Tissue degeneration in Japanese black pine (Pinus thunbergii) mother trees and seedlings that are resistant to pine wilt disease

To determine the factors affecting the resistance of Japanese black pine mother trees to pine wilt disease, the time required for full browning and the water content of the branches obtained from 15 mother trees (clones) were analyzed. The time required for full browning of branch pieces inoculated with the pine wood nematode (PWN) Bursaphelenchus xylophilus ranged from 7.9̃19.3 days and significantly differed among 15 clones examined in this study (ρ<0.01). Because sampling direction and the position of the mother trees in the seed orchard had little effect on the browning of branch pieces, the time required for tissue degeneration was considered to be a property of the clone. The water content of branch pieces ranged from 116̃138% but did not significantly differ among the clones (ρ<0.05). The time required for full browning of branch pieces obtained from the mother trees significantly correlated with that from the seedlings of these trees(rs=0.579, Spearman's rank test; ρ<0.05); therefore, the property of degeneration of the mother trees was thought to be inherited by their seedlings. However, neither the time required for the full browning of branch pieces from mother trees nor that from their seedlings correlated with the degree of resistance of the seedlings, suggesting that the time required for tissue degeneration may not be an important factor influencing the resistance of a whole tree.