It is important to improve the understanding of the interactions between the trees and pathogens and integrate this knowledge about disease resistance into tree breeding programs. The conifer Norway spruce (Picea abies) is an important species for the forest industry in Europe. Its major pathogen is Heterobasidion parviporum, causing stem and root rot.In this study, we identified 11 Norway spruce QTLs (Quantitative trait loci) that correlate with variation in resistance to H. parviporum in a population of 466 trees by association genetics. Individual QTLs explained between 2.1 and 5.2% of the phenotypic variance. The expression of candidate genes associated with the QTLs was analysed in silico and in response to H. parviporum hypothesizing that (a) candidate genes linked to control of fungal sapwood growth are more commonly expressed in sapwood, and; (b) candidate genes associated with induced defences are respond to H. parviporum inoculation. The Norway spruce laccase PaLAC5 associated with control of lesion length development is likely to be involved in the induced defences. Expression analyses showed that PaLAC5 responds specifically and strongly in close proximity to the H. parviporum inoculation. Thus, PaLAC5 may be associated with the lignosuberized boundary zone formation in bark adjacent to the inoculation site.

Ash dieback (ADB) caused by the pathogen Hymenoscyphus fraxineus is the cause of massive mortality of Fraxinus spp. in Europe. The aim of this work was to check for the presence of the molecular marker for ADB tolerance in mapped healthy‐looking F. excelsior trees, and to compare its occurrence in trees exhibiting severe ADB symptoms. Monitoring of 135 healthy‐looking F. excelsior on the island of Gotland, Sweden, showed that after 3–4 years 99.3% of these trees had 0%–10% crown damage, thus remaining in a similar health condition as when first mapped. After 5–6 years, 94.7% of these trees had 0%–10% crown damage. Molecular analysis of leaf tissues from 40 of those showed the presence of the molecular marker in 34 (85.0%) trees, while it was absent in 6 (15.0%) trees. Analysis of leaf tissues from 40 severely ADB‐diseased trees showed the presence of the molecular marker in 17 (42.5%) trees, but its absence in 23 (57.5%) trees (p < .0001). The results demonstrated that monitoring of healthy‐looking F. excelsior is a simple and straightforward approach for the selection of presumably ADB‐tolerant ash for future breeding. The cDNA‐based molecular marker revealed moderate capacity on its own to discriminate between presumably ADB‐tolerant and susceptible F. excelsior genotypes.

Common ash (Fraxinus excelsior L.) is a tree species of significant ecological and economic importance that has suffered a devastating decline since the 1990s in Europe. Native ash species are being threatened by the alien invasive fungus Hymenoscyphus fraxineus, which causes ash dieback. The main goal of the study was to develop markers for traits related to tolerance to ash dieback and to investigate whether genotypes selected for tolerance were genetically different from susceptible wild populations. We phenotyped 326 ash trees from Sweden for disease severity and genotyped them using 63 amplicon-derived single-nucleotide polymorphism (SNP) markers derived from genes in 40 scaffolds spanning 8 MB in total, which represents approximately 1% of the ash genome. We used a mixed linear model to test for an association between genotypic variation at these loci and disease severity of ash. In total, two SNPs were found to have significant associations. One non-synonymous SNP associated with the disease severity of ash was found in a gene predicted to encode a subtilisin-related peptidase S8/S53 domain. A second marginally significant marker was associated with an LRR gene. Our results demonstrate an inexpensive time-effective method for generating genomic data that could have potential for use in future tree breeding programs and provide information for marker-assisted selection. Our study also showed a low differentiation between genotypes selected for disease tolerance and the wild population of ash representing a range of susceptibilities to ash dieback, indicating opportunities for further selection without significantly losing genetic diversity in the ash population.

No abstract