Taxonomy, biogeography and evolution of thirty-four taxa in Phytophthora Clade 7a including eleven new species from natural forests and updated descriptions for P. fragariae, P. rubi and the 'P. ×alni hybrid complex'.

Abstract

Over the past three decades, surveys in natural and managed ecosystems, driven mainly by forest biosecurity issues, have greatly expanded knowledge of the global diversity and biogeography of the plant pathogen genus Phytophthora. In Phytophthora subclade 7a alone, 16 new species were described raising the number of validly described species in this subclade to 18. Recently, in further surveys conducted in natural forests of Japan, Vietnam, Europe and North America, 11 more new Clade 7a species and one other 7a taxon have now been identified. All are considered to be endemic to the surveyed sites. They are formally described here, including their morphological properties, colony and temperature-growth characteristics and their phylogenetic relationships. Informal, updated descriptions without nomenclatural act are also presented for the known species P. fragariae, P. rubi, P. uniformis, P. ×alni and P. ×multiformis. With now 34 taxa including hybrids, Clade 7a is currently the largest Phytophthora subclade. It is essentially a Northern Hemisphere subclade, with a disjunct distribution in North America, Europe and Asia which probably reflects an original Laurasian ancestral distribution ~90-80 Mya and subsequent post-Laurasian migrations and radiations. It is comprised of predominantly soilborne and aquatic species with non-papillate non-caducous sporangia, though some may have facultative adaptations to a degree of aerial dispersal. The largest diversity hotspots are in Southeast and East Asia, and both adaptive sympatric and non-adaptive radiations are indicated in the data. All 22 'pure' or non-hybrid species are self-fertile and exhibit relatively low nuclear heterozygosity levels. In contrast, high heterozygosity levels, relatively high numbers of tri- and partly tetra-allelic sites and maternal lines partly shared with other Clade 7a taxa are consistent with ten of the taxa (30 %) originating via interspecific hybridisation. Of these five are self-fertile, but unusually, given their likely self-fertile parents, the other five, including the 'P. ×cambivora hybrid complex', have an A1/A2 or outcrossing breeding system. Some of the hybrids, such as the 'P. ×alni hybrid complex', are known to have emerged as a result of anthropogenic disturbance events. Numerous Phytophthora species, including several in Clade 7a, have become highly damaging pathogens of trees and agricultural crops as a result of historical introductions via global plant trade. However, for the 11 new species described here, no symptoms were observed at the natural sample sites and their natural and potential hosts are unknown. The same applies to well over 150 Phytophthora species in other Clades and subclades, representing a significant plant health evidence gap. It is recommended that the biosecurity risk to forests, natural ecosystems and agriculture posed by these previously scientifically unknown taxa be actively investigated. Taxonomic novelties: New species: Phytophthora algida T. Jung, N.M. Chi, I. Milenković & M. Horta Jung, Phytophthora arta T. Jung, K. Kageyama, C.M. Brasier, Z.Á. Nagy & M. Horta Jung, Phytophthora facultativa T. Jung, T. Corcobado, Z.Á. Nagy & S. Raghuwinder, Phytophthora grandoogonia T. Jung, N.M. Chi, Z.Á. Nagy & M. Horta Jung, Phytophthora infundibuliformis T. Jung, T. Májek, I. Milenković & M. Ferreira, Phytophthora irregularis T. Jung, K. Kageyama, S. Uematsu, J.F. Webber & M. Horta Jung, Phytophthora laevigata I. Milenković, T. Jung, Z. Tomić & Z. Stanivuković, Phytophthora sessilis T. Jung, N.M. Chi, I. Milenković & M. Horta Jung, Phytophthora ×grandisporangia T. Jung, N.M. Chi, I. Milenković & M. Horta Jung, Phytophthora ×leptopleura T. Jung, H. Masuya, K. Kageyama, A. Hieno & M. Horta Jung, Phytophthora ×nodosa T. Jung, A. Hieno, H. Masuya, K. Kageyama, & S. Uematsu Citation: Nagy ZÁ, Horta Jung M, Milenković I, Kudláček T, Bourret T, Kageyama K, Hieno A, Masuya H, Uematsu S, Chi NM, Thu PQ, Chang T-T, Fu C-H, Corcobado T, Janoušek J, Májek T, Mullett M, Tomšovský M, Ferreira M, Singh R, Maia C, Stanivuković Z, Tomić Z, Bakonyi J, Webber JF, Brasier CM, Jung T (2026). Taxonomy, biogeography and evolution of thirty-four taxa in Phytophthora Clade 7a including eleven new species from natural forests and updated descriptions for P. fragariae, P. rubi and the 'P. ×alni hybrid complex'. Studies in Mycology 114: 554-661. doi: 10.3114/sim.2026.114.05.