Phytophthora siskiyouensis

Species profile prepared by E. M. Hansen, P. Reeser, and S. Rooney-Latham

To cite, see Forest Phytophthoras (2011) doi: 10.5399/osu/fp.1.1.1826

Disease History: 

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.

Phytophthora collar rot on Italian alder trees; bleeding canker with a sprinkler emitter adjacent to the trunk (top), grove of dying trees in a commercial landscape in Foster City, CA (bottom)

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Impacts in the Forest: 

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