Phytophthora alni Brasier & S.A. Kirk (2004) was discovered in 1993 in southern England causing lethal root and collar rot in alders. It has subsequently been reported in many European countries including the Netherlands, Germany, France, Sweden, Belgium, Austria and Hungary, and has recently been found in Alaska and Oregon. P. alni is unusually variable, perhaps as a result of recent hybridizations among related species. There are three subspecies: the most commonly isolated and most virulent subspecies P. alni ssp. alni and two less commonly isolated subspecies, P. alni ssp. uniformis and P. alni ssp. multiformis. Etymology: From Alnus, the tree from which the pathogen was first isolated.
The typical form of P. alni, P. alni ssp. alni, is homothallic, with gametangia usually frequent. Oogonia have tapered stalks, and are variably warty with bullate protuberances (similar to those of P. cambivora). Diameters of mature oogonia commonly range from 43 to 50 um. Other oogonia are only partially developed and sometimes distorted or with beak-like or tube-like projections. Commonly many oogonia abort or produce thin-walled oospores. Antheridia are predominantly two-celled and amphigynous. Sporangia are not seen on carrot agar (CA) but are produced sparsely in pea broth or when plugs from margins of actively growing cultures on CA are partially submerged in pond water or soil leachate. Sporangia are borne singly on long sporangiophores, ellipsoid, non-papillate, and noncaducous. They exhibit nested or extended internal proliferation. Average sporangial lengths range from 48–60 mm and widths range from 31 to 43 mm. The length/width ratio ranges from about 1.3 to 1.6. No chlamydospores are observed. The other subspecies of P. alni are recognizably similar to P. alni ssp. alni, but are often more variable in growth and morphology. Many isolates appear to be unstable in culture.