The pinewood nematode, Bursaphelenchus xylophilus, originating from North America (NA), is a major invasive pine pest in Eurasia. It was first detected in Portugal in 1999 associated with maritime pine, Pinus pinaster, and has been differently affecting the main local pine species, P. pinaster and P. pinea. Field studies and direct inoculation experiments in Pinus spp. seedlings, under controlled conditions, were performed to assess whether the differences in constitutive and inducible defences are determining the different susceptibility of pine host species to B. xylophilus. Host co-evolution with the pathogen was also assessed, including the NA P. radiata, widely used in forestry in the northeast of the Iberian peninsula. Pine mortality in the field was positively related with the abundance of B. xylophilus, and concentration of phenolics and condensed tannins in pines. In the greenhouse assay, seedling tissues were analysed for constitutive investment in defences, as well as the potential inducibility of those defences as driven by B. xylophilus inoculation. Slower growing P. pinea presented higher levels of constitutive defences than faster growing P. pinaster, with only P. pinaster being affected by B. xylophilus. Furthermore, co-evolution with the pathogen is important, with the fast-growing NA P. radiata presenting an inducible and effective response to B. xylophilus. Results point to the importance of integrating data on pine life history traits, including growth rate, and production of constitutive and inducible defences, into predictive models for this invasive forest pest.

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The pinewood nematode, Bursaphelenchus xylophilus, native to North America (NA), is now a major invasive pine pest in Eurasia. Pine species differ greatly in their susceptibility to B. xylophilus, and differences in susceptibility might be related to differences in phytochemistry. This study addressed two major questions: (i) which plant chemical profiles (including compounds that act as plant defences and those important for nutrition) favour pinewood nematode development; and (ii) are NA and Portuguese pinewood nematodes different in their capacity to subsist on plant tissues? For this, B. xylophilus isolates from NA and Portugal were tested on pine phloem and xylem of different pine species. It was found that unaltered pine phloem can sustain the population growth of B. xylophilus, but only in a few cases, and this was clearly related to distinct chemical profiles in some pine species. No evidence was found of evolutionary divergence between B. xylophilus populations in Portugal and NA. The pine species with phloem on which pinewood nematodes could grow tended to have low levels of total phenols, condensed tannins, flavonoids and lignin. Evidence was also found that nutrients such as N may be important.

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Bursaphelenchus xylophilus, the pinewood nematode and causal agent of pine wilt disease (PWD), is a globally destructive pine pest. PWD is favoured by high temperatures and drought, but B. xylophilus can also persist in symptomless hosts in areas not prone to PWD. We compared seedlings of different Pinus species that were inoculated with B. xylophilus when well‐watered and under moderate temperatures, conditions that favour sublethal infections. We compared isolates of B. xylophilus in south‐west Europe (Portugal) and south‐east USA (Louisiana). P. pinea, P. pinaster, and P. radiata were challenged in both areas with inoculations of local B. xylophilus. P. sylvestris in Portugal and P. palustris in Louisiana were also tested. Seedling mortality was low, as expected, but nematode establishment was common. There were higher nematode densities and more damage in seedlings of P. sylvestris and P. radiata, followed by P. pinaster, with P. pinea and P. palustris proving to be more tolerant. Life history of hosts was a better predictor of resistance to B. xylophilus than geographical origin, even under sublethal conditions. Fast‐growing species (P. radiata and P. pinaster) appeared most likely to be symptomless carriers in areas not prone to the development of PWD. Cold‐adapted species (P. sylvestris) may still be suitable hosts for the pathogen, even at the sublethal conditions of their distribution areas. There were more nematodes per gram of host using nematode isolates from Portugal. Nonetheless, our results opposed the hypothesis that North American pinewood nematodes have such low virulence that they are obligatorily saprophytic.