Sudden oak death
589513
225796920
2008-07-15T13:15:35Z
84.92.90.120
/* A native organism */ clarification
{{Taxobox
| color = khaki
| name = Sudden Oak Death
| image = Sudden_Oak_Death.jpg
| image_width = 240px
| image_caption = Canker on an infected oak
| domain = [[Eukaryota]]
| unranked_phylum = [[Chromista]]
| phylum = [[Heterokont]]ophyta
| classis = [[Water mould|Oomycetes]]
| ordo = [[Pythiales]]
| familia = [[Pythiaceae]]
| genus = ''[[Phytophthora]]''
| species = '''''P. ramorum'''''
| binomial = ''Phytophthora ramorum''
| binomial_authority = Werres et al. 2001
}}
[[Image:IMG 0223.JPG|A hillside in Big Sur, California, devastated by Sudden Oak Death|thumb|right|225px]]
'''Sudden Oak Death''' is the common name of a disease caused by the [[oomycete]] [[plant pathogen]] '''''Phytophthora ramorum'''''. The disease kills [[oak]] and other species of tree and has had devastating effects on the oak populations in [[California]] and [[Oregon]] as well as also being present in [[Europe]]. Symptoms include bleeding cankers on the tree’s [[trunk]] and dieback of the [[foliage]], in many cases eventually leading the death of the tree.
''P. ramorum'' also infects a great number of other plant species, significantly [[rhododendron]]s, causing a non-fatal foliage disease known as ramorum dieback. Such plants can act as a source of the [[inoculum]] for the disease, with the pathogen producing [[spore]]s that can be transmitted by [[wind]] and [[rainwater]].
''P. ramorum'' was first reported in [[1995]], and the origins of the pathogen are still unclear but most evidence suggests it was introduced as an exotic species. Very little control mechanisms exist for the disease, and they rely upon early detection and proper disposal of infected plant material.
==Presence==
The disease is known to exist in California's coastal region between [[Big Sur]] (in [[Monterey County, California|Monterey County]]) and southern [[Humboldt County, California|Humboldt County]]. It is confirmed to exist in all coastal counties in this range, as well as in all immediately inland counties from [[Santa Clara County, California|Santa Clara County]] north to [[Lake County, California|Lake County]]. It has not been found east of the [[California Coast Ranges]], however. It was reported in [[Curry County, Oregon]] (just north of the California border) in 2002. [http://kellylab.berkeley.edu/SODmonitoring/maps/PDF/SODCalifornia_conf12-20-06page.pdf] About the same time, a similar disease in continental [[Europe]] and the [[United Kingdom|UK]] was identified as ''Phytophthora ramorum''.
==Hosts and symptoms==
It was first discovered in [[California]] in [[1995]] when large numbers of [[tan oak]]s (''Lithocarpus densiflorus'') died mysteriously, and was described as a new species of ''[[Phytophthora]]'' in [[2000]]. It has subsequently been found in many other areas including [[Great Britain|Britain]], [[Germany]], and some other [[United States|U.S.]] states, either accidentally introduced in nursery stock, or already present undetected.
In tan oaks, the disease may be recognized by [[wilting]] new [[shoot]]s, older leaves becoming pale green, and after a period of two to three weeks, foliage turns brown while clinging to the branches. Dark brown [[sap]] may stain the lower trunk's bark. Bark may split and exude gum, with visible discoloration. After the tree dies, suckers will sprout next year, but their tips soon bend and die. [[Ambrosia beetle]]s (''Monarthrum scutellare'') will most likely infest a dying tree during midsummer, producing piles of fine white dust near tiny holes. Later, [[bark beetle]]s (''Pseudopityophthorus pubipennis'') produce fine red boring dust. Small black domes, the fruiting bodies of the [[Hypoxylon]] fungus, may also be present on the [[bark]]. Leaf death may occur more than a year after the initial infection and months after the tree has been girdled by beetles.
In [[Coast Live Oak]]s and [[Black Oak]]s, the first symptom is a burgundy-red to tar-black thick sap bleeding from the bark surface. These are often referred to as bleeding [[canker]]s.
