Characteristics of Japanese Red Pine (Pinus Densiflora) Families Resistant to Pine Wilt

In the trunks of Pinus species susceptible to pine wilt disease, blockage of sap ascent becomes extensive after infection by the pine wood nematode (PWN, Bursaphelenchus xylophilus) because many tracheids are filled with gas. Infected trees die from a deficit of water supplied to the shoots. In Japan, small numbers of trees of susceptible species, Pinus densiflora and P. thunbergii, survive in forests that are extensively damaged by pine wilt disease. Trees were selected from many districts and clones are being maintained in seed orchards. Seedlings from selected families express low mortality rates after inoculation with the pathogen. We discuss the factors preventing development of the symptoms in resistant families of P. densiflora based on an investigation of the reaction of tree tissues to the infection. In the stems of resistant families, cavitated areas emerged, but those areas did not enlarge. Nematode populations were small in the main stems. There must be systems that prevent nematode migration and reproduction in pine tissue. Numerous branches are a visible characteristic of some resistant families. The complicated arrangement of resin canals may be a physical barrier to PWN migration. Based on the hypothesis that a chemical barrier may be activated in pine tissue, a systemic chemical was tested on non-resistant seedlings. The chemical induced nematicidal stilbenoids in the base of the branches. Symptoms were partially developed or delayed by application of the chemical prior to PWN inoculation. Thus, the base of branches plays an important role in preventing the advance of the nematode.