Phytophthora siskiyouensis Reeser & E. M. Hansen, 2008

California Pest Rating for
Phytophthora siskiyouensis Reeser & E. M. Hansen, 2008
Pest Rating: B


Initiating Event:  

In October 2014, Suzanne Latham, CDFA plant pathologist detected the oomycete, Phytophthora siskiyouensis which was isolated from a diseased alder tree with a bleeding trunk canker in Mill Valley, Marin County, California.  A few months earlier, the pathogen had been detected in some diseased Italian alder trees in Richmond, Contra Costa County, and originally in 2006 from a large planting of Italian alder trees in Foster City, San Mateo County. These detections in California first noted the capability of P. siskiyouensis to cause detrimental disease in alder. Subsequently, there is a need to reevaluate the current status and pest rating of P. siskiyouensis for the proposal of a permanent rating.

History & Status:

Background: Since the discovery of Phytophthora ramorum, causal organism for the Sudden Oak Disease, there has been an increase of surveys throughout the world, for Phytophthora spp. which resulted in the identification of several new species, including P. siskiyouensisPhytophthora siskiyouensis was first discovered in 2007 in streams and soils in native forest within sudden oak death epidemic regions of coastal southwest Oregon (Reeser, et al., 2007).  It appears to occur naturally in native forests in south west Oregon and was later found associated with a blighted myrtlewood shoot growing near ground level, and infrequently from tanoak bark cankers.  Furthermore, it was found in Oregon nurseries as a pathogen of alder planting stock but was not observed to cause disease on red alder in natural environments where it was detected in soil and streams (Hansen, et al., 2011; Reeser, et al., 2007).

In California, Phytophthora siskiyouensis was first found associated with bleeding trunk cankers in dying Italian alder trees in Foster City, San Mateo County (Rooney-Latham et al., 2009). In pathogenicity tests, the pathogen was confirmed to be pathogenic on Italian alder trees and potentially pathogenic to red and white alder trees.  Furthermore, it was only detected in tree bark and vascular tissue lesions and not from associated soil and root samples of the diseased trees.  The pathogen was also reported to be associated with dying alder trees in a garden in Melbourne, Australia, but as in California, was not detected in soil around the diseased trees (Smith et al., 2004).  Presently, it is not known if Phytophthora siskiyouensis is endemic to or was introduced to California.

Hosts: Presently, the full range of Phytophthora siskiyouensis is not known.  Reported hosts include, Italian alder (Alnus cordata), European common alder (A. glutinosa), White alder (A. rhombifolia), tanoak (Notholithocarpus densiflorus), California bay laurel/Myrtlewood (Umbellularia californica), and in forest streams or soil (Hansen, et al., 2011; Rooney-Latham et al., 2009; Smith et al., 2004). In pathogenicity tests, Smith et al., (2004) determined that P. siskiyouensis was a potential weak pathogen of Citrus, Acacia, and Eucalyptus seedlings.

Symptoms:  The pathogen causes collar rot in Italian alder trees.  Predominant symptoms produced on alder trees include sparse foliage, dieback in the canopy, and bleeding cankers on the trunks. Canker form primarily at the bases of trunks near the soil line and extend upwards.  Few isolated cankers may form about 2 meters above the soil line.  Trunks may have several small spots or large areas with bleeding lesions which are dark-brown to black on the outer bark.  Below the outer bark, diseased, dark orange-brown tissue is present.  A cinnamon-brown margin is separates cream-colored healthy tissue from diseased tissue which extends through the bark to the vascular cambium and sapwood interface.   Trees with large cankers may show dieback of the canopy however, cankers are not always produced on trees showing dieback (Rooney-Latham, et al., 2009; Sims, 2012). Reeser et al., (2007) noted that in Oregon the pathogen was associated with occasional symptoms on a variety of plants.

Damage Potential: Currently, there are no reports on quantitative economic losses in plant production caused by Phytophthora siskiyouensis. While infected native trees in Oregon occasionally resulted in apparent symptoms on a variety of associated plants, including trunk cankers on tanoaks, the pathogen was found to kill alder trees in California and Australia.  Infestations may result in significant damage and loss in production and stands of host plants by causing collar rot of infected plants. The full host range for this pathogen is yet not known, nevertheless, nursery alders in particular, and plants grown in natural ecosystems are particularly affected.  In general for Phytophthora spp., young seedlings of trees and annual plants may be killed within a few day, weeks or months (Agrios, 2005).