[[Image:Phytophtora ramorum.png|thumb|Leaf death caused by ''P. ramorum'']]
In addition to oaks, many other forest species may be hosts for the disease, in fact it was observed in the USA that nearly all woody plants in some [[Californian]] [[forest]]s were susceptible to ''P. ramorum''.<ref>Rizzo, D.M., et al. Phytophthora ramorum and Sudden Oak Death in California 1: Host Reletionships. in Fifth Symposium on Oak Woodlands: Oaks in California's Changing Landscapes. 2001. San Diego, California.</ref> including [[rhododendron]], [[Madrone]] (''Arbutus menziesii''), Evergreen [[Huckleberry]] (''Vaccinium ovatum''), [[California Bay Laurel]] (''Umbellularia californica''), [[Buckeye]] (''Aesculus californica''), [[Bigleaf Maple]] (''Acer macrophyllum''), [[Toyon]] (''Heteromeles arbutifolia''), [[manzanita]] (''Arctostaphylos spp.''), [[Coast Redwood]] (''Sequoia sempervirens''), [[Douglas Fir]] (''Pseudotsuga menziesii''), [[Coffeeberry]] (''Rhamnus californica''), [[Honeysuckle]] (''Lonicera hispidula'') and [[Shreve's Oak]] (''Quercus parvula'' v. ''shrevei''). ''P. ramorum'' more commonly causes a less severe disease known as Ramorum dieback/leaf blight on these [[Host (biology)|host]]s. Characteristic symptoms are dark spots on [[foliage]] and in some hosts the dieback of the stems and twigs.<ref>Werres, S., et al., Phytophthora ramorum sp. nov., a new pathogen on Rhododendron and Viburnun. Mycological Research, 2001. 105(10): p. 1155-1165.</ref> The disease is capable of killing some hosts, such as Rhododendron, but most survive. Disease progression on these species is not well documented but hikers have observed dead Douglas Firs with massive quantities of red frass surrounding their base. Redwoods exhibit needle discoloration and cankers on small branches, with purple lesions on sprouts that may lead to sprout mortality.
==Transmission==
''P. ramorum'' produces both resting [[spore]]s (chlamydospores) and [[zoospore]]s, which have [[flagella]] enabling [[swimming]]. ''P. ramorum'' is spread by air;<ref name="transmission">Davidson, J.M., et al. Phytophthora ramorum and Sudden Oak Death in California: II. Transmission and Survival. in Fifth Symposium on Oak Woodlands: Oaks in California's Changing Landscape. 2001. San Diego, California.</ref> one of the major mechanisms of [[dispersal]] is [[rainwater]] splashing spores onto other susceptible plants, and into [[watercourse]]s to be carried for greater distances.<ref name="transmission">Davidson, J.M., et al. Phytophthora ramorum and Sudden Oak Death in California: II. Transmission and Survival. in Fifth Symposium on Oak Woodlands: Oaks in California's Changing Landscape. 2001. San Diego, California.</ref> [[Chlamydospores]] can withstand harsh conditions and are able to overwinter.<ref name="transmission"/> The pathogen will take advantage of wounding, but it is not necessary for [[infection]] to occur.<ref> Anon, Phytophthora ramorum and Phytophthora kernoviae: Key findings from research, DEFRA, Editor. 2005, DEFRA.</ref>
As mentioned above, ''P. ramorum'' does not kill every plant that can be used as a [[host (biology)|host]], and it is these plants that are most important in the [[epidemiology]] of the disease as they act as sources of [[inoculum]].<ref>Garbelotto, M., et al., Non-oak native plants are the main hosts for sudden oak death pathogen in California. Californian Agriculture, 2003. 57(1): p. 18-23.</ref> In the USA [[bay laurel]] seems to be the main source of inoculum in forests. Green waste, such as leaf litter and tree stumps are also capable of supporting ''P. ramorum'' as a [[saprotroph]] and acting as a source of inoculum. Because ''P. ramorum'' is able to infect many [[ornamental plant]]s, it can be transmitted by ornamental plant movement.
[[Hiker]]s, [[mountain bike]]rs, [[Equestrianism|equestrian]]s, and other people engaged in various outdoor activities may also unwittingly move the pathogen into areas where it was not previously present. If you are travelling in an area known to be infested with SOD, you can help prevent the spread of the disease by cleaning your (and your animal's) feet, tires, tools, camping equipment, etc. before returning home or entering another uninfected area, especially if you have been in muddy soil.
==The two mating types==
[[Image:Phytophtora reproduction.png|thumb|Mating Structures]]
''P. ramorum'' is [[heterothallic]] and has two [[mating type]]s identified: A1 and A2. Interestingly A1 is found almost exclusively in [[Europe]] and A2 in [[North America]]. Genetics of the two isolates indicate that they are reproductively [[isolated]].<ref>Ivors, K., et al., AFLP and phylogenetic analyses of North American and European populations of Phytophthora ramorum. Mycological Research, 2004. 108(4): p. 378-392.</ref> On average the A1 mating type is more virulent than the A2 mating type but there is more variation in the pathogenicity of A2 isolates.<ref>Brasier, C., et al. Pathogenicity of Phytophthora ramorum isolates from North America and Europe to bark of European Fagaceae, American Quercus rubra and other forest trees. In Sudden Oak Death Science Symposium. 2002. Marriott Hotel, Montery.</ref> Because of the genetic and pathological differences it is believed that if the two mating types remain reproductively isolated then two [[sub-species]] will be formed.
