マツノザイセンチュウに対するマツ属の抵抗性

Abstract

西南日本の海岸線を中心に, 日本の各地でマツ林に激しい被害をもたらしているマツノザイセンチュウに対するマツ属各種間の抵抗性の違いを調査するために1977, 1978の両年に, 京都大学農学部附属演習林上賀茂試験地および白浜試験地に植栽されているマツ属30種, のべ約600本に対してマツノザイセンチュウの接種試験を行なった。接種にあたっては1本の供試木あたり2, 000頭のマツノザイセンチュウを接種したが, さらに, P. strobus, P. taedaには接種密度を変えて, 1本につき2, 000頭ずつ3ヶ所に計6, 000頭を接種した。接種後2および5週目に早期症状の調査のため樹脂浸出量を測定した。その後, 経時的に1年間供試木の外見的異常を観察し, しかる後に供試木からの線虫の再分離を試みた。これらの調査・観察の結果の大要は次のようである。(1) マツノザイセンチュウを接種された木の樹脂量はその後の外見的症状の有無とは無関係に減少する傾向が見られた。(2) 外見的病徴にもとづく異常発生率の供試樹種間における違いは, 育種学的知見にもとづいて築きあげられた Critchfield & Little の分類体系で比較的うまく類別できる。すなわち, Australes 亜節に含まれる種は最も抵抗性が強く, Contortae 亜節の種がこれに準じる。Ponderosae, Oocarpae 両亜節の種はいずれも感受性であり, 日本産のクロマツやアカマツが含まれる Sylvestres 亜節の中には強度の感受性樹種から抵抗性樹種まで, さまざまな反応が見られた。また Strobus 亜属の各種の異常発生率は高かったが, いくつかの種では Pinus 亜属の感受性反応と異なり異常を部分で食い止め, 全体としては健全性を保ち枯れない可能性をうかがわせる反応が見られた。(3) 接種密度が高くなると抵抗性の P. taeda でも異常発生率が高まり, これらの樹種の抵抗性が本質的には絶対的なものではないことを示唆した。(4) 1977年度の接種試験で生き残った個体を1978年度, 再度接種に供したところ, いくらかの種で, それらの異常発生率は新規に接種した場合の異常発生率より低い傾向がうかがわれた。これはそれらの種内に抵抗性の個体間差が存在することを示唆している。(5) 供試木から接種一年後に線虫を再分離したところ, 異常を発現し, 枯死したような木や部位からは普遍的にマツノザイセンチュウが分離された。一方健全なまま生存した個体や部位からはマツノザイセンチュウは分離されず, マツ属内に見られる抵抗性と樹体内での線虫の増殖の密接な関係が明らかになった。

The most severe damage of pine trees which is caused by the pine wood nematode, Bursaphelenchus lignicolus, is widespread throughout central to southwestern Japan. Through a series of inoculation tests conducted to prove the pathogenicity of this nematode, Kiyohara and Tokushige elucidated the difference in resistance among pine species to this nematode. Since then, many inoculation trials were carried out to find the resistant strains or resistant plants. For this purpose, it is important to survey the difference of resistance among pine species, and so the present inoculation experiments was carried out with about 600 trees from 30 pine species planted at Kamigamo Experimental Station in Kyoto and Shirahama Experimental Station in Wakayama. The pine trees to be tested were inoculated with about 2, 000 nematodes per tree, except for some trees of Pinus strobus and P. taeda to which about 2, 000 nematodes were inoculated at three different sites, about 6, 000 nematodes in all. Two and five weeks after the inoculation, degree of oleoresin exudation, an early symptom of this disease, was examined. Wilt symptoms were also observed at intervals for one year, and then nematodes were recovered from inoculated trees. Results obtained from the present experiments were as follows: 1) In most of the trees tested, amount of oleoresin exudation decreased regardless of the symptoms following. 2) Whether susceptible or resistant, which were judged from external symptoms, correlated fairly well with a taxonomical group of the pine classified by Clitchfield and Little (1966). That is, three species of subsection Australes tested, (P. elliottii, P. rigida and P. taeda) were highly resistant to this nematode, and P. contorta and P. banksiana of subsection Contortae moderately resistant. Resistance of subsection Ponderosae, subsiction Oocarpae and subgenus Strobus seemed to be low, though P. excelsa and P. strobus showed partial symptoms that they had signs of latent resistance. Responses of species in subsection Sylvestres ranged widely from highly resistant to highly susceptible. 3) When about 2, 000 nematodes were inoculated, most of P. taeda trees showed high resistance and did not die: whereas a few P. taeda trees inoculated with 2, 000 nematodes at three different sites, 6, 000 in all, fell in sick to death. This fact means that the resistances of host pine varies with the density of pine wood nematodes inoculated, and so the resistant species may not be safe from the attack of this parasite when transferred by beetle at high density. 4) In inoculation tests conducted in 1978, the pine trees which had been inoculated in 1977 but yet survived in good health were served for the test once again in addition to the newly used trees. Generally speaking, the number of weakened and dead trees tended to be fewer in the twice-inoculated group than in the once-inoculated group. 5) About 300 days after inoculation, part of each pine tree inoculated with nematodes were sampled to recover the progenies of the inoculated nematodes. Pine wood nematodes were recovered from most of the dead trees and dead parts of the trees showing partial symptoms, but not from the healthy trees or healthy parts of the partially dead pine trees.