Phytophthora siskiyouensis was discovered first from streams and soil 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 bole cankers on tanoak and only once from necrotic sprouts on myrtlewood. It appears to be part of the native forest mycobiota of SW Oregon, associated with occasional symptoms on a variety of plants. It is also apparently present in the nursery industry as a pathogen on horticultural alder planting stock.
P. siskiyouensis was discovered in the course of P. ramorum survey and detection activities in southwestern Oregon. It was originally found in soil and stream water samples. Later it was found once in association with blighted myrtlewood shoots growing near ground level, and several times in tanoak bark cankers. It is also recovered occasionally from water traps placed beneath tanoak canopies. P. siskiyounsis is of great concern as a pathogen of Italian alder (Alnus cordata (Loisel.) Duby) and the native white alder (A. rhombifolia) in California (Rooney-Latham et al. 2009), and European alder (Alnus glutinosa (L.) Gaertn) in Australia (Smith et al. 2006). Although found in Oregon in soil and streams in an area where red alder (Alnus rubra) is abundant in the landscape, and demonstrated experimentally capable of infecting red alder stems, P. siskiyouensis has not to date been observed causing disease on red alder in the known area of natural overlap.
|Host Latin Name||Host Common Name||Symptoms||Habitat||Region|
|Alnus cordata||Italian alder||Canker||Residential||USA|
|Alnus glutinosa||European common alder||Canker||Gardens||Australia - Melbourne|
|Alnus rhombifolia||White alder||Canker||Residential||USA - Pacific Northwest|
|Alnus rhombifolia||White alder||Root rot||Forest||USA - Oregon|
|Alnus rubra||Red alder||Canker, Root rot||Forest||USA - Oregon|
|Host unknown||None, found in streams or soil||None, found in streams or soil||Forest||USA - Pacific Northwest|
|Notholithocarpus densiflorus||Tanoak||Canker||Forest||USA - Pacific Northwest|
|Umbellularia californica||California bay laurel, Myrtlewood||Blight, Dieback, Leaf necrosis, Twig canker||Forest||USA - Pacific Northwest|