==Possible origins==
''P. ramorum'' is a relatively new disease, and there have been several debates about where it may have originated or how it evolved.
===Introduction as an exotic species===
[[Evidence]] exists that suggests ''P. ramorum'' may be an introduced [[exotic species]], and that these introductions occurred separately for the European and NA populations, hence why only one mating type exists on each continent – this is called a [[founder effect]].<ref> Brasier, C., Evolutionary Biology of Phytophthora PART I: Genetic System, Sexuality and the Generation of Variation. Annual Review of Phytopathology, 1992. 30: p. 153-171.</ref> The differences between the two populations are thus caused by adaptation to separate climates. Evidence includes little genetic variability, as ''Phytophthora ramorum'' has not had time to diversify since being recently introduced. What variability there is may be explained by multiple introductions with a few individuals adapting best to their respective environments.<ref>Garbelotto, M., et al. ''Phytophthora ramorum'' and Sudden Oak Death in California: III. Preliminary Studies in Pathogen Genetics. in Fifth Symposium on Oak Woodlands: Oaks in California's Changing Landscape. 2001. San Diego, California.</ref> The behavior of the pathogen in California is also indicative of being introduced; it is assumed that such a high mortality rate of trees would have been noticed sooner if ''P. ramorum'' was [[native]].
Where ''Phytophthora ramorum'' did originate remains unclear but most researchers feel [[Asia]] is the most likely, since many of the hosts of ''P. ramorum'' originated there.<ref>Martin, F.N. and P.W. Tooley, Phylogenetic relationships of ''Phytophthora ramorum'', ''P. nemorosa'' and ''P. pseudosyringae'', three species recovered from areas in California with sudden oak death. Mycological Research, 2003. 107(12): p. 1379-1391.</ref> Since certain climates are best suited to ''P. ramorum'', the most likely sources are the Southern [[Himalayas]], [[Tibet]] or [[Yunnan province]].<ref>Nicholls, H., Stopping the Rot. PLoS Biology, 2004. 2(7): p. 891-895.</ref>
===Hybridization events===
Species of ''[[Phytophthora]]'' have been shown to have evolved by the [[Hybrid (biology)#Interspecific hybrids|interspecific hybridization]] of two different species from the [[genus]].<ref>Brasier, C. and E.M. Hansen, Evolutionary Biology of Phytophthora PART II: Phylogeny, Speciation, and Population Structure. Annual Review of Phytopathology, 1992. 30: p. 173-200.</ref> When a species is introduced into a new [[Natural environment|environment]] it causes [[episodic selection]]. The invading species is exposed to other resident [[taxon|taxa]], and hybridization may occur to produce a new species. If these hybrids are successful, they may out compete their parent species.<ref> Brasier, C., Episodic Selection as a force in fungal microevolution, with special reference to clonal speciation and hybrid introgression. Canadian Journal of Botany, 1994. 73 (suppl. 1.): p. S1213-S1220.</ref> Thus it is possible that ''Phytophthora ramorum'' is a hybrid between two species. Its unique [[morphology (biology)|morphology]] does support this. Also, 3 sequences that were studied to establish the phylogeny of ''Phytophthora'': ITS, cox II and nad 5, were identical supporting ''Phytophthora ramorum'' having recently evolved.
===A native organism===
It is possible that ''Phytophthora ramorum'' is native to the USA. Infection rates could have previously been at a low level, but changes in the environment caused a change to the [[population]] structure.<ref>Rizzo, D.M., et al., Phytophthora ramorum as the Cause of Extensie Mortality of Quercus spp. and Luthocarpus densiflorus in California. Plant Disease, 2002. 86(3): p. 205-214.</ref> Alternatively, the symptoms of ''Phytophthora ramorum'' may have been mistaken for that of other pathogens. When SOD first appeared in the USA, many other pathogens and conditions were blamed before ''P. ramorum'' was found to be the causal agent.<ref>Garbelotto, M., P. Svihra, and D.M. Rizzo, Sudden oak death syndrome fells 3 oak species. Californian Agriculture, 2001. 55(1): p. 9-19.</ref> With many of the most seriously affected plants being in the forest, the likelihood of seeing diseased trees is also low.
