Abstract: The pine wood nematode (PWN, Bursaphelenchus xylophilus) is the main pathogen causing pine wilt disease. Direct contact killing method was used to determine the nematocidal activity and stability of Bacillus thuringiensis fermentation broth to screen highly toxic B. thuringiensis strain. The screened strain was identified with 16S rDNA fragment. The results showed that the nematocidal activity of strain D fermentation broth was the highest among four strains. The mortality of PWN treated by strain D fermentation broth was over 70% at 8 h and reached 100% at 24 h. The toxicities of strain D fermentation broth against PWN were more than 85% when the strain was treated to temperatures of -20, 25, 40, 60, 80 and 121℃. At pH of 5.0-10.0, toxicities of strain D fermentation broth against PWN ranged between 85%-94%, which showed no significant difference. When treated with protease K, the strain D fermentation liquid still showed a toxicity above 98%. When the strain D fermentation liquid was stored at 4℃ for 50 d, no obvious decrease in its toxicity was observed. The sequence analysis of the 16S rDNA 1.5 kb fragment showed that that the strain D may be B. thuringiensis KNU-07. It can be concluded that the strain D fermentation broth is heat resistant, shelf stable and not degradable by protease K, and thus is a highly virulent strain with potential for development and application.

Key words: Bacillus thuringiensis, fermentation broth, pine wood nematode, toxicity