Vulnerable juniper populations show adaptive potential in the face of a highly damaging invasive tree pathogen

Invasive tree pathogens pose a significant and increasing threat to natural ecosystems. The outcome of these novel host‐pathogen interactions depends largely on the presence and nature of resistance in host populations, which will govern the host's potential to respond through natural selection and adaptation to the new threat. This study assessed the adaptive potential of juniper (Juniperus communis) in the face of the invasive tree pathogen Phytophthora austrocedri through two inoculation experiments: an excised‐shoot trial using six different P. austrocedri isolates and a progeny‐provenance trial that inoculated whole trees with a single highly virulent isolate. We found evidence for both qualitative and quantitative resistance in juniper populations, with lesion length (quantitative resistance) showing moderate to high heritability and lesion development (qualitative resistance) showing very high heritability. There was a significant genotype‐by‐genotype interaction between pathogen isolate and host genotype, lowering the estimate of heritability to moderate values when calculated across six different isolates. Finally, we found evidence that P. austrocedri is imposing natural selection on juniper populations, with individuals originating from highly exposed populations having a lower predicted probability of developing a lesion. Based on the results of this study, we recommend that the most effective management strategy for vulnerable UK juniper populations is to promote natural regeneration within populations, making use of existing genetic diversity in resistance within natural populations without risking the introduction of new P. austrocedri genotypes through the planting of nursery‐grown juniper.