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Fungi

Sudden oak death
Phytophthora ramorum Werres, De Cock & Man In’t Veld Pythiales, Oomycetes

Status
Exotic (absent from Australia) High Impact Pest Species

Host Symptoms

Caption: Died plant.

Source: Joseph O'Brien (www.forestryimages.org) USDA Forest Service

APHIS Diagnostic Protocols

Symptoms:

Phytophthora ramorum has a wide host range, over 40 plant genera, causing two types of symptoms: bark cankers and foliar symptoms. The latter includes localised necrotic leaf spots, leaf blight, and twig dieback. Infections on Rhododendron cause brown lesions on leaves and young stems. On Viburnum, the infection usually occurs at the stem base causing the plants to collapse (Henricot & Prior 2004).

The most consistent and diagnostic symptoms on trees are cankers that develop before foliar symptoms become evident. Cankers have brown or black discoloured outer bark often with an ooze of dark red sap. They occur on the trunk at the root crown up to 20 m above the ground, but do not enlarge below the soil line into the roots. Individual cankers are delimited by thin black lines in the inner bark and can be over 2 m in length (Rizzo et al. 2002).

Although P. ramorum is deadly on certain oaks (Quercus spp.) and tanoak, on other hosts, such as Umbellularia californica, Aesculus californica and Acer macrophyllum, P. ramorum is primarily a leaf pathogen with very limited stem infection (Garbelotto et al. 2003). Symptoms on non-tree woody plant species include twig wilt and branch dieback, stem lesions, necrosis of the midrib from the petiole, leaf tip necrosis and aerial dieback (Appiah et al. 2004).

The pathogen:

Colonies on carrot piece agar (CPA), V8 agar, and cornmeal agar submerged, showing concentric rings; on cherry decoction agar with appressed aerial mycelium and an indistinct rosette pattern. Main hyphae up to 8 µm wide. Chlamydospores abundant on agar, commonly intercalary and terminal, occasionally lateral, (sub-)globose, 22—53.8—72 µm in diameter. Sporangia abundantly produced on agar as well as on hemp seeds in soil extract, single or in sympodia, terminal or lateral after proliferation of the subtending hypha, ellipsoidal, spindle-shaped or elongate-ovoid, with rounded or occasionally tapering base, with a single, narrow and indistinct papilla, 20—32 x 40—80 µm, average 52 x 24 µm, average length:width ratio 2.16, but on hemp seeds in soil extract lower; caducous with a short pedicel, occasionally without pedicel. Zoospores produced in water at temperatures below 20 ^oC. Heterothallic. Oogonia only developed in dual cultures with a complementary strain, subglobose, 28—31.2—38 µm in diameter, wall smooth, colourless, up to 2 µm thick. Antheridia single, terminal, diclinous, amphigynous, spherical to barrel-shaped, approx. 10—18 x 14—16 µm. Oospores plerotic. Cardinal temperatures for growth: minimum 2 ^oC, optimum 20^oC, maximum 26^oC. Daily radial growth on CPA at 20 ^oC 2.8 mm (Werres et al. 2001).

Notes:

1. Werres & Kaminski (2005) studied 94 isolates from different hosts and geographical distribution and found the minimum temperature for vegetative growth was 2—5 ^oC, the optimum temperature was between 15—25 ^oC for most of the isolates, and the maximum temperature varied between 26 and 30 ^oC. Growth rate at optimum temperature was between 1.1—4.4 mm d^-1 with a mean of 2.9 mm d^-1. The mean sporangium length was 43.6 ± 5.3 μm with a range from 20—79 μm and the mean sporangium width was 23.9 ± 2.6 μm, with a range from 12—40 μm. The mean length:breadth ratio was 1.8:1 ± 0.12, the minimum was 1.5:1; and the maximum was 2.2:1. Chlamydospore size ranged from 22—88 μm with a mean size of 53.4 ± 4.7 μm.

2. Two distinct populations of P. ramorum have been characterized. In the coastal forests of California and Oregon (USA), P. ramorum causes serious bark cankers and has been killing thousands of trees of native oak species while surviving and causing damage on a wide range of other shrubs and trees. A second population has been found in Europe mainly on Rhododendron and Viburnum spp. used as ornamental plants in nurseries and landscapes, but has been found rarely in forest ecosystems (Kroon et al. 2004, Henricot & Prior 2004). Although the host ranges of the two populations are similar, they appear to be differentially adapted (Brasier, 2003). All European isolates (with a single exception) have been of the A1 mating type, whereas North American forest isolates have been of A2 (Rizzo et al. 2005). Genetic analysis of microsatellite loci of 151 isolates of P. ramorum revealed three lineages of the pathogen. Two occur in USA and one in Europe (Ivors et al. 2006).

3. P. ramorum genome was sequenced by Tyler et al. (2005). The pathogen can be detected by polymerase chain reaction (PCR) techniques (Kong et al. 2004, Martin et al. 2004, Osterbauer & Trippe 2005). Kroon et al. (2004) developed a single-nucleotide polymorphism protocol to distinguish between isolates of P. ramorum originating in Europe and those originating in the United States.

Corresponding author: Dr JR Liberato

Citation: Liberato JR, O’Brien JG & Werres S (2009) Sudden oak death (Phytophthora ramorum ) Pest and Diseases Image Library. Updated on 2/9/2009 2:06:16 PM. Available online: http://www.padil.gov.au

Created Date: 6/23/2006 2:09:46 PM

Last Updated: 2/9/2009 2:06:16 PM

Diagnostic Images
Host Symptoms	Host symptoms - stem	Host symptoms - stem	Host symptoms - stem
Overview Images
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Host Symptoms	Host Symptoms	Host Symptoms	Host Symptoms
Host Symptoms	Host Symptoms	Host Symptoms	Host Symptoms
Host symptoms - leaves	Host symptoms - leaves	Host symptoms - leaves	Host symptoms - leaves
Host symptoms - stem	Host symptoms - stem	Host symptoms - stem	Host symptoms - stem
Host symptoms - stem