Pine wilt disease (PWD) results from the complex interaction of three main factors: the pinewood nematode (PWN, Bursaphelenchus xylophilus (Steiner & Buhrer), Nickle), a vector (Monochamus spp.) and a host (mainly Pinus spp.), along the influence of external environmental factors. Human-mediated dispersal through the timber trade is also involved in the long-term spread of the disease (Robinet et al., 2011). Native to North America, PWN was first introduced into several Asian countries (Japan, 1905; China, 1982; Taiwan, 1985; Korea, 1988) through the importation of infested timber. In 1999, the disease was first detected in Europe, on the Setubal Peninsula in Portugal (Mota et al., 1999), and has since spread throughout Portuguese territory (Naves et al., 2015). In neighbouring Spain, the first outbreak was declared in 2008 (Robertson et al., 2011) and several outbreaks are still being eradicated in southern Pontevedra

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The occurrence of new recent B. xylophilus outbreaks in Spain and the adverse disease expansion forecasts require a rapid advance in genetic breeding against this pathogen (Ikegami and Jenkins 2018; de la Fuente et al., 2018). The optimum for breeding more PWD resistant trees is to use the most virulent isolates in the inoculation assays (Akiba et al., 2012).

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Key message The resistance of Pinus pinaster Aiton to pinewood nematode Bursaphelenchus xylophilus (Steiner and Buhrer) varied among populations from the Iberian Peninsula, with survival rates for inoculated seedlings ranging from 5 to 100%. These differences in resistance were paralleled by differences in some constitutive chemical defenses. Populations from the southeastern Iberian Peninsula displayed higher resistance than northern populations. Context The presence of the pinewood nematode (PWN), Bursaphelenchus xylophilus (Steiner and Buhrer), in Portugal threatens Mediterranean pine species such as Pinus pinaster Aiton. Aims We have focused on assessing the resistance of P. pinaster populations to B. xylophilus, looking for any relationship between the PWN resistance and some constitutive chemical defenses and geoclimatic parameters. Methods Two-year-old seedlings from 32 provenances and two seed orchards were evaluated in an experiment of artificial inoculation following a randomized complete block design under greenhouse conditions. We measured growth-related traits, response to B. xylophilus inoculations, and constitutive chemical compounds in needles of the evaluated seedlings and compiled geoclimatic data for each population. Mixed models, nonparametric tests, correlations, and PCA were used to analyze the data. Results Survival, wilting symptoms, morphological traits, and nematode density varied significantly among populations. Lower concentrations of constitutive polyphenols, lipid-soluble substances, and tannins were related to higher PWN resistance. Populations from the southeast of the Iberian Peninsula showed higher survival rates than those from further north. Additionally, we observed that populations to warm, dry climates showed higher resistance to B. xylophilus than populations originating from humid, temperate climates. Conclusion Higher susceptibility to PWN is related to lower growth traits, to lower levels of certain constitutive chemical compounds, and to adaptations to harsher climate.