Phytophthora alni species complex - Phytophthora alni species complex
Effective: March 10, 2020
Taxonomic Position: Peronosporales : Pythiaceae
Pest Type: Fungi
Pest Code (NAPIS): FICBPUT
These Approved Methods are appropriate for: 2025, 2024, 2023, 2022, 2021, 2020
Pest Information
CAPS Pest Datasheet: https://download.ceris.purdue.edu/file/4336
Pest Tracker: https://pesttracker.org/pest/FICBPUT
Hosts Identified in the CAPS Host Matrix:
Alder
Pest is vectored by:
No known vector.
Survey
Approved Method(s)
| Method | Detail | NAPIS Survey Method |
|---|---|---|
| Water Sample | In situ water sampling with rhododendron leaf baits (preferred method), see detailed instructions below. | 3014 - General Water Sample |
Method Notes:
Use rhododendron leaves as bait by cutting the leaves in a herringbone pattern. Place 3-4 cut leaves into a mesh bag. Place the mesh bag into the water source for a minimum of 48 hours to 1 week (preferable).
Alder twigs, apple, eucalyptus, and oak leaflets have also been used as baits for P. alni.
A baiting technique using rhododendron leaves has been used in Alaska (Adams, 2007; Adams et al., 2010).
Alder twigs, apple, eucalyptus, and oak leaflets have also been used as baits for P. alni.
A baiting technique using rhododendron leaves has been used in Alaska (Adams, 2007; Adams et al., 2010).
Survey Recommendations
The following are recommendations for executing the survey using the approved methods for pest surveillance. The recommendations are developed through literature review and consultation with subject matter experts.
Signs:
No specific signs are present.
Symptoms:
P. alni causes a serious disease of alder (Alnus spp.), including lower stem bark lesions, root and collar necrosis, and crown dieback typical of other Phytophthora diseases.
In mid to late summer, diseased alder exhibit thin crowns and small, pale leaves; the fungus may completely girdle the tree, resulting in death; or narrow strips of bark may remain alive and support limited growth; bleeding cankers on trunks. Leaves frequently fall prematurely, leaving the tree bare. On severely affected trees, tarry or rusty spots are present on the bark at the base of the tree.
Crown decline and the tarry spot symptoms occurring together reliably indicate the presence of a basal stem necrosis produced by Phytophthora spp. After removing the outer bark layers around the tarry spots a red-brown to black discolored necrotic area is exposed. It is mostly tongue-shaped, growing upwards as well as in a periclinal direction.
Although not a specific symptom, the development of adventitious roots can be a useful indication of the presence of a bark lesion further down the stem.
Early and often excessive fructification with unusually small cones is also observed.
General information on detecting Phytophthora spp. is available in O"Brien et al. (2009).
In mid to late summer, diseased alder exhibit thin crowns and small, pale leaves; the fungus may completely girdle the tree, resulting in death; or narrow strips of bark may remain alive and support limited growth; bleeding cankers on trunks. Leaves frequently fall prematurely, leaving the tree bare. On severely affected trees, tarry or rusty spots are present on the bark at the base of the tree.
Crown decline and the tarry spot symptoms occurring together reliably indicate the presence of a basal stem necrosis produced by Phytophthora spp. After removing the outer bark layers around the tarry spots a red-brown to black discolored necrotic area is exposed. It is mostly tongue-shaped, growing upwards as well as in a periclinal direction.
Although not a specific symptom, the development of adventitious roots can be a useful indication of the presence of a bark lesion further down the stem.
Early and often excessive fructification with unusually small cones is also observed.
General information on detecting Phytophthora spp. is available in O"Brien et al. (2009).
Key Diagnostic or Identification
Approved Method(s)
ID/Diagnostic:
Diagnostic Resources:
IDphy: molecular and morphological identification of Phytophthora, is a tool developed by PPQ in collaboration with Phytophthora experts that supports morphological and molecular identification of Phytophthora spp. including P. alni.
https://idtools.org/id/phytophthora/
Serological: An Enzyme-Linked ImmunoSorbent Assay (ELISA) Reagent Set for Phytophthora (AGDIA, Cat# SRA 92600/1000) at the genus level for primary screening. A positive does not indicate P. alni.
