other inoculation series. (2) Nematodes used as inoculum were multiplied on the colony of Pestalotia sp. isolated from dead pine wood. As inoculum suspension was prepared by washing the Bursaphelenchus-Pestalotia culture, it contained nematodes and conidia of the fungus. However, conidia of Pestalotia and filtrate from the mixed suspension proved to be harmless to healthy pine trees. Only inoculum containing nematodes, pure suspension of Bursaphelenchus sp. and mixed suspension of Bursaphelenchus-Pestalotia, manifested the ability to kill healthy pine trees (Table 3). (3) Inoculation of nematodes into the xylem resulted high mortality rate of the trees, regardless the part of the tree where it was injected; primary branch, base of stem, or thick root. Negative result was obtained from the inoculation on unwounded bark, and low mortality rate was recorded in the inoculation to wounded one. Five trees, to which disks of pine wood colonized with the nematode and the fungus were placed and hurried around their root without wound, were all killed (Table 5). (4) The following results were obtained from the monthly inoculation experiments from February through October: Highest mortality resulted in the inoculation made from June to August, wherein the latent period, from the injection of inoculum nematodes to the stop of oleoresin exudation or to the appearance of wilt symptom, shortened by 15_??_30 days or 2_??_3 months. In the trees inoculated in the spring season, the latent period was lengthened and their death did not occur before summer. Mortality in this case was lower than that in the summer inoculations. No dead tree was observed up to December 20th in the case of inoculation carried out from September to October (Tables 4, 6). (5) Two inoculation series were given in the three pine stands showing different rates of natural infestation. In the inoculation series carried in March, pine trees of the highly infested stand tend to die more rapidly and in higher rate than those of the moderately or slightly infested stands. On the other hand, no such tendency was recognized among three stands in the inoculation series carried in June (Figure 1).

Inoculation Experiments of a Nematode, <i>Bursaphelenchus</i> sp., onto Pine Trees

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