Full understanding and control of pine wilt disease (PWD) is a work in progress and breeding for disease resistance constitutes an essential management strategy for reducing its impact, as evidenced by advanced breeding programs in countries such as Japan. Since Pinus radiata is one of the most commercially relevant species in northern Spain, we designed a study to assess genetic variation in susceptibility to this pathogen using 44 P. radiata half-sib families from the Galician breeding program. Three Bursaphelenchus xylophilus (pinewood nematode, PWN) inoculation experiments were performed to evaluate disease-related variables, estimate genetic parameters, and study sources of genotype by environment interaction (G × E). We also looked at differences in the constitutive chemical compounds of susceptible and non-susceptible individuals. The results showed great variation in PWN susceptibility, with survival rates for P. radiata families ranging from 0% to 90%. In addition, heritability estimates (h_i² = 0.43, h_f² = 0.72) and genetic gain (>26% selecting 50% of the families) were both moderately high for survival. Significant differences in several constitutive chemical compounds were found between susceptible and non-susceptible seedlings in the two susceptibility groups considered. These results confirm the potential of breeding to obtain P. radiata genotypes that are resistant to pine wilt disease and open possibilities for mitigating its future impact on P. radiata stands.

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.

Different studies have emphasized the influence of resin canal traits in the susceptibility of pine trees to the pine wood nematode (PWN) Bursaphelenchus xylophilus. Resin canals can facilitate the PWN migration through the stem and are involved in the accumulation of volatile terpenes in the xylem in response to the pathogen, inducing tracheid embolisms. In this work, we conducted a PWN inoculation experiment under greenhouse conditions to investigate the anatomical traits of constitutive resin canals among seven Pinus species with different degrees of susceptibility to the PWN: P. canariensis, P. halepensis, P. pinea and P. taeda were grouped into a ‘low-susceptible group’, and P. pinaster, P. radiata, and P. sylvestris were grouped into a ‘high-susceptible group’. The high-susceptible group presented higher xylem radial growth, wider constitutive canals in the cortex, lower frequency of constitutive canals in the cortex, and smaller constitutive canals in the xylem than the low-susceptible group. The size of constitutive cortical canals was positively related to the number of seedlings colonized by the PWN, suggesting that wider canals facilitated migration. The inoculation of the PWN increased the frequency and diminished the mean area of canals in the xylem, and the high-susceptible group showed more frequency of induced xylem canals than the low-susceptible group. Additionally, the high-susceptible group presented larger radial growths in the xylem than the low-susceptible group. These results suggest a role of resin canal traits on Pinus spp. susceptibility to the PWN. Nevertheless, the high interspecific variability found in these traits within each susceptibility group evidences the importance of other factors in the susceptibility to the PWN.

Aim of the study: The Breeding Program of wild cherry (Prunus avium) developed by Lourizán Forest Research Center (NW Spain), aims for the creation of the Main Breeding Population, that is formed by a large number of plus trees and for obtaining an Elite Population generated from controlled crosses of a number of plus trees selected by, at least, one trait of economic importance. The aim of this study was to genotype 131 accessions of Prunus avium plus trees, included in the breeding program. Area of study: The Prunus avium plus trees are located in the North, Northwest and Central Spain. Material and Methods: The Prunus avium plus trees were genotyped with nine microsatellites. Several genetic parameters were calculated. Genetic data were analyzed with STRUCTURE and the genetic distance between the plus trees were calculated. Main results: A total of 122 multilocus genotypes were detected. Several accessions with the same genotype were identified, which could be due to clonality or to labelling errors. The nine microsatellites are useful for identifying individuals because the combined probability of identity was low (PI = 5.19X10-9). Bayesian methods detected two genetic clusters in the sampled plus trees. Research highlights: The unique genotypes identified in this work are suitable for being included in the elite breeding population for economic traits. Keywords: Prunus avium; breeding program; microsatellite; genetic distance.

Pine wilt disease was detected in Galicia (North West of Spain) in 2010. Two-year-old seedlings of seven pine species were inoculated with B. xylophilus, and three different groups were stablished: non-susceptible (P. canariensis, P. taeda, P. halepensis, and P. pinea); susceptible (P. pinaster, P. radiata), and highly-susceptible (P. sylvestris). We aimed to determine the interspecific variation of constitutive compounds levels, groups and species, and their relationships with nematode multiplication and mortality. Needles of the non-susceptible group had significantly less water and more nitrogen, potassium, iron, and starch than the others groups; the cortex and phloem of the non-susceptible group had more nitrogen, phosphorus, manganese, and starch, and less potassium, calcium, iron, total polyphenols, condensed tannins and liposoluble substances than the highly susceptible group. The xylem of the non-susceptible group had more N, P, Mg, Mn, total polyphenols, and starch than the other groups. Higher levels of constitutive N and/or starch in any tissue was related with less mortality and nematode multiplication; higher P in the three tissues was also correlated with less nematode multiplication. Moreover, liposoluble substances, soluble carbohydrates and condensed tannin concentration in the needles were negatively correlated with nematode multiplication. On the contrary, in the needles, water content and K were positively correlated with mortality and nematode invasion.

Aim of the study: To determine if the decline of a Pinus sylvestris L. stand is due to pine wilt disease. Area of study: This research was conducted in the Agricultural and Phytopathological Laboratory of Galicia, LAFIGA (AGACAL, Xunta de Galicia). Material and methods: Six decaying P. sylvestris trees were sampled for Bursaphelenchus xylophilus detection. Morphological and molecular tools were applied for nematode identification according to the EPPO protocols. Main results: The nematode B. xylophilus was detected for the first time in P. sylvestris trees under natural conditions in Spain. Research highlights: It is confirmed the serious threat posed by pine wilt disease to P. sylvestris stands.