Virulence of seven pathogenic bursaphelenchus xylophilus isolates in pinus pinaster and pinus radiata seedlings and its relation with multiplication

Recent new outbreaks of Bursaphelenchus xylophilus in Spain and adverse pine wilt disease expansion forecasts must be met with rapid advances in genetic breeding against this pathogen. The optimal approach for breeding trees that are more resistant to pine wilt disease is to use the most virulent isolates in inoculation assays. A series of controlled inoculation experiments on Botrytis cinerea cultures, along with Pinus pinaster and P. radiata branch sections and seedlings, was conducted to investigate differences among seven virulent B. xylophilus isolates of diverse geographic origin (USA745 from USA, KA4 and S10 from Japan, Pt52T and Pt72CH from Portugal, SpPO1 and SpSA1 from Spain). Results from the experiments showed significant differences among B. xylophilus isolates. The virulence classification of the isolates studied differed for P. pinaster and P. radiata, indicating that PWN isolates vary according to host specificity. Isolates Pt52T and SpP01 proved the most virulent to P. pinaster, so either of them could be used as 'test isolates' in future assays to identify resistant or tolerant genotypes. For P. radiata, any isolate except SpSA1 could be used, as all the others were more virulent and all presented the same significant level of virulence. The results indicate that use of P. pinaster branch sections, rather than P. radiata branch sections or multiplication assays on B. cinerea, could be an accurate, time-saving method for evaluating virulence levels of new B. xylophilus isolates.