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Figure 1. Culture of Phytophthora ramorum grown on V-8 juice agar. (Courtesy Jean B. Ristaino)

Figure 2. Sporangia of Phytophthora ramorum. (Courtesy Sabine Werres; Reproduced from Werres et al., 2001)

Figure 3. Terminal chlamydospore of Phytophthora ramorum. (Courtesy Sabine Werres; Reproduced from Davidson et al., 2003)

Figure 4. Oospore of Phytophthora ramorum. (Courtesy Sabine Werres; Reproduced from Davidson et al., 2003)

Figure 5. Tan oak with phloem canker on main trunk caused by Phytophthora ramorum. (Courtesy Matteo Garbelotto; Reproduced from Garbelotto et al., 2003a)

Figure 6. Stem canker caused by Phytophthora ramorum on tan oak stops at the soil line. (Courtesy Matteo Garbelotto; Reproduced from Garbelotto et al., 2003a

Figure 7A. Canker on coast live oak caused by Phytophthora ramorum. Outer bark shows rust-colored seepage. (Courtesy David M. Rizzo; Reproduced from Davidson et al., 2003)

Figure 7B. Canker on coast live oak caused by Phytophthora ramorum. Outer bark shows rust-colored seepage. (Courtesy David M. Rizzo; Reproduced from Davidson et al., 2003)

Figure 8. Rhododendron showing branch dieback and foliar lesions caused by Phytophthora ramorum. (Courtesy David M. Rizzo; Reproduced from Garbelotto et al., 2003b)

Figure 9. Bay laurel with leaf tip necrosis. (Courtesy David M. Rizzo; Reproduced from Garbelotto et al., 2003a)

Introduction

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

Phytophthora ramorum is an aggressive forest pathogen and the cause of sudden oak death. It was originally described as the cause of a twig and branch blight of Rhododendron and Viburnum spp. in Germany and the Netherlands (Werres et al., 2001). Sudden oak death was observed in the San Francisco Bay area in 1994–1995 and subsequently caused major epidemics in northern California in native tree stands of tan oak (Lithocarpus densiflorus), coast live oak (Quercus agrifolia), and California black oak (Quercus kelloggii) in 2000 (Rizzo and Garbelotto, 2003; Rizzo et al., 2002). It is believed that P. ramorum is an exotic species and was accidentally introduced into California and Europe (Brasier et al., 2004). It is speculated that the pathogen originated in Asia, potentially in Nepal or areas in south China, although this has not been definitively confirmed (Brasier, 2003). The pathogen is heterothallic and requires two mating types to form the sexual oospore (Werres and Zielka, 2003). It is believed that pathogen populations in the United States and Europe are clonal (Ivors et al., 2004; Kroon et al., 2004). Initially, all European isolates were the A1 mating type, whereas U.S. isolates were the A2 mating type (Brasier, 2003; Werres and Zielka, 2003; Werres et al., 2001). It was suspected that separate introductions of these different types occurred into the United States and Europe (Ivors et al., 2004). Unlike P. infestans, pairings of P. ramorum with the opposite mating type do not readily result in oospore formation in culture (Werres and Kaminski, 2005; Werres and Zielka, 2003). Oospores are formed more abundantly on live tissue and in interspecific pairings (Brasier, 2003). In 2003, A2 strains of P. ramorum were found in Europe (Werres and De Merlier, 2003), and in the same year, the first A1 isolates were found in Oregon in a survey of nurseries (Hansen et al., 2003). Pairings of the European and U.S. strains result in oospore formation. Although the potential is there for sexual reproduction in this pathosystem, fortunately, this has not been documented yet in the field. Amplified fragment length polymorphism (AFLP) analysis of a large number of U.S. and European isolates also revealed a high level of genetic similarity within populations and no evidence of recombination between the U.S. and European populations (Ivors et al., 2004). Three lineages are currently described.

Cultural Characteristics

Colonies on V-8 juice agar, cornmeal agar, and carrot agar show concentric rings (Fig. 1). Those on cherry decoction agar show appressed aerial mycelium and indistinct rosette pattern. The optimal temperature for growth is 20°C, the minimum temperature for growth is 2°C, and the maximum temperature for growth is 26°C.

Reproductive Structures

Asexual Structures

 

Sporangiophores:

Sporangia are borne either singly or on simple sympodia. There is terminal or lateral proliferation of the subtending hyphae.

