Molecular characterization of <Emphasis Type="Italic">Phytophthora porri</Emphasis> and closely related species and their pathogenicity on leek (<Emphasis Type="Italic">Allium porrum</Emphasis>)

White tip, caused by Phytophthora porri, is a destructive disease in the cultivation of European leek (Allium porrum). P. porri and closely related species such as P. brassicae, P. primulae and P. syringae belong to the phylogenetic clade 8b within the genus Phytophthora. The objectives of this study were to establish the position of P. porri and closely related species within the Phytophthora clade 8b; to study genetic variation among P. porri isolates from leek and closely related species and to test the hypothesis that host-driven speciation has occurred within this clade. AFLP analysis could clearly make a distinction between isolates of P. porri from Allium species and related Phytophthora species such as P. brassicae, P. syringae and P. primulae. DNA similarity and cluster analysis based on 353 markers demonstrated little genetic diversity within the P. porri population from Allium species although Belgian and Dutch P. porri isolates from leek could be distinguished from Japanese P. porri isolates from other Allium species and the P. porri isolate from carrot. Our results point to incipient speciation within the P. porri isolates, which could have been driven by the host plant or by geographic isolation. ITS sequence analysis confirmed the results obtained by AFLP and showed a close relationship between P. porri isolates from Allium and P. primulae and between the P. porri isolate from carrot and P. brassicae. We hypothesize that interspecific hybridization has occurred within this clade.