Etiological Study of Pine Wilt Disease

1989

S. 127-176

12 Abb., 19 Tab., 86 Lit. Ang.

Unselbständiges Werk

200062300

In this paper, results of experiments proving the pathogenicity of this nematode are reported. Pathogenic variability within the species is alos investigated and its significance is discussed in relation to the epidemiology of pine wilt disease. I. Discovery of a pathogenic nematode and proof of its pathogenicity. (1) Intensive isolatin trials have been made from wilted pine roots and rhizosphere to discover the cause of pine weakening. Many kinds of fungi were isolated from them; of those, some species including Pythium sp., Fusarium sp., Cylindrocarpon sp. caused some wilting of 20-day-old P. thunbergii seedlings (Table 1). A few fungi also caused some discoloration of roots of 3-year-old plants in wond inoculations, but they were anable to weaken or kill the plants (Table 1). As stated above, we noted a species of Bursaphelenchus nematode reproducing on cultures of several fungi isolated from the diseased pines, such as Pestalotia sp., Fusarium spp. blue stain fungi and so on. Preliminary inoculation suggested pathogenicity of this nematode on pine plants (Table 2) . To determine if the nematode, Bursaphelenchus sp. is a causal agent of pine wilt disease, we must follow the scientific principles as Koch's postulate. (2). Dead or dying pine trees from 48 damaged stands in Kyushu were inspected for the pesence of Bursaphelenchus sp. 96% of the examined stands were infested with the nematode and 84% of 99 examined trees were parasitized by Bursaphelenchus sp. (Table 3). Detailed examinations on 3 representative trees revealed that nematodes were present throughout all the organs and tissues of a wilted pine tree except for the foliage (Fig. 1). (3) A series of inoculation tests to healthy pine trees was conducted to elucidate the pathogenicity of this nematode. When 20 red pine trees of 16 to 24 yeas of age were inoculated with 30x10 monoxenically cultured nematodes, all inoculated trees wilted and died within 3 months, showing characteristic disease development (Table 4). The symptoms which appeared on the inoculated trees can be summarized as follows; a reduction of oleoresin exudation flow in the early stage of disease development (Plate 1. B). This flow reduction may be characteristic of the disease, since Pine plants usually exhibit resinosis symptoms when infected by other pathogens such as fungi; the cessation of oleoresin exudation was observed over the subsequent several weeks, when the plants displayed an external symptom of yellowing and wilting of foliage. The discoloration of foliage usually appeared first on older needle leaves (Plate 1, C). The desiccation of wood is a characteristic of this stage; death of diseased trees occurred 2 to 3 months after inoculation. The brown wilted foliage remained on the tree. The symptoms observed on inoculated trees were identical to those of naturally diseased trees. (4) From all inoculated .