Do silver fir woolly adelgids (Dreyfusia nordmannianae) facilitate pathogen infestation with Neonectria neomacrospora on Christmas trees (Abies nordmanniana)?

In Europe, the most widely used Christmas tree species, Abies nordmanniana, has a long history of infestation with the adelgid Dreyfusia nordmannianae. Since 2011, the species has furthermore been increasingly damaged by the fungus Neonectria neomacrospora. The objective was to study whether infestations by N. neomacrospora were facilitated by the presence of adelgids in Nordmann fir and to explore the genetic components of this potential interplay. The damage caused by D. nordmannianae and N. neomacrospora in two Nordmann fir clonal seed orchards in Denmark was evaluated in 2014, and an experimental infestation was conducted to test the performance of Nordmann fir somatic embryogenesis-derived trees exposed to N. neomacrospora and D. nordmannianae individually or in combination by setting up an artificial infestation chamber. The genetic correlation between the two diseases was 0.54 at two clonal seed orchards, indicating that the trees susceptible to N. neomacrospora were also susceptible to D. nordmannianae or vice versa. In the experimental test, only trees in the treatment with both D. nordmannianae and N. neomacrospora developed needle loss and dieback symptoms. Thus, both the field evaluations and the experimental test revealed an interplay between D. nordmannianae and N. neomacrospora damage on Nordmann fir trees. Future studies should explore in more detail how D. nordmannianae interacts with N. neomacrospora. Our findings and future studies on this topic could have important implications for suitable breeding programs for Nordmann fir Christmas trees and forest pest management.