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Review
. 2008 Nov;9(6):729-40.
doi: 10.1111/j.1364-3703.2008.00500.x.

Phytophthora ramorum: a pathogen with a remarkably wide host range causing sudden oak death on oaks and ramorum blight on woody ornamentals

Niklaus J Grünwald  1 , Erica M Goss, Caroline M Press
Affiliations
Review

Phytophthora ramorum: a pathogen with a remarkably wide host range causing sudden oak death on oaks and ramorum blight on woody ornamentals

Niklaus J Grünwald et al. Mol Plant Pathol. 2008 Nov.

Abstract

Phytophthora ramorum is an oomycete plant pathogen classified in the kingdom Stramenopila. P. ramorum is the causal agent of sudden oak death on coast live oak and tanoak as well as ramorum blight on woody ornamental and forest understorey plants. It causes stem cankers on trees, and leaf blight or stem dieback on ornamentals and understorey forest species. This pathogen is managed in the USA and Europe by eradication where feasible, by containment elsewhere and by quarantine in many parts of the world. Genomic resources provide information on genes of interest to disease management and have improved tremendously since sequencing the genome in 2004. This review provides a current overview of the pathogenicity, population genetics, evolution and genomics of P. ramorum.

Taxonomy: Phytophthora ramorum (Werres, De Cock & Man in't Veld): kingdom Stramenopila; phylum Oomycota; class Peronosporomycetidae; order Pythiales; family Pythiaceae; genus Phytophthora.

Host range: The host range is very large and the list of known hosts continues to expand at the time of writing. Coast live oak and tanoak are ecologically, economically and culturally important forest hosts in the USA. Rhododendron, Viburnum, Pieris, Syringa and Camellia are key ornamental hosts on which P. ramorum has been found repeatedly, some of which have been involved in moving the pathogen via nursery shipments. Disease symptoms: P. ramorum causes two different diseases with differing symptoms: sudden oak death (bleeding lesions, stem cankers) on oaks and ramorum blight (twig dieback and/or foliar lesions) on tree and woody ornamental hosts.

Useful websites: http://nature.berkeley.edu/comtf/, http://rapra.csl.gov.uk/, http://www.aphis.usda.gov/plant_health/plant_pest_info/pram/index.shtml, http://genome.jgi-psf.org/Phyra1_1/Phyra1_1.home.html, http://pamgo.vbi.vt.edu/, http://pmgn.vbi.vt.edu/, http://vmd.vbi.vt.edu./, http://web.science.oregonstate.edu/bpp/labs/grunwald/resources.htm, http://www.defra.gov.uk/planth/pramorum.htm, http://www.invasive.org/browse/subject.cfm?sub=4603, http://www.forestry.gov.uk/forestry/WCAS-4Z5JLL.

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Figures

Figure 1
Figure 1
(A) Tanoak mortality near Bolinas Ridge, Mt. Tamalpais, Marin County, California. Photo courtesy: Janet Klein, Marin Municipal Open Space District. (B) Eradication of sudden oak death in Curry County, Oregon, through slash and burn. Photo courtesy: Oregon Department of Forestry.
Figure 2
Figure 2
Phylogeny of the genus Phytophthora highlighting clade 8. Clade locations of other Phytophthora species with published or forthcoming whole genome sequences are also indicated (modified from Blair et al., 2008).
Figure 3
Figure 3
Generic life cycle of Phytophthora ramorum, broadly outlining the major life stages. Note that the sexual cycle to date has only been described under controlled conditions in the laboratory and has not been documented in nature (Brasier and Kirk, 2004; Werres and Kaminsky, 2005).
Figure 4
Figure 4
Typical symptoms observed on different hosts of Phytophthora ramorum. (A) Bleeding canker of tanoak in the exposed inner bark (phloem) after removal of the outer bark: note typical lesions with clearly differentiated margins; (B) leaf tip necrosis or leaf spots on leaf margins of California bay laurel; (C) rhododendron—arrow marks necrosis of leaf petiole; (D) leaf necrosis on nursery ornamental rhododendron.
Figure 5
Figure 5
Difference in aggressiveness of P. ramorum clonal lineages EU1 and NA1. (A) Leaf lesion area 10 days post inoculation on two Rhododendron cultivars as a function of clonal lineage. The difference between EU1 and NA1 was significant in trials 2 and 3 (P < 0.005 and P < 0.05, respectively) (V. T. McDonald and N. J. Grünwald, unpublished data). (B) Mean lesion area in wound inoculated lower stems of Quercus rubra (data from Brasier et al., 2006). The differences between EU1 and NA1 were significant in trials 1 and 3 (P < 0.001 and P < 0.0012, respectively).
Figure 6
Figure 6
Relative number of genes potentially involved in infection in P. ramorum compared with P. sojae (grey bars) and the diatom Thalassiosira pseudonana (white bars). For three gene families, the number of genes in T. pseudonana was not determined (nd). Data are from Tyler et al. (2006: table 1).
Figure 7
Figure 7
Maximum likelihood genealogy of a subfamily of RXLR‐class effector genes (avr gene homologues, Avh) in Phytophthora clade 8c. PrAvh gene sequences are from P. ramorum isolate Pr102. For heterozygous genes, the two alleles are distinguished by the suffixes ‘a’ and ‘b’. Homologous sequences are in bold typeface. The P. lateralis and P. hibernalis homologues to PrAvh60, PrAvh68, PrAvh108 and PrAvh205 (genes in box) appear to be syntenic to PrAvh205 based on flanking sequence similarity. Bootstrap support is shown for all branches (E. M. Goss and N. J. Grünwald, unpublished data).
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