| Summary: | The oomycete genus <i>Phytophthora</i> includes many plant pathogens important in agricultural and environmental systems. Natural interspecific hybridization has been reported several times in <i>Phytophthora</i>, and although the fundamental processes of interspecific hybridization and the consequences of subsequent ecological distribution are poorly understood, reports suggest some hybrids can infect a broader host range and display enhanced virulence compared to the putative parental species. During a survey carried out at the University of Aberdeen in 2014–2015, of oomycetes present in ornamental plants purchased via the internet, a batch of oomycete isolates remained unidentified, showing, in some isolates, features generally related to hybridization. The aim of this study was to determine whether hybridization events had occurred between endemic and introduced oomycetes, probably/possibly facilitated through the international plant trade. The list of isolates examined included a putative hybrid closely related to <i>Phytophthora cryptogea</i>. The putative hybrid isolate was further characterized, and pathogenicity were tests carried out on <i>Eucalyptus globulus</i>, using an isolate of <i>P. cryptogea</i> as a positive control. Cloning of ITS, COXI and β-tubulin genes resulted in different sequence versions of the putative hybrid isolate; after mapping and a polymorphism position comparison, it was concluded that the studied isolate contained genetic information from <i>P. cryptogea</i>, <i>P. erythroseptica</i>, <i>P. kelmanii</i>, <i>P. sansomeana</i> and <i>Phytopythium chamaehyphon</i>. A PCR-RFLP assay, a NEBcutter analysis and flow cytometry analysis (genomes ranged between 0.168 to 0.269 pg/2C) added further evidence of the hybrid nature of this isolate. The putative hybrid presented complex growing patterns ranging from rosaceous to chrysanthemum-like and had an optimum growth temperature of 25 °C. Although the putative hybrid produced visible symptoms of disease on <i>E. globulus</i> seedlings, assessment of the relative susceptibility of <i>E. globulus</i> to <i>P. cryptogea</i> and the putative hybrid indicated that <i>P. cryptogea</i> was significantly more virulent than the putative hybrid, based on mortality, disease severity and foliar symptoms.
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