Population dynamics and pathogenicity of Bursaphelenchus xylophilus in seven- toeight-year-old Pinus sylvestris trees

Abstract

The pinewood nematode, Bursaphelenchus xylophilus, is \nthe most harmful plant parasitic nematode on trees in \nAsia and Europe and is the causal agent of the so-called \npine wilt disease of susceptible conifer species, mainly \nPinus spp. The disease has occurred only outside its natural \nrange of distribution (North America) or on nonnative \npine species. In greenhouse trials using saplings, \nPinus sylvestris was one of the most susceptible European \npine species. To examine the significance of these sapling- \nbased analyses concerning the population dynamics \nand pathogenicity for mature P. sylvestris trees, investigations \nusing seven- to eight-year-old trees were carried \nout. The trees were artificially inoculated using a suspension \nof 10,000 B. xylophilus in 600 μl of tap water per \ntree. For the population dynamics investigation, the pines \nwere divided into 48 segments for nematode extraction. \nThe development of wilt symptoms as well as physiological \nchanges were observed until tree death. \nDuring the population dynamics investigation, B. \nxylophilus was located at the inoculation site and in adjacent \nsegments during the first 11 days after inoculation. \nOn day 16, B. xylophilus was distributed throughout the \nentire stem, adjacent branch segments, root collar and \nroots before any external wilt symptoms appeared. With \nincreasing pine wilt disease, B. xylophilus was finally \nfound in all wood and root segments. High nematode \ndensities appeared. Shortly before tree death, the treetop \nshowed several nematode-free segments. The rest of the \nstem and adjacent branch segments and root collar were \nhighly nematode-infested. During the pathogenicity \ninvestigation, all B. xylophilus-inoculated pines died within \n84 days. The significant decline in the water potential in \nthe needles was steeper and more strongly correlated \nwith increasing wilt symptoms compared to a droughtstressed \nvariant. However, the decline in the water potential \nin the needles started earlier in the drought-stressed \npine variant. In conclusion, the population dynamics of \nB. xylophilus in seven- to eight-year-old P. sylvestris trees \nand the pathological reactions of the pines were comparable \nto those observed in assays with saplings, although \ndelayed in reaching a population peak and developing \nwilt symptoms. Therefore, P. sylvestris saplings are good \nindicator trees for B. xylophilus population dynamics and \npathogenicity investigations.