Genetic, Phenotypic, and Pathogenic Variation in Phytophthora pseudosyringae: Implications for Larch Dieback in Britain

The oomycete Phytophthora pseudosyringae is mainly associated with rhizosphere infections of declining oak, beech, and alder in mainland Europe where it is considered native. However, the species has recently come to prominence in Britain, initially by causing significant damage to the heathland and woodland dwarf shrub Vaccinium myrtillus, then more recently associated with aerial infections on larch (Larix). The emergence of P. pseudosyringae on conifers raises concerns for forestry, particularly of larch in Europe, plus questions about its significance compared to P. ramorum, which has killed millions of plantation-grown larch in Britain since 2009. To understand the drivers underlying the emergence of P. pseudosyringae, we investigated the genetic, morphological, and pathological diversity of P. pseudosyringae isolates obtained from various hosts across Britain. Major gene sequence analyses identified two major genetic lineages within the British population of P. pseudosyringae. Within these lineages, three host-associated groups (Fagus–Nothofagus, Vaccinium and Larix) could be differentiated based on colony morphology, sporangial size, and in vitro growth characteristics. Moreover, pathogenicity varied markedly between groups with Larix-associated isolates producing larger lesions on Japanese larch (L. kaempferi) comparable to those formed by P. ramorum, whereas Fagus-Nothofagus- and Vaccinium isolates caused smaller or negligible lesions, respectively. Our findings suggest that P. pseudosyringae comprises genetically differentiated lineages that differ in host association, morphology, growth characteristics and pathogenicity. The Larix-associated group produced lesions on Japanese larch comparable in size to those caused by P. ramorum, suggesting that it has the potential to be an important pathogen of larch in Britain.