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Invasive, exotic plant pathogens pose a major threat to native and agricultural ecosystems. Phytophthora× cambivorais an invasive, destructive pathogen of forest and fruit trees causing severe damage worldwide to chestnuts (Castanea), apricots, peaches, plums, almonds and cherries (Prunus), apples (Malus), oaks (Quercus), and beech (Fagus). It was one of the first damaging invasive Phytophthoraspecies to be introduced to Europe and North America, although its origin is unknown. We determined its population genetic history in Europe, North and South America, Australia and East Asia (mainly Japan) using genotyping-by-sequencing. Populations in Europe and Australia appear clonal, those in North America are highly clonal yet show some degree of sexual reproduction, and those in East Asia are partially sexual. Two clonal lineages, each of opposite mating type, and a hybrid lineage derived from these two lineages, dominated the populations in Europe and were predominantly found on fagaceous forest hosts (Castanea, Quercus, Fagus). Isolates from fruit trees (Prunusand Malus) belonged to a separate lineage found in Australia, North America, Europe and East Asia, indicating the disease on fruit trees could be caused by a distinct lineage of P.× cambivora, which may potentially be a separate sister species and has likely been moved with live plants.The highest genetic diversity was found in Japan, suggesting that East Asia is the centre of origin of the pathogen. Further surveys in unsampled, temperate regions of East Asia are needed to more precisely identify the location and range of the centre of diversity.

Phytophthora ramorumis a recently introduced pathogen in Europe and North America consisting of three clonal lineages. Due to the limited intralineage genetic variation, only a few polymorphic markers are available for use in studies involving the epidemiology and evolution of P. ramorum. A total of 159 primer pairs for candidate polymorphic SSR loci were tested with universal labeling. Four polymorphic microsatellite loci were identified within the NA1 lineage and one within the NA2 lineage, demonstrating the power and flexibility of the screening technique. The markers may significantly increase the number of genotypes that can be identified and as such can help better characterize the North American lineages of P. ramorum.

The genus Phytophthoracomprises many economically and ecologically important plant pathogens. Hybrid species have previously been identified in at least six of the 12 phylogenetic clades. These hybrids can potentially infect a wider host range and display enhanced vigour compared to their progenitors. Phytophthorahybrids therefore pose a serious threat to agriculture as well as to natural ecosystems. Early and correct identification of hybrids is therefore essential for adequate plant protection but this is hampered by the limitations of morphological and traditional molecular methods. Identification of hybrids is also important in evolutionary studies as the positioning of hybrids in a phylogenetic tree can lead to suboptimal topologies. To improve the identification of hybrids we have combined genotyping-by-sequencing (GBS) and genome size estimation on a genus-wide collection of 614 Phytophthoraisolates. Analyses based on locus- and allele counts and especially on the combination of species-specific loci and genome size estimations allowed us to confirm and characterize 27 previously described hybrid species and discover 16 new hybrid species. Our method was also valuable for species identification at an unprecedented resolution and further allowed correct naming of misidentified isolates. We used both a concatenation- and a coalescent-based phylogenomic method to construct a reliable phylogeny using the GBS data of 140 non-hybrid Phytophthoraisolates. Hybrid species were subsequently connected to their progenitors in this phylogenetic tree. In this study we demonstrate the application of two validated techniques (GBS and flow cytometry) for relatively low cost but high resolution identification of hybrids and their phylogenetic relations.