No known vector.

Method	Instructions	NAPIS Survey Method
Visual	Collect bark lesions or soil from symptomatic trees.	3031 - General Visual Observation
Water Sample	In situ water sampling with rhododendron leaf baits (preferred method), see detailed instructions below.	3014 - General Water Sample

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).

No specific signs are present.

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.

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.

ID/Diagnostic: Multiple

1. Serological: An ELISA test is available for Phytophthora at the genus level for primary screening. A positive does not indicate P. alni. ID must be confirmed by other methods. 2. 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 20°C (68°F). 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). 3. Molecular methods are being validated by the CPHST Beltsville laboratory. The approved methods will be updated after the validation is completed.

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.

General information on detecting Phytophthora spp. is available in O'Brien et al. (2009). Characteristic symptoms: 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. Serological: Lane et al. (2007) discuss the development of a rapid on-site antibody-based testing device (lateral flow device) by Forsite Diagnostics (York, UK) for the detection of Phytophthora spp. in plant tissue samples in the field. The test would only be useful for identification at the genus level for primary screening similar to the ELISA test discussed above. Olsson (1999) developed an improved DAS-ELISA technique for Phytophthora spp. from strawberry and raspberry that is suitable for mass testing of plant material and for diagnosing the alder Phytophthora in Sweden. The sensitivity of this method was found to be comparable to that of DNA-based methods using PCR, although specific data are not given. Molecular: De Merlier et al. (2005) developed a PCR test to identify P. alni subsp. alni and P. alni subsp. uniformis. However, this test did not detect P. alni subsp. multiformis. Ioos et al. (2005) developed a PCR-based test that detects and differentiates the various subspecies of P. alni. Where the De Merlier et al. (2005) method used DNA extracted from laboratory grown cultures, the Ioos et al. (2005) method extracted DNA directly from soil, water, or wood. Bakonyi et al. (2006) used SAP and SWAP PCR primers to identify all three subspecies of P. alni from pure culture and from plant tissue. This test can detect a minimum of 20 pg of DNA from pure cultures or DNA extracted from as few as 10 zoospores. Single-strand conformation polymorphism (SSCP) analysis of PCR-amplified ribosomal DNA internal transcribed region I has now been used to identify and provide a molecular fingerprints for 59 Phytophthora species, including P. alni subsp. alni (Gallegly and Hong, 2008).

Phytophthora alni is an emergent hybrid pathogen of alder (Alnus spp.). Because this Phytophthora hybrid does not consist of a single entity but comprises a range of phenotypically diverse allopolyploid genotypes, P. alni was split into three subspecies: P. alni subsp. alni (Paa), P. alni subsp. uniformis (Pau), and P. alni subsp. multiformis (Pam). The 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. Phytophthora alni subsp. uniformis was found in Alaska in 2007 and Oregon in 2012.