 

Sporangia:

Sporangia are abundant on agar as well as on hemp seeds in soil extract. Sporangia are ellipsoid, spindle shaped, or elongate-ovoid, with a rounded or occasionally tapered base. Sporangia have a single narrow, indistinct papilla (semipapillate). Sporangia are caducous with a short pedicel and occasionally without a pedicel. Sporangia are 20–32 × 40–80 µm (average 24 × 52 µm) (Fig. 2). The length–breadth ratio is 2.16 (Werres et al., 2001).

 

Chlamydospores:

Chlamydospores are abundant on agar, commonly intercalary and terminal, occasionally lateral, subglobose, and 20–91 µm (average 46–60 µm) (Fig. 3). They are brown and produced abundantly.

 

Hyphae:

Main hyphae are up to 8 µm wide. 

 

Sexual Structures

 

P. ramorum is heterothallic.

 

Antheridia:

Antheridia are single, terminal, diclinous, amphigynous, spherical to barrel shaped, and 10–18 × 14–18 µm.

 

Oogonia:

Oogonia are subglobose, 28–31.2 µm in diameter, smooth walled, colorless, and up to 2 µm thick.

 

Oospores:

Oospores are plerotic and 21.1–22.5 µm in diameter, with a wall thickness of 3.5 µm (Fig. 4).

Host Range and Distribution

Host	Common Name	Disease	Geographical Distribution
Camellia japonica, C. sasanqua	Japanese camellia, sasanqua camellia	Leaf blight	Canada, Europe, United States
Kalmia spp.	Mountain laurel	Twig and leaf blight	Canada, Europe, United States
Leucothoe spp.	Fetterbush	Twig and leaf blight	Canada, Europe, United States
Lithocarpus densiflorus	Tan oak	Sudden oak death	United States (CA, OR)
Pieris spp.	Andromeda, pieris	Twig and leaf blight	Canada, Europe, United States
Quercus agrifolia, Q. kelloggii, Q. parvula var. shrevei	Coast live oak, California black oak, Shreve oak	Sudden oak death	United States (CA)
Rhododendron spp.	Rhododendron	Twig and leaf blight	Canada, Europe, United States
Syringa vulgaris	Lilac	Twig and leaf blight	Canada, Europe, United States
Umbellularia californica	Oregon myrtlewood, California bay laurel, pepperwood	Leaf blight	Canada, Europe, United States
Viburnum spp.	Viburnum	Twig and leaf blight	Canada, Europe, United States

The host range of the pathogen has expanded rapidly, and a complete and current list of the species considered hosts of P. ramorum is maintained online by the USDA-APHIS (2010) (Cline et al., 2008; Inman et al., 2003). Species of Camellia, Rhododendron, Pieris, Kalmia, Viburnum, Umbellularia, and tan oak (Lithocarpus densiflorus) are considered the most important known hosts for P. ramorum.

Symptoms

P. ramorum is dispersed by windblown rain. It attacks the bark and lower stems of tan oak, causing bleeding cankers, but does not infect below the soil line (Brasier, 2003; Rizzo et al., 2002) (Figs. 5 and 6). It infects the bark of beech trees. Bark infections typically result in large cankers that are brown to black with discolored outer bark that seeps dark red to blue-black sap (bleeding cankers) (Figs. 7A and 7B). Cankers can girdle the trunk and kill the tree. Leaf infections occur as brown necrotic spots, often at the edge or tip of the leaf. The pathogen causes a twig blight of Rhododendron spp. and sporulates and causes foliar symptoms on Rhododendron, Viburnum, Pieris, Hamamelis, and Parrotia spp. (Rizzo et al., 2002) (Fig. 8). Symptoms include blackening of the petiole, leaf base, and leaf tip (Fig. 9). Twig cankers can lead to the wilting of shoots. Roots are unaffected. The pathogen can be detected by both molecular diagnostic assays and isolation (Osterbauer and Trippe, 2005).

References

Brasier, C. 2003. Sudden oak death: Phytophthora ramorum exhibits transatlantic differences. Mycol. Res. 107:257-259.

 

Brasier, C. M., Denman, S., Brown, A., and Webber, J. F. 2004. Sudden oak death (Phytophthora ramorum) discovered on trees in Europe. Mycol. Res. 108:1107-1110.

