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Phytophthora austrocedri has very little genetic diversity in Britain and is therefore almost certainly a fairly recent introduction. However, genome-wide differences among British isolates suggest multiple introductions of this lineage, likely via the plant trade. The British linage of the pathogen is genetically and morphologically distinct from the lineage causing widespread mortality of Chilean cedar in Argentina, and the geographical origin of the source population, where both lineages originated, remains unknown.

P. austrocedri grows best at 15°C and is killed at temperatures of 22.5°C and higher. This pathogen is widespread on juniper in many parts of Scotland and northern and southern England, and its DNA is readily detected in soil at infected sites. However, lack of a consistent finding of P. austrocedri DNA in 2m rain traps located on a heavily infected juniper site suggests that the pathogen is not capable of effective aerial dispersal.

It is thought that once the pathogen has been introduced to a site (for example, via supplementary planting of infected stock), it has a slow natural spread in soil, requiring high moisture conditions; thus, juniper populations on wet sites are predisposed to infection. Once established at a site the pathogen can be spread readily in soil locally, vectored by animal movements and/or human activities.

Observations of healthy trees within high-mortality sites suggest that there might be resistance in juniper to P. austrocedri. Inoculation of a clonal trial of rooted cuttings collected from 17 such ‘survivor’ trees showed that many were indeed resistant to the pathogen. A provenance/progeny trial of juniper was established in 2015 with seed collected from 15 geographically separate locations across Britain. The long-term goal of this trial is to assess genetic diversity in juniper and the heritability of resistance to P. austrocedri, which could be exploited to enable population recovery.

Findings of P. austrocedri in the UK plant trade, and its presence in some UK nurseries, raises the risk posed by conservation plantings. A recommendation arising from this research is to avoid supplementary planting of existing juniper sites and instead instigate management actions that optimise natural regeneration.