Phytophthora × alni

Name and publication

Phytophthora alni subsp. alni Brasier & Kirk (2004) 

Brasier CM, Kirk SA, Delcan J, Cooke DEL, and Jung T. 2004. Phytophthora alni sp. nov. and its variants: designation of emerging heteroploid hybrid pathogens spreading on Alnus trees. Mycol. Res. 108: 1172–1184.

Corresponding author:

Note: reclassified as Phytophthora x alni (Brasier & S.A. Kirk) Husson, Ioos & P. Frey (2015) (SMML). Reclassification under nomenclature evaluation.

Husson C, Aguayo J, Revellin C, Frey P, Ioos R, and Marcais B. 2015. Evidence for homoploid speciation in Phytophthora alni supports taxonomic reclassification in this species complex. Fungal Genet. Biol. 77: 12–21.

Phytophthora alni species complex (alder Phytophthora) Invasive Species Compendium 

The alder Phytophthora species complex of oomycetes encompasses the hybrid P. × alni and its two parental species, P. uniformis and P. × multiformis.


from Brasier et al. (2004) 




from Alnus meaning alder


Type: UNITED KINGDOM, Yorkshire, Doncaster, isolated from Alnus bark (alder, Alnus glutinosa), 1994, G. MacAskill. IMI 392314, dried culture on carrot agar (reported as holotypus)

Ex-type: Isolate P772 at the Forestry Commission UK Phytophthora culture collection

Sequences for ex-type in original manuscript: Isolate P772 ITS rDNA AY689133

Ex-type in other collections

P772 = P772 (Brasier - DEFRA), IMI 392314, 2001 9550 (PPS Wageningen), PD 20019550, P16203 (WPC), CPHST BL 1 (Abad)

Molecular identification

Voucher sequences for barcoding genes (ITS rDNA and COI) of the ex-type (see Molecular protocols page)

Phytophthora alni subsp alni isolate CPHST BL 1 (= P16203 WPC) ITS rDNA MK496513, COI MH136991

Sequences for ex-type in other sources
Position in ITS phylogenetic tree

Clade 7a

Morphological identification

adapted from Brasier et al. (2004)

Colonies and cardinal temperatures

Colonies in V8 agar, potato dextrose agar, and malt extract agar with no distinctive pattern. Minimum growth temperature 3°C, optimum 23–25°C, and maximum 29°C.

Conditions for growth and sporulation

Sporangia form on V8 agar flooded with 10% soil extract; oogonia are usually produced on hemp seed agar amended with fatty oils.

Asexual phase

Sporangia nonpapillate; persistent; globose, ovoid, obpyriform (22–65 L x 19–46 W µm), showing nested and extended internal proliferation and borne in long unbranched or simple sympodial sporangiophores. Hyphal swellings absent. Chlamydospores absent.

Sexual phase

Homothallic. Oogonia globose (24–62 µm diam) with ornamented walls showing bullate protuberances and tapered bases; antheridia predominantly amphigynous, most single-celled but some two-celled; oospores plerotic (18–52 µm diam).

Most typical characters

P. alni subsp. alni is homothallic and characterized by the presence of oogonia with ornamented walls showing bullate protuberances and tapered bases.

Additional specimen(s) evaluated

P. alni subsp. alni CPHST-BL # 1 (Abad) = P16203 [World Phytophthora Collection (WPC) California, USA] duplicate of the ex-type P772

Hosts and distribution

Distribution: Sweden, Latvia, Germany, Austria, Italy, Hungary, probably present in or spreading to other parts of Europe. In some locations, such as southern Germany, overlaps with other P. alni subspecies and varieties.
Substrate: Alnus spp., stem/roots
Disease note: associated with root and collar rot of riparian, nursery, and shelterbelt Alnus glutinosa and other Alnus species, including A. incana and A. cordata
Hosts: Alnus glutinosa and other Alnus species, including A. incana and A. cordata

Retrieved January 19, 2018 from U.S. National Fungus Collections Nomenclature Database.

Quarantine status

USA:  Phytophthora alni is on the National Cooperative Agricultural Pest Survey (CAPS) Priority Pest Lists for 2017, 2018, and 2019.

Phytophthora alni is listed in the U.S. Regulated Plant Pest Table (last modified Nov. 15, 2017).

P. alni was listed as a species of concern during the 2009 Phytophthora prioritization project conducted by USDA APHIS PPQ CPHST PERAL (Schwartzburg et al.). 

EU: none

EPPO: none

Additional references and links

Brasier CM, Cooke DE, and Duncan JM. 1999. Origin of a new Phytophthora pathogen through interspecific hybridization. Proc. Natl. Acad. Sci. 96: 5878–5883.

Metzler B. 2011. Alder rot caused by Phytophthora alni (in German).

Solla A, Pérez-Sierra A, Corcobado T, Haque MM, Diez JJ, and Jung T. 2010. Phytophthora alni on Alnus glutinosa reported for the first time in Spain. Plant Pathology. 59: 798.



Fact sheet author

Z. Gloria Abad, Ph.D., USDA-APHIS-PPQ-S&T Beltsville Laboratory, United States of America