Has Scots pine (Pinus sylvestris) co‐evolved with Dothistroma septosporum in Scotland? Evidence for spatial heterogeneity in the susceptibility of native provenances

Abstract

Spatial heterogeneity in pathogen pressure leads to genetic variation in, and evolution of, disease‐related traits among host populations. In contrast, hosts are expected to be highly susceptible to exotic pathogens as there has been no evolution of defence responses. Host response to pathogens can therefore be an indicator of a novel or endemic pathosystem. Currently, the most significant threat to native British Scots pine (Pinus sylvestris) forests is Dothistroma needle blight (DNB) caused by the foliar pathogen Dothistroma septosporum which is presumed to be exotic. A progeny–provenance trial of 6‐year‐old Scots pine, comprising eight native provenances each with four families in six blocks, was translocated in April 2013 to a clear‐fell site in Galloway adjacent to a DNB‐infected forest. Susceptibility to D. septosporum, measured as DNB severity (estimated percentage nongreen current‐year needles), was assessed visually over 2 years (2013–2014 and 2014–2015; two assessments per year). There were highly significant differences in susceptibility among provenances but not among families for each annual assessment. Provenance mean susceptibility to D. septosporum was negatively and significantly associated with water‐related variables at site of origin, potentially due to the evolution of low susceptibility in the host in response to high historical pathogen pressure.

Planting exotic relatives has increased the threat posed by Dothistroma septosporum to the Caledonian pine populations of Scotland

“…Circumstantial evidence that D. septosporum has had a long‐term endemic presence in the Caledonian pine populations comes from studies of geographic variation in their DNB susceptibility. Pine populations from areas of high rainfall, where conditions are most favourable to D. septosporum , show significantly lower susceptibility than those in low rainfall areas, the pattern expected under long‐term co‐evolution (Perry, Brown, Cavers, Cottrell, & Ennos, ).…”

“…Analysis of the outcome of natural inoculations by the SR race present at Torrs Warren showed high genetic variation for susceptibility both within and among populations of Caledonian pine (Perry, Brown et al., ). Thus, although SR is likely to cause some damage to Caledonian pine populations, this could be mitigated if populations are able to naturally regenerate, allowing the evolution of greater resistance to SR by natural selection (Cavers & Cottrell, ).…”

Comparison of host susceptibilities to native and exotic pathogens provides evidence for pathogen‐imposed selection in forest trees

“…), and populations from environments with greater climatic suitability for disease outbreaks are generally less susceptible to the native pathogens. Such trends have been reported previously in E. globulus (Hamilton et al ., ) and other forest trees (Ades et al ., ; Perry et al ., ). In E. globulus , physiological tolerance (Park, ) and disease risk modelling (Pinkard et al ., ) suggest that the probability of infection by TLD will increase with increasing temperature and precipitation in the near‐coastal region of southeastern Australia, which E. globulus inhabits.…”

Lecanosticta acicola: A growing threat to expanding global pine forests and plantations

“…This implies that under favourable conditions this host could be infected by the pathogen. Investigations on the impact of DNB on P. sylvestris revealed that there is high intraspecific variability of P. sylvestris in Europe and that susceptibility of the host to the pathogen varies between individuals (Perry et al ., ,b) and this could also influence the potential importance of L. acicola . Unusually high humidity associated with climate change could increase pathogen pressure on P. sylvestris (Perry et al ., ) and the single incidences in Europe should carefully be monitored.…”

“…Investigations on the impact of DNB on P. sylvestris revealed that there is high intraspecific variability of P. sylvestris in Europe and that susceptibility of the host to the pathogen varies between individuals (Perry et al ., ,b) and this could also influence the potential importance of L. acicola . Unusually high humidity associated with climate change could increase pathogen pressure on P. sylvestris (Perry et al ., ) and the single incidences in Europe should carefully be monitored. Caution must also be taken when planting susceptible exotic hosts alongside native forests, as this could influence the vulnerability of native forests (Piotrowska et al ., ).…”