A study on pathogenicity of bacteria carried by pine wood nematodes

Three bacterial strains Njh, Njt and Njw, have been isolated with a high frequency from the xylem of the wilted black pine (Pinus thuntergii) and from the surface of the pine wood nematodes (Bursaphelenchus xylophilus). These bacteria have been identified as Pseudomonas fluorescens biotype I, P. fluorescens biotype II and a species of the genus Pantoea respectively. To determine pathogenicity, callus and aseptic black pine seedlings were inoculated as follows: aseptic nematodes only (each of B. xylophilus and B. mucronatus); bacterium only (each of bacterial strain Njh, Njt, Njw) and aseptic nematodes plus bacterium (each of B. xylophilus and B. mucronatus in combination with each of Njh, Njt, Njw). The results showed that inoculation with either aseptic B. xylophilus or B. mucronatus did not lead to browning of the callus or wilt of aseptic black pine seedlings. But those inoculated with aseptic nematodes plus any one of Njh, Njt, Njw showed some wilting or browning symptoms. The combination of nematodes plus either Njh or Njt caused severe symptoms but the combination of nematodes plus Njw had fewer symptoms. The bacteria carried by pine wood nematodes played an important role in pathogenicity. In addition, bacteria were cultured in liquid media through shaking. The filtered liquid was directly applied to the callus and the ability of each kind of bacterium to induce browning was determined. The results showed that Njh and Njt caused severe browning but Njw did not. It is suspected that this is due to wilt related toxins in the bacterial culture fluid. Therefore, it's suggested that the disease was caused by co-infection of both pine wood nematodes and bacteria and possible toxic effect of bacteria.