Abstract:

Objective: The aim of this study is to explore the roles of Bursaphelenchus xylophilus (pine wood nematode,PWN) and the bacteria associated with PWN in the development of pine wilt disease (PWD),to provide theory gist for research on PWN pathogenic mechanism and PWD prevention. Method: Five-year-old healthy Pinus massoniana seedlings grown in green house were artificially inoculated with aseptic PWNs and non-aseptic PWNs,including 8000 PWNs per seedling,and then the status of seedling wilting was observed. The cross section was taken to detect the resin secretion after 5,10 and 30 days of inoculation. The anatomical structure of stem section was observed under a microscope after 20 days of inoculation. The PWNs were re-isolated from wilting seedlings after 30 days of inoculation. Result: Both aseptic PWNs and non-aseptic PWNs wilted 5-year-old seedlings of P. massoniana, expressing similar symptom.The wilting rate reached 90% after 35 days of inoculation,while the control seedlings remained healthy. The observation of cross sections showed that resin stopped secreting after 5 days of aseptic PWNs inoculation,a small amount of cavities began to appear after 10 days,and serious cavitation appeared after 30 days. The observation of cross sections showed that resin decreased secretion but not stopped yet after 5 days of non-aseptic PWNs inoculation,the secretion stopped after 10 days and cavities formed after 30 days. While the control pines secreted normally resin and there is no cavity in their stems. The results of paraffin section showed that the histological and anatomical structure of the stem segments of wilted seedlings changed remarkably in the both treated groups. Most of the cells,in cortex,xylem and pith,were deformed and the epithelial cells near the resin canal were broken and disintegrated. The multiplication rate of aseptic PWNs in wilted seedlings was higher than that of non-aseptic PWNs. The average number of nematodes re-isolated from wilted seedlings was (8 105±5 661)·g^-1 for aseptic PWNs and (4 317±1 896)·g^-1 for non-aseptic PWNs,with no significant difference between the two groups (P = 0.3619). Conclusion: Both of the aseptic and non-aseptic PWNs are able to infect P. massoniana. Asepsis does not make PWNs lose their pathogenicity on P. massoniana. PWNs are necessary factor in the PWD development while,the pathogenicity has nothing to do with the associated bacteria.

Key words: Bursaphelenchus xylophilus, asepsis, pathogenicity, Pinus massoniana, histophology