Since the start of breeding Nordmann fir Christmas trees in Denmark in 1992, new challengeshave arisen in the field of Christmas tree production. One of the most important is the challengeposed by the fungal pathogen Neonectria neomacrospora, which causes canker disease in truefirs (genus Abies). This pathogen developed into an epidemic in 2011, affecting many Christmastree stands in Denmark. Management of this canker disease is essential to ensure profitableChristmas tree production. In addition, the silver fir woolly adelgid, Dreyfusia nordmannianae,traditionally the most important pest in Nordmann fir Christmas trees in Central and NorthernEurope, makes it very hard to avoid the use of pesticides. A way to deal with this adelgid is toattain more resistant plant material. Abies bornmülleriana seems less susceptible to this adelgid,making it a promising alternative species for Christmas tree production in Europe.Knowledge about genetic variation that can be used to develop breeding strategies (based on thetesting of clones or progeny) for selecting more resistant individuals for deployment in the nextbreeding cycle is highly demanded. The present thesis includes five papers aimed at providingsuch knowledge based on a combination of quantitative genetics and tree resistance breedingstudies.In paper I we studied the natural damage of N. neomacrospora among the 39 Abies taxa,including 32 species, in the Hørsholm Arboretum. Artificial infestation of the same materialswas conducted to validate the findings from the natural field damage. The results showedsignificant differences of natural infection by N. neomacrospora between taxa. Furthermore,they also showed this fungus is an aggressive pathogen that can infect nearly all species exceptone, the Abies cephalonica, and that all species could be infected using the artificial infestationof detached twigs. The damage observed in the infestation test could explain 30% of thevariation in field observations based on species mean values. Our results indicate that, for thecultivation and conservation of the Abies species, N. neomacrospora requires attention.Paper II describes the study of genetic variation and genotype by environment (G×E) interactionin the susceptibility of Nordmann fir to N. neomacrospora. This study was based on anevaluation of the naturally occurring damage caused by N. neomacrospora in three Nordmann firclonal seed orchards in Denmark in 2014. Again, this study showed N. neomacrospora is anaggressive pathogen which caused substantial damage in all three sites (all clones being infectedby this fungus). Luckily, a pronounced genetic variation in the susceptibility detected amongclones opens up prospects for adapting the Nordmann fir to this canker disease through theselection of less susceptible trees. The presence of a significant G×E interaction for susceptibilitysuggests the need to implement breeding and deployment programs to select superior clones forspecific environments.In paper III the aim was to reveal whether the presence of adelgids can assist infestations by N.neomacrospora in Nordmann fir. In this study, we combined a natural damage evaluation of D.nordmannianae and N. neomacrospora in two Nordmann fir clonal seed orchards in Denmarkwith an experimental infestation test on small potted Nordmann fir trees. In the latter, weexposed the trees to these two important damaging agents individually or in combination. Boththe natural damage evaluation and the experimental infestation test showed an interplay betweenthe insect and the fungus on the Nordmann fir. It suggests that an effective pest managementagainst D. nordmannianae might also reduce the risk of N. neomacrospora infections, and futurestudies on this topic could have important implications for suitable breeding programs forNordmann fir Christmas trees and forest pest management.In paper IV we estimated the narrow-sense heritability of the susceptibility of the Nordmann firto N. neomacrospora based on an artificial infestation trial on full-sib offspring from polymixcrossings of Nordmann fir. Furthermore, several additional traits relevant to Christmas treeproduction were measured in both the full-sib and half-sib offspring of the Nordmann fir fieldtrial to study the genetic relationship between the susceptibility to N. neomacrospora and otherChristmas tree traits. The results showed significant additive genetic variation and a highnarrow-sense heritability for susceptibility to N. neomacrospora in the Nordmann fir. Amoderate positive genetic correlation between susceptibility to N. neomacrospora and flushingshowed that selecting less susceptible trees will also achieve late flushing genotypes, which ispreferable since it can help avoid late spring frost damage. In terms of a link to the Christmastree industry, the high narrow sense heritability and the large genetic variation favors substantialgenetic gains through breeding.In paper V breeding values for various Christmas tree characteristics for the A. bornmüllerianaclones in the clonal seed orchard FP.267 were estimated, as well as for offspring which can beused for forward selection. Quasi-field trials (QFTs) using offspring from first-generation plustrees, were established on two sites in Denmark via DNA markers and parentage analysis. Nineimportant Christmas tree traits were evaluated for all offspring individuals in designated areas oftwo production stands, and, moreover, the individuals with the best Christmas tree qualities wereidentified in the entire stand at both sites. Postharvest needle retention was evaluated in theparent population in both 2016 and 2017. Combining modern quantitative genetic breedingmethods and parentage analysis via DNA-markers (QFTs) to improve the genetic materialavailable for Christmas tree production proved efficient in A. bornmülleriana. Backwardselection is preferred due to the benefit of having a shorter time horizon for the delivery ofimproved genetic material, and using postharvest needle retention evaluations to potentiallymodify the selection is highly recommended. Here we propose to combine the results based onQFTs with selective seed harvest and paternity tests to give the best possible starting point forthe production of clonal A. bornmülleriana plants using somatic embryogenesis. Tests of N.neomacrospora susceptibility are desirable to confirm the suitability of using