Effect of family, crown position, number of winter buds, fresh weight and the length of needle on rooting ability of Pinus thunbergii Parl. cuttings

As a measure for contrasting pine wilt disease, which caused serious damage in Japanese black pine (Pinus thunbergii Parl.) by the pine wood nematode (Bursaphelenchus xylophilus (Steiner et Buhrer) Nickle), resistant trees have been widely planted in Japan. The propagation of resistant trees using cuttings obtained from healthy stock plants and inoculated with pine wood nematode is expected to further increase in the next future. To improve the cutting propagation of Japanese black pine trees resistant to pine wilt disease, the factors associated with rooting and root volume were investigated. The type of cutting and the crown position of stock plants from which cuttings were taken, were markedly associated with rooting. The crown position did not show significant interactions with any other investigated factor, while fresh weight of cuttings and their number of winter buds did not affect rooting. The rooting percentage was markedly higher for cuttings taken from the lower crown than for those from the upper crown, as already reported for other coniferous tree species. The length of the longest needle was significantly correlated with the root volume of cuttings, and showed significantly interactions with crown position and fresh weight of cuttings. Such correlation suggests that the growth of needles can be considered a useful predictor in the assessment of the root volume of cuttings during the propagation period, allowing growers to transplant rooted cuttings at the appropriate time without excavating or uprooting. These findings may contribute to the improvement of cutting propagation of Japanese black pine.