Disease Cycle: Generally, species of Phytophthora that cause root and stem rots survive cold winters or hot and dry summers as thick-walled, resting spores (oospores and chlamydospores) or mycelium in infected roots, stems or soil.  During spring, the oospores and chlamydospores germinate to produce motile spores (zoospores) that swim around in soil water and roots of susceptible hosts. The pathogen infects the host at the soil line causing water soaking and darkening of the trunk bark. This infected area enlarges and may encircle the entire stem of small plants which wilt and eventually die.  On large plants and trees, the infected, necrotic area may be on one side of the stem and become a depressed canker below the level of the healthy bark.  Collar rot canker may spread down the root system. Roots are invaded at the crown area or at ground level.   Mycelium and zoospores grow in abundance in cool, wet weather causing damage where the soil is too wet for normal growth of susceptible plants and low temperatures (15-23°C) prevail (Agrios, 2005).

Transmission:  The pathogen may be spread to non-infected sites through infected plants, nursery and planting stock, seedlings, soil, run-off and splash irrigation and rain water, and contaminated cultivation equipment and tools.

Worldwide DistributionNorth America: USA; Oceania: Australia.

In the USA, Phytophthora siskiyouensis has been found in California and Oregon (CABI, 2014).

Official Control:  None reported.

California Distribution:  Phytophthora siskiyouensis has been detected on alder in Contra Costa, Marin, and San Mateo Counties (see ‘Initiating Event’).

California Interceptions:  The pathogen has not been intercepted in quarantine shipments of plants.

The risk Phytophthora siskiyouensis would pose to California is evaluated below.

Consequences of Introduction: 

1)  Climate/Host Interaction: Evaluate if the pest would have suitable hosts and climate to establish in California. Score:

– Low (1) not likely to establish in California; or likely to establish in very limited areas
– Medium (2) may be able to establish in a larger but limited part of California
High (3) likely to establish a widespread distribution in California

Risk is High (3) Phytophthora siskiyouensis has already been detected in few counties.  Within California, it is likely to establish in cool, wet climates in susceptible hosts.  Thus far it has only been detected in alder trees in California.

2)  Known Pest Host Range: Evaluate the host range of the pest:

Low (1) has a very limited host range
 Medium (2) has a moderate host range
– High (3) has a wide host range

Risk is Low (1) The full host range of Phytophthora siskiyouensis is yet not known.  Reported hosts include, Italian alder, European common alder, white alder, tanoak, and California bay laurel.  It has also been detected in forest streams or soil in Oregon.  Experimentally, it has been shown to be a weak pathogen of Citrus, Acacia and Eucalyptus seedlings.

3)  Pest Dispersal Potential: Evaluate the dispersal potential of the pest:

– Low (1) does not have high reproductive or dispersal potential
– Medium (2) has either high reproductive or dispersal potential
High (3) has both high reproduction and dispersal potential

Risk is High (3) Phytophthora siskiyouensis is primarily spread artificially via infested soils, plants, nursery and planting stock, seedlings, run-off and splash irrigation water, cultivation equipment and tools that may spread contaminated soil and plant materials to non-infected sites. Thus far, in California, the pathogen has not been isolated from roots and associated soil of infected alder trees.

4)  Economic Impact: Evaluate the economic impact of the pest to California using these criteria:

A.   The pest could lower crop yield.
B.   The pest could lower crop value (includes increasing crop production costs).
C.   The pest could trigger the loss of markets (includes quarantines).
D.   The pest could negatively change normal cultural practices.
E.   The pest can vector, or is vectored, by another pestiferous organism.
F.   The organism is injurious or poisonous to agriculturally important animals.
G.   The organism can interfere with the delivery or supply of water for agricultural uses.

– Low (1) causes 0 or 1 of these impacts
– Medium (2) causes 2 of these impacts
High (3) causes 3 or more of these impacts

Risk is High (3) – Although quantitative economic losses in plant production have not reported, the potential for infected alder  plants to result in collar rot, trunk cankers, and shoot dieback could decrease stands on non-infected plants, increase production costs and cause loss of market of infected  nursery stocks. The potential for the pathogen to survive and spread in infected soils and irrigation water could require changes in normal cultivation practices of host plants.

5)  Environmental Impact: Evaluate the environmental impact of the pest on California using these criteria:

A.   The pest could have a significant environmental impact such as lowering biodiversity, disrupting natural communities, or changing ecosystem processes.
B.   The pest could directly affect threatened or endangered species.
C.   The pest could impact threatened or endangered species by disrupting critical habitats.
D.   The pest could trigger additional official or private treatment programs.
E.   The pest significantly impacts cultural practices, home/urban gardening or ornamental plantings.

Score the pest for Environmental Impact:

– Low (1) causes none of the above to occur
Medium (2) causes one of the above to occur
– High (3) causes two or more of the above to occur

Risk is Medium (2) – Currently, the host range and geographic distribution of P. siskiyouensis are not fully known.  The few known host plants (see ‘Hosts’ above) can be found in natural ecological habitats as well as in nursery environments.  Alder trees are particularly affected by this pathogen.  Subsequently, under favorable climate conditions, natural plant communities and ecosystems, as well as home/urban gardening and ornamentals may be negatively impacted.

