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

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.

Phytophthora himalsilva sp. nov. an unusually phenotypically variable species from a remote forest in Nepal.

The Himalaya have received little investigation for Phytophthora species. In a remote forest in Western Nepal ten isolates of an unknown Phytophthora were recovered from the rhizosphere of Quercus, Castanopsis, Carpinus and Cupressus spp. The Phytophthora, formally named here as a P. himalsilva sp. nov., is homothallic with either amphigynous or paragynous antheridia and papillate, highly variable sporangia which may also be facultatively caducous. Based on ITS, ß-tubulin, and cox I sequences Phytophthora himalsilva falls within Phytophthora Clade 2c together with Phytophthora citrophthora, Phytophthora meadii, Phytophthora colocasiae, and Phytophthora botryosa. It is suggested that Clade 2c has radiated within Asia. Molecular and sporangial characters indicate that P. himalsilva and P. citrophthora may share a recent common ancestor although they have diverged in their breeding systems. Although highly local the P. himalsilva isolates exhibited significant variation in growth rates and optimum temperatures for growth. This may reflect adaptation to different niches within a heterogeneous sub-tropical to temperate forest environment. Their cox I polymorphisms were also rather variable, including possible clustering for subsite. The occurrence of a previously unknown Phytophthora in a remote forest in Nepal highlights once again the plant health risk associated with moving rooted plants and soil between different bio-geographical regions of the world and the need for rapid pathological screening of potential risk organisms.

Characterization and distribution of mating type genes in the dothistroma needle blight pathogens.

ABSTRACT Dothistroma septosporum and D. pini are the two causal agents of Dothistroma needle blight of Pinus spp. in natural forests and plantations. Degenerate primers amplified portions of mating type genes (MAT1-1-1 and MAT1-2) and chromosome walking was applied to obtain the full-length genes in both species. The mating-type-specific primers designed in this study could distinguish between the morphologically similar D. pini and D. septosporum and between the different mating types of these species. Screening of isolates from global collections of D. septosporum showed that only MAT2 isolates are present in Australian and New Zealand collections, where only the asexual form of the fungus has been found. In contrast, both mating types of D. septosporum were present in collections from Canada and Europe, where the sexual state is known. Intriguingly, collections from South Africa and the United Kingdom, where the sexual state of the fungus is unknown, included both mating types. In D. pini, for which no teleomorph is known, both mating types were present in collections from the United States. These results provided new insights into the biology and global distribution of two of the world's most important pine pathogens and should facilitate management of the diseases caused by these fungi.