Early Symptom Development and Histological Changes Associated with Migration of Bursaphelenchus xylophilus in Seedling Tissues of Pinus thunbergii

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Abstract
In order to clarify the mechanism of pine wilt caused by the pinewood nematode (PWN), Bursaphelenchus xylophilus, nematode migration in tissues and disease symptoms in Pinus thunbergii seedlings were investigated. One-year-old seedlings were inoculated with different pathogenic isolates of PWN under two different temperatures. At an early stage of symptom development, a virulent isolate of PWN multiplied in both bark and xylem and was distributed in cortical resin canals, cortical tissue, and xylem resin canals at 30°C. Cell death and disease symptoms developed in both bark and xylem. The virulent isolate of PWN at 25°C and the avirulent isolate of PWN at 30°C were distributed mainly in cortical resin canals, but rarely in xylem resin canals and cortical tissue. Disease symptoms and cell death occurred in cortical resin canals and rarely occurred in other tissues. These results demonstrated that the virulent isolate of PWN at low temperature and avirulent nematodes could not easily migrate to xylem resin canals and cortical tissue. It was shown that cell death and early symptom development coincided with PWN migration and, therefore, PWN invasion induces cell death and early symptom development.