| Summary: | The pine wood nematode (PWN) <i>Bursaphelenchus xylophilus</i> is recognized as a major invasive species in many countries and causes widespread mortality in pine trees. Pine wood nematode disease (PWD) has spread northward from southern China to several areas of Liaoning Province, which has temperatures outside of the optimal range for this disease. To determine whether obvious variations in the population adaptability of PWN are involved in its rapid spread from southern to northern China, this study compared the differences in morphology of eight southern strains and eight northern strains and the pathogenicity of the 16 strains to <i>Pinus thunbergii</i>, the pine species that is the most susceptible to PWD in China, and to <i>P. tabuliformis</i>, the main PWN host in northern Liaoning Province. The southern-strain females were smaller than the northern-strain females, except for strain GD32. The size differences between the males of the different strains were not significant. The difference in pathogenicity between the northern and southern strains to <i>P. tabuliformis</i> was more significant than the difference in their pathogenicity to <i>P. thunbergii</i>. The pathogenicity differentiation among northern strains was lower than that among southern strains, and the northern strains showed stronger pathogenicity to <i>P. tabuliformis</i>. The <i>P. tabuliformis</i> inoculation experiment showed that the pathogenicity of GD32, JS27, FJ14, LN13, and LN06 was significantly higher than that of FJ13. The results suggest that some PWN populations in the southern region, which are better adapted to <i>P. tabuliformis</i>, were likely directly transmitted to the northern region, resulting in the spread of PWD in the northern region. The spread of PWN from the south did not necessarily require a process of adaptation to the host or to the northern climate.
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