 

Cline, E. T., Farr, D. F., and Rossman, A. Y. 2008. A synopsis of Phytophthora with accurate scientific names, host range, and geographic distribution. Plant Health Progress doi:10.1094/PHP-2008-0318-01-RS.

 

Davidson, J. M., Werres, S., Garbelotto, M., Hansen, E. M., and Rizzo, D. M. 2003. Sudden oak death and associated diseases caused by Phytophthora ramorum. Plant Health Progress doi:10.1094/PHP-2003-0707-01-DG.

 

Garbelotto, M., Davidson, J. M., Ivors, K., Maloney, P. E., Hüberli, D., Koike, S. T., and Rizzo, D. M. 2003a. Non-oak native plants are main hosts for sudden oak death pathogen in California. Calif. Agric. 57:18-23.

 

Garbelotto, M., Davidson, J. M., Ivors, K., Maloney, P. E., Hüberli, D., Koike, S. T., and Rizzo, D. M. 2003b. Bay laurel and native plants other than oaks are the main hosts for the sudden oak death pathogen, P. ramorum, in California. APSnet Feature. American Phytopathological Society, St. Paul, MN. www.apsnet.org/publications/apsnetfeatures/Pages/SuddenOak.aspx.

 

Hansen, E. M., Reeser, P. W., Sutton, W., Winton, L. M., and Osterbauer, N. 2003. First report of A1 mating type of Phytophthora ramorum in North America. Plant Dis. 87:1267.

 

Inman, A. J., Townsend, V. C., Barnes, A. V., Lane, C. R., Hughes, K. J. D., Griffin, R. L., and Eales, S. J. 2003. First report of ramorum dieback (Phytophthora ramorum) on Pieris in England. Plant Pathol. 52:785.

 

Ivors, K., Hayden, K. J., Bonants, P., Rizzo, D. M., and Garbelotto, M. 2004. AFLP and phylogenetic analysis of North American and European populations of Phytophthora ramorum. Mycol. Res. 108:378-392.

 

Kroon, L. P. N. M., Verstappen, E. C. P., Kox, L. F. F., Flier, W. G., and Bonants, P. J. M. 2004. A rapid diagnostic test to distinguish between American and European populations of Phytophthora ramorum. Phytopathology 94:613-620.

 

Osterbauer, N., and Trippe, A. 2005. Comparing diagnostic protocols for Phytophthora ramorum in rhododendron leaves. Plant Health Progress doi:10.1094/PHP-2005-0314-01-HN.

 

Rizzo, D. M., and Garbelotto, M. 2003. Sudden oak death: Endangering California and Oregon forest ecosystems. Front. Ecol. Environ. 1:197-204.

 

Rizzo, D. M., Garbelotto, M., Davidson, J. M., Slaughter, G. W., and Koike, S. T. 2002. Phytophthora ramorum as the cause of extensive mortality of Quercus spp. and Lithocarpus densiflorus in California. Plant Dis. 86:205-214.

 

United States Department of Agriculture-Animal and Plant Health Inspection Service (USDA-APHIS). 2010. APHIS lists of regulated hosts and plants proven or associated with Phytophthora ramorum. USDA-APHIS, Plant Protection Quarantine. www.aphis.usda.gov/plant_health/plant_pest_info/pram/downloads/pdf_files/usdaprlist.pdf.

 

Werres, S., and De Merlier, D. 2003. First detection of Phytophthora ramorum mating type A2 in Europe. Plant Dis. 87:1266.

 

Werres, S., and Kaminski, K. 2005. Characterization of European and North American Phytophthora ramorum isolates due to their morphology and mating behavior in vitro with heterothallic Phytophthora species. Mycol. Res. 109:860-871.

 

Werres, S., and Zielka, B. 2003. First studies on the pairing of Phytophthora ramorum. J. Plant Dis. Prot. 110:129-130.

 

Werres, S., Marwitz, R., Man in 't Veld, W. A., de Cock, A. W. A. M., Bonants, P. J. M., De Weerdt, M., Themann, K., Ilieva, E., and Baayen, R. P. 2001. Phytophthora ramorum sp. nov., a new pathogen on Rhododendron and Viburnum. Mycol. Res. 105:1155-1165.