Consequences of Introduction to California for Phytophthora siskiyouensis:

Add up the total score and include it here. (Score)

Low = 5-8 points
Medium = 9-12 points
High = 13-15 points

Total points obtained on evaluation of consequences of introduction of Phytophthora siskiyouensis to California = (12).

6)  Post Entry Distribution and Survey Information: Evaluate the known distribution in California. Only official records identified by a taxonomic expert and supported by voucher specimens deposited in natural history collections should be considered. Pest incursions that have been eradicated, are under eradication, or have been delimited with no further detections should not be included. (Score)

-Not established (0) Pest never detected in California, or known only from incursions.
Low (-1) Pest has a localized distribution in California, or is established in one suitable climate/host area (region).
-Medium (-2) Pest is widespread in California but not fully established in the endangered area, or pest established in two contiguous suitable climate/host areas.
-High (-3) Pest has fully established in the endangered area, or pest is reported in more than two contiguous or non-contiguous suitable climate/host areas.

Evaluation is Low (-1). To date, Phytophthora siskiyouensis has been detected in three California coastal counties (Contra Costa, Marin and San Mateo Counties) on alder (Alnus sp.) under similar climate (north coastal environment).

Final Score:

7) The final score is the consequences of introduction score minus the post entry distribution and survey information score: (Score)

Final Score:  Score of Consequences of Introduction – Score of Post Entry Distribution and Survey Information = 11.


The full host range and in-state distribution of Phytophthora siskiyouensis is not currently known.  To date, in California, the pathogen has only been detected from alder trees in three northern coastal counties.  Continued statewide surveys for Phytophthora spp. occurring in nurseries and natural ecosystems (e.g. restoration sites) will contribute to the present knowledge of this pathogen group as well as that of P. siskiyouensis.  Also, it is not known if P. siskiyouensis is endemic to California or was introduced. Related information gained through further research may affect the current proposed rating of P. siskiyouensis.  

Conclusion and Rating Justification:

Based on the evidence provided above the proposed rating for Phytophthora siskiyouensis is B.


Agrios, G. N.  2005.  Plant Pathology fifth edition.  Elsevier Academic Press, Massachussetts, USA.  922 p.

CABI.  2014.  Phytophthora siskiyouensis datasheet (basic) report.  Crop Protection Compendium.

Farr, D.F., & Rossman, A.Y.  2014.  Fungal Databases, Systematic Mycology and Microbiology Laboratory, ARS, USDA. Retrieved November 10, 2014, from .

Hansen, E. M., P. Reeser and S. Rooney-Latham.  2011.  Phytophthora siskiyouensis.  Forest Phytophthoras 1(1). doi: 10.5399/osu/fp.1.1.1826

Reeser, P. W., E. M. Hansen and W. Sutton.  2007.  Phytophthora siskiyouensis, a new species from soil, water, myrtlewood (Umbellularia californica) and tanoak (Lithocarpus densiflorus) in southwestern Oregon.  Mycologia 99:639-643.

Reeser, P. W., Sutton, W., and Hansen, E. M. 2008. Phytophthora species causing tanoak stem cankers in southwestern Oregon. Plant Dis. 92:1252.

Rooney-Latham, S., C. L. Blomquist, T. Pastalka and L. R. Costello.  2007.  First report of Phytophthora siskiyouensis causing disease on Italian alder in Foster City, California.  Phytopathology 97:S101.

Rooney-Latham, S., C. L. Blomquist, T. Pastalka and L. Costello.  2009.  Collar rot on Italian alder trees in California caused by Phytophthora siskiyouensis.  Online. Plant Health Progress doi:10.1094/PHP-2009-0413-01-RS. .

Sims, L.  2012.  Alder (Alnus spp.) – Collar Rot {Phytophthora Canker}.  PNW Plant Disease Management Handbook Printed page URL:

Smith, I. A., Cunnington, J., and Pascoe, I. 2004. Another new? species of Phytophthora on alder down under (Australia). P. of the IUFRO Res. Work. Gr. Conf. on Phytophthora Forests and Natural Ecosystems. 11-17 September 2004. Freising, Germany.

Responsible Party:

John J. Chitambar, Primary Plant Pathologist/Nematologist, California Department of Food and Agriculture, 3294 Meadowview Road, Sacramento, CA 95832. Phone: 916-262-1110,[@]

Comment Period:  CLOSED

The 45-day comment period opened on June 1, 2015 and closed on July 16, 2015.


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