Fifty strains of bacteria were isolated from six isolates of the nematode Bursaphelenchus mucronatus (Bm) from China and Russia and identified using the BioMerieux Vitek 32 system. In bioassay, 3 bacterial strains showed the high levels of phytotoxin production while 19, 16, and 12 strains showed moderately, low and no phytotoxin production, respectively. Inoculation of 2-month-old Pinus thunbergii seedling with each of the six Bm isolates showed that the mean number of days from inoculation to death of 80% of the seedlings was significantly related to the ratio of the total number of bacterial strains for a nematode isolate to the number of pathogenic bacterial strains of the nematode isolate. The results of inoculation of 3-year-old P. thunbergii seedlings showed that inoculation with either axenic Bm (ABm) or axenic B. xylophilus (ABx) and the pathogenic bacterial strain together were essential for inducing pine wilt. These findings demonstrate that wilt symptoms caused by Bm conform to our earlier hypothesis (Zhao et al., 2003) that pine wilt disease, induced by certain Bx or Bm isolates, is caused by a complex of both the nematodes and their associated pathogenic bacteria. The results also account for the variation in pathogenicity of Bm populations from different parts of the world.
Keywords: Acinetobacter; Aeromonas; Pinus thunbergii; Pseudomonas; axenic nematode; bioassay; pathogenicity.