Phytophthora siskiyouensis Reeser & E.M. Hansen 2007 was discovered first in streams and soil in Curry County, Oregon in areas dominated by native forest, and most isolates continue to come from these sources. Only later, and still infrequently, were isolates of the new species recovered from diseased forest plants. Recent reports that the same fungus was killing alders planted in cities in California (Rooney-Latham et al. 2007) and Australia (Smith et al. 2006) were a surprise.
Phytophthora siskiyouensis appears to be part of the native forest mycoflora of SW Oregon, causing occasional symptoms on a variety of associated plants, including bole cankers on tanoaks. Its appearance as an aggressive pathogen of alder in urban settings (Rooney-Latham et al. 2007, Smith et al. 2006), however, suggests that it has a larger distribution and greater pathogenic potential than has been observed in Oregon. Red alder trees of all ages grow on the sites and adjacent to the streams that yielded P. siskiyouensis in Oregon, but careful examination and isolations yielded no Phytophthora.
Etymology: ‘‘siskiyouensis’’ refers to the Siskiyou Mountains in SW Oregon where the new species was first identified.
With its semi-papillate sporangia and predominately paragynous antheridia, Phytophthora siskiyouensis would be placed in Waterhouse’s (1963) Group III. Sporangia (average 46–70 µm by 30–51 µm) formed in water are ovoid, reniform, or misshapen, on simple, unbranched sporangiophores, terminal, subterminal or intercalary, and attached basally, subbasally, or laterally. Sporangia are semi-papillate with papillae apical, subabical, or lateral, and sometimes more than one are present. Sporangia are weakly caducous with variable length (0–133 µm) pedicels. A small hyphal swelling is sometimes seen on the sporangiophore. Oogonia (average 25 µm to 30 µm), are formed in agar in single strain culture. They are globose to subglobose, terminal on long or short stalks or often sessile or laterally intercalary, with predominately paragynous (about 10 percent amphigynous), capitate antheridia. Oospores (average ca. 23–26 µm) are globose to subglobose, and slightly to markedly aplerotic. Hyphal swellings or chlamydospores are not formed in agar.