Diversity and composition of pathogen and endophyte assemblages in a widespread tree decline disease (beech bark disease)

Background/Question/Methods

Plant-pathogen interactions are often considered in a pairwise manner with minimal consideration of the impacts of the broader endophytic community on disease progression and/or outcomes for disease agents and hosts. Community interactions may be especially relevant in the context of disease complexes (i.e, interacting or functionally redundant causal agents) and decline diseases (where saprobes and weak pathogens synergize the effects of primary infections and hasten host mortality). Further, pathogen strain level diversity may be associated with disease aggressiveness in disease complexes, particularly where ecologically redundant causal agents co-infect hosts. Here we describe the bark endophyte communities associated with a widespread decline disease of American beech, beech bark disease (BBD), caused by an invasive scale insect (Cryptococcus fagisuga) and two fungal pathogens, Neonectria faginata and N. ditissima. Ten sites were established across the current range of BBD forming a latitudinal transect ranging from Maine to North Carolina, and a longitudinal transect ranging from Maine to Wisconsin. We used ITS2 amplicon sequencing to assess the prevalence of the two primary pathogens and composition of bark endophyte communities. We further performed single-spore isolations of N. faginata from individual trees and whole-genome re-sequencing of 72 isolates to assess genetic diversity and population structure.

Results/Conclusions

The two primary fungal pathogens were both observed across the range of disease, with N. faginata occurring in all ten sites and 71 of 102 trees, and N. ditissima occurring in nine of ten sites and 32 of 102 trees. Importantly, the two species co-occurred in 27 trees and in 38 of the 170 total bark plugs collected (22%). Further, we identified fungi that were consistently associated with diseased trees including saprotrophs, additional pathogens, mycoparasites, and entomopathogens, such as the mycoparasite Clonostachys rosea and the plant pathogen/entomopathogen Fusarium babinda. Preliminary population genomics results indicate N. faginata populations are structured geographically with strong phylogenetic clustering of individuals isolated from NY state, but potential gene flow linkages between northeasterly populations in ME and NH and discrete satellite populations in PA, WV, and NC. We show that while N. faginata does increase in prevalence with infection duration, contrary to the prevailing understanding of species dynamics N. ditissima maintains a substantial foothold throughout the range. Neonectria ditissima presence may contribute to enhanced disease severity and/or influence N. faginata evolutionary dynamics. Co-occurring endophytes may also modulate disease progression by acting as parasites/pathogens of the primary disease agents or hastening tree decline as saprotrophs/plant pathogens.