Inhibiting factors of symptom development in several Japanese red pine (Pinus densiflora) families selected as resistant to pine wilt

Abstract

Pinus densiflora and P. thunbergii, native to Japan, are highly susceptible to pine wilt disease caused by infection with a pine wood nematode (Bursaphelenchus xylophilus). Trees of these susceptible species have occasionally been found surviving in forests that are extensively damaged by this disease. Seedlings from a part of surviving trees that were selected as resistant families indicate lower mortality rates after the infection. The factors that prevent the symptoms from developing in resistant families of a susceptible species, P. densiflora, as based on the analysis of the pathogen's behavior in the tree tissue and the anatomy of the resistant families, are presented in this paper. Nematode populations remained lower in the stems of seedlings from resistant families of P. densiflora than in the stems of non-resistant families. Areas dysfunctional in water transport developed in the stems of resistant families, but did not reach a size large enough to seriously block the ascent of sap. These results suggest that there are systems within the seedlings that prevent nematode migration and reproduction. The 2-year-old seedlings from resistant families of P. densiflora, however, did not suppress the pathogen activity. Numerous branches are a visible characteristic in the seedlings of some resistant families. The arrangement of the resin canals, the only channels in the tree to the pathogen migration, was disoriented at the joints between the branches and the main stem. Such a structure may be effective as a barrier to nematode dispersal.

Key words:

• Bursaphelenchus xylophilus
• Pine wood nematode
• Resistant family
• Xylem sap
• Dysfunction