In the stems of pine species resistant to pine wilt, such as Pinus taeda and R strobes growing in North America, migration and propagation of the pinewood nematode (PWN) are suppressed and the nematodes disappear from pine tissue in contrast to the highly susceptible Japanese pine species, P. thunbergii and P densiflora. Resistant cultivars of these susceptible species have been found in heavily damaged forests. Although they are potential saviors of pine forests in Japan, certain proportions of seedlings obtained from those cultivars are susceptible and are killed after infection. To obtain reliable seedlings with stable high resistance, it is important to find some criteria that can be used to select truly resistant trees for seed orchards. In the tissue of resistant cultivars, mechanisms that prevent nematode activities must be present even if the effect is weaker than those in P. taeda. The initial migration of PWN in the shoots was investigated on the cuttings of non-resistant and resistant cultivars of P densiflora and compared with that in P. taeda. PWN was inoculated on the apices of 20cm long cuttings. Every day or two, cuttings of each cultivar were sectioned into short segments (less than 5 cm). Nematodes were extracted from each segment and were counted. PWN in the cortex and xylem tissue was counted separately for the cuttings of P densiflora. Then the anatomical characteristics were investigated on seedlings inoculated with PWN. In P. taeda cuttings, the PWN distribution was restricted to the inoculated area during 4 days from inoculation. On the other hand, suppression of nematode migration was not detected in resistant cultivars of P. densiflora judging from the PWN numbers in each stem segment. When PWN population in xylem tissue was compared, a tendency was detected: In resistant cultivars, PWN populations during 5 days from inoculation were smaller in the area more than 5 cm below from inoculated sites. In contrast, PWN population in cortex indicates no specific tendency in resistant cultivars. These results Suggested that xylem tissue contributes to the defense system in the early period of infection although it is not yet clear whether the structural barrier is effective or toxic substances exist in xylem.
