Testing of selected Pinus pinaster half-sib families for tolerance to pinewood nematode (Bursaphelenchus xylophilus)

Pine wilt disease (PWD) is caused by the nematode Bursaphelenchus xylophilus, an invasive pest that currently constitutes the greatest threat to pine forests worldwide. Breeding for resistance to the nematode is one of the main strategies for coping with this disease in most severely affected countries. This work evaluates genetic variation in susceptibility to B. xylophilus among 91 half-sib families from a Pinus pinaster breeding population that was selected for growth and stem form in Galicia (NW Spain). We performed artificial inoculations on these families to evaluate mortality, disease development, and their relationships both to physiological variables measured throughout the experiments and morphological traits assessed prior to inoculation. The results showed significant differences among families in almost all estimated variables and reasonably high (varying from 0.45 to 0.87) family heritability for morphological traits, mortality and wilting symptoms. Some of the families presented high tolerance to B. xylophilus, suggesting that a breeding programme for tolerance to pinewood nematode could be an important control strategy for managing PWD in Spain. However, mortality correlated positively with tree height and negatively with the number of branches, which may indicate that current breeding selection criteria could be negatively affecting tolerance to PWD. Chlorophyll fluorescence and oleoresin flow were useful tools for assessing the physiological condition of the trees after pinewood nematode infection.