Identification must be confirmed by other methods.
Identification must be confirmed by other methods.
Morphological: Samples of inner bark (phloem) tissue from outer edges of necrotic lesions may be washed in running water and plated directly onto a selective medium and incubated at 20C (68F). Letting samples soak for 2-5 days and replacing the water four times per days has been used to remove excess polyphenols and bacteria, which can be inhibitory to P. alni.
The pathogen can be isolated from water using alder twigs as bait, but the efficiency of this method is low (Streito et al., 2002).
The pathogen can be isolated from water using alder twigs as bait, but the efficiency of this method is low (Streito et al., 2002).
Molecular: Conventional polymerase chain reaction (cPCR) is approved for species identification. There are three subspecies of P. alni (see Notes below). Additional testing by the S&T Beltsville laboratory are necessary to confirm the subspecies.
To request a copy of these protocols, email the S&T Beltsville laboratory at APHIS-PPQCPHSTBeltsvilleSampleDiagnostics@aphis.usda.gov and use the subject line "Diagnostic protocol request".
To request a copy of these protocols, email the S&T Beltsville laboratory at APHIS-PPQCPHSTBeltsvilleSampleDiagnostics@aphis.usda.gov and use the subject line "Diagnostic protocol request".
Diagnostic Resources:
IDphy: molecular and morphological identification of Phytophthora, is a tool developed by PPQ in collaboration with Phytophthora experts that supports morphological and molecular identification of Phytophthora spp. including P. alni.
https://idtools.org/id/phytophthora/
Mistaken Identities:
Reliable diagnosis of P. alni in the field is not possible; Symptoms are similar to other root rot Phytophthora species.
Morphologically the gametangia of P. alni are similar to P. cambivora, but mycologists can distinguish these pathogens in a laboratory through colony type, their ability to self-fertilize (homothallic), oogonia shape, lower optimal growth temperature, and other characteristics.
Morphologically the gametangia of P. alni are similar to P. cambivora, but mycologists can distinguish these pathogens in a laboratory through colony type, their ability to self-fertilize (homothallic), oogonia shape, lower optimal growth temperature, and other characteristics.
Notes:
The Phytophthora alni species complex is an emergent hybrid pathogen of alder (Alnus spp.). Molecular diagnostics are necessary to confirm the ID of any suspected detections.
This species complex is split into three subspecies: P. alni subsp. alni (Paa), P. alni subsp. uniformis (Pau), and P. alni subsp. multiformis (Pam). These variants appear to range in their virulence and pathogenicity on European alders.
Phytophthora alni subsp. alni appears to be the most aggressive and pathogenic to European alder species and is not present in the United States.
Phytophthora alni subsp. uniformis was found in Alaska in 2007 and Oregon in 2012 and appears to be common in streams and alder stands without causing noticeable damage.
This species complex is split into three subspecies: P. alni subsp. alni (Paa), P. alni subsp. uniformis (Pau), and P. alni subsp. multiformis (Pam). These variants appear to range in their virulence and pathogenicity on European alders.
Phytophthora alni subsp. alni appears to be the most aggressive and pathogenic to European alder species and is not present in the United States.
Phytophthora alni subsp. uniformis was found in Alaska in 2007 and Oregon in 2012 and appears to be common in streams and alder stands without causing noticeable damage.
References
- Adams, G.C. 2007. Baiting methodology for 2007 Phytophthora surveys in Alaska.
- Adams, G.C., Catal, M., Trummer, L. 2010. Distribution and severity of alder Phytophthora in Alaska. In: Frankel, S.J., Kleijunas, J.T., and Palmieri, K.M. Proceedings of the Sudden Oak Death Fourth Science Symposium. Gen. Tech. Rep. PSW-GTP-229. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station. pp: 29-49.
- O'Brien, P.A., Williams, N., and St. J. Hardy, G.E. 2009. Detecting Phytophthora. Critical Reviews in Microbiology 35(3): 169-181.
- Streito, J.C., Jarnouen De Villartay, G., and Tabary, F. 2002. Methods for isolating the alder Phytophthora. Forest Pathology 32: 193-196.
If you are unable to find a reference, contact STCAPS@usda.gov. See the CAPS Pest Datasheet for all references.