Phytophthora ramorum

Name and publication

Phytophthora ramorum Werres, de Cock & Man in ’t Veld (2001)

Werres S, Marwitz R, Man in 't Veld WA, de Cock AWAM, Bonants PJM, de Weerdt M, Themann K, Ilieva E, and Baayen RP. 2001. Phytophthora ramorum sp. nov., a new pathogen on Rhododendron and Viburnum. Mycol. Res. 105: 1155–1165.

Corresponding author:


from Werres et al. (2001)




"ramus", branch: referring to the pathogenicity to twigs and branches


Type: GERMANY, Niedersachsen, isolated from Rhododendron catawbiense ‘Grandiorum’ collected in a nursery by S. Werres during July 1995, BBA 9/95 (CBS 101553 - holotypus; BBA 9/95 - isotypus); herbarium number: CBS H-7707

Ex-type: CBS 101553 (mating type A1)

Sequences for ex-type in original manuscript: Although in the original manuscript of Werres et al. (2001), CBS 101553 is shown in figures 5a (ITS1) and 5b (ITS2) (pg 1163), no accession numbers are indicated for the species in the referred manuscript.

Ex-type in other collections

BBA 9/95 = CBS 101553 = P10103 (WOC/WPC), CPHST BL 55G (Abad), Pr-164

Molecular identification

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

Phytophthora ramorum isolate CPHST BL 55G (= P10103 WPC) = ITS rDNA MG865581, COI MH136973

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

Clade 8c

Genome sequence

Phytophthora ramorum strain ex-type CPHST BL 55G. Accession genome CPHST_55G Assem3, BioProject PRJNA510798, USDA-APHIS-PPQ-S&T (2019), Srivastava et al. 2020

Morphological identification

adapted from Werres et al. (2001)

Colonies and cardinal temperatures

Colony morphology on PDA, V8, and MEA with non-distinct pattern. Minimum growth temperature 9°C, optimum 15–21°C, and maximum 27°C.

Conditions for growth and sporulation

Sporangia abundantly produced on agar as well as on plugs of culture media in soil extract. Sexual structures produced mostly in culture pairings with opposite types of other species (i.e. Phytophthora capsici, and Phytophthora cryptogea).

Asexual phase

Sporangia semipapillate; caducous with short pedicels; ellipsoid, ovoid (22–71 L x 15–40 W µm) sometimes with tapered bases and with conspicuous basal plugs; originated in unbranched or in simple sympodial sporangiophores. Hyphal swellings absent. Irregular coralloid mycelia are observed. Chlamydospores globose, subglobose (24–58 µm diam), produced commonly terminal or intercalary and occasionally lateral.

Sexual phase

Heterothallic. Oogonia smooth-walled, globose (23–38 µm diam) some with tapered bases; antheridia amphigynous, spherical, ovoid, club-shaped, sometimes with digitate projections; oospores plerotic to slightly aplerotic (20–33 µm diam).

Most typical characters

Phytophthora ramorum is characterized by the ellipsoid to elongate sporangia with short pedicels and the large chlamydospores (>30 µm diam).

Additional specimen(s) evaluated

Phytophthora ramorum ex-type CPHST BL 55G, duplicate of P10103 (World Phytophthora Collection)

Hosts and distribution

Distribution: Europe, North America (Canada, USA: WA, OR, CA). It was recently introduced via nursery shipments from western states to many other states in the USA; strict quarantine restrictions have been implemented to combat its spread.
Substrate: twigs, branches
Disease note: an aggressive forest pathogen, cause of sudden oak death; originally described as a twig and branch blight of Rhododendron
Host: at least 26 genera in 17 families, including Rhododendron spp. (Ericaceae) and Quercus spp. (Fagaceae)

Retrieved February 01, 2018 from U.S. National Fungus Collections Nomenclature Database.

Additional info
Substrate: trunk/foliage

Quarantine status

USDA/APHIS: Phytophthora ramorum regulations

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

EPPO: Phytophthora ramorum was previously listed on the EPPO alert list (at the bottom of this page).

Additional references and links

Fact sheet author

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