==Control==
===Early detection===
Early detection of ''P. ramorum'' is essential for its control, as most studies have shown that treatments work best if applied preventively to stop infection.<ref>Rizzo, D.M., M. Garbelotto, and E.M. Hansen, Phytophthora ramorum: Integrative Research and Management of an Emerging Pathogen in California and Oregon Forests. Annual Review of Phytopathology 2005. 43: p. 309-335.</ref> Techniques were developed to detect ''P. ramorum'' even before it becomes symptomatic. One of the most successful techniques uses a real time [[polymerase chain reaction]] [[assay]]. Researchers developed PCR [[Primer (molecular biology)|primer]]s specific to the [[Internal transcribed spacer|ITS]] region of the [[genome]] so that [[DNA]] is only amplified if it belongs to ''P. ramorum''. The method is more sensitive than direct [[Cell culture|culturing]], the classical method of pathogen identification. Because [[fluorescence]] occurs every time the DNA is replicated, it also allows an accurate [[quantification]] of the level of infection.<ref>Hayden, K., et al., Detection and Quantification of Phytophthora ramorum from Californian Forests Using a Real-Time Polymerase Chain Reaction Assay. Phytopathology, 2004. 94(10): p. 1075-1083.</ref> A Pocket Diagnostic on-site test ([[lateral flow device]]) is available that can detect any members of the genus ''[[Phytophthora]]'' and this is useful for field situations. This test is used by plant health inspectors worldwide at the point of inspection. <ref> Lane, C.R. et al Evaluation of a rapid diagnostic field test kit for identification of Phytophthora species, including P. ramorum and P. kernoviae at the point of inspection. Plant Pathology 2007 </ref> Researchers are investigating [[microsatellite]] markers in the pathogens genome, that can be used to detect the pathogen and establish the mating type.<ref> Prospero, S., J.A. Black, and L.M. Winton, Isolation and charcaterisation of microsatellite markers in Phytophthora ramorum, the causal agent of sudden oak death. Molecular Ecology Notes, 2004. 4: p. 672-674.</ref>
Researchers have built computer models that [[predict]] areas at high risk of epidemic ''P. ramorum'' based on the preferred [[climate]] and hosts of the pathogen. The models predict infections before they emerge, and areas at high risk can be closely monitored and given preventative treatment.<ref> Meentemeyer, R. Predicting the Risk of Establishment of Phytophthora ramorum Across California. in Spring 2004 Meeting of the California Oak Mortality Task Force. 2004. Sonoma State University, Rohnert Park.</ref>
===Chemical control===
Several compounds have been identified that are effective against ''P. ramorum.'' The most successful compound to be tested was [[metalaxyl]], but at least one [[Drug resistance|resistant]] strain of ''P. ramorum'' has emerged.<ref name="chemical control">Garbelotto, M., A report on a comprehensive series of experiments, both in vitro and in planta, to develop treatments for ''P. ramorum'', the cause of sudden oak death, Department of Environmental Science, Policy and Management. Ecosystem Sciences Division, University of California, Berkeley. p. 1 - 20.</ref> Because of this it is recommended metalaxyl is not used. One of the most effective treatments involves injecting [[phosphonate]] into the [[trunk]] of the plant. This works best preventively, but also is able to reduce canker size and prolong the life of the tree if applied soon after the symptoms develop. Injection or application directly to the bark when coupled with an [[organosilicate]] is the only known viable way to deliver the [[fungicide]].<ref name="chemical control">Garbelotto, M., A report on a comprehensive series of experiments, both in vitro and in planta, to develop treatments for ''P. ramorum'', the cause of sudden oak death, Department of Environmental Science, Policy and Management. Ecosystem Sciences Division, University of California, Berkeley. p. 1 - 20.</ref> [[Copper]] compounds may also be used for a preventative coating of the trunk.<ref> Garbelotto, M., D.M. Rizzo, and L. Marais. Phytophthora ramorum and Sudden Oak Death in California: IV. Preliminary Studies on Chemical Control. in Fifth Symposium on Oak Woodlands: Oaks in California's Changing Landscape. 2001. San Diego, California.</ref>
===Disposal of infected plant material===
In general, when plants are found to be infected with ''P. ramorum'' they are cut and burned, along with any surrounding host plants regardless of infection status.<ref> Kanaskie, A., et al. An Update on Sudden Oak Death in Oregon. in Spring 2004 Meeting of the Californian Oak Mortality Task Force. 2004. Sonoma State University, Rohnert Park.</ref> This technique has its problems, as the pathogen can survive in the tree stumps, soil, and water as well as re-infecting any sprouts. One solution involves spraying stumps with [[herbicide]]s to prevent such sprouting. Another way to deal with infected plant material, more specifically [[green waste]], utilizes [[composting]]. Because ''P. ramorum'' is a cool climate pathogen, the high heat involved in the incubation of compost kills the pathogen after two weeks, including the resting spores.<ref> Garbelotto, M., Composting as a Control for Sudden Oak Death Disease. BioCycle -Journal of Composting and Organics Recycling, 2003.</ref>
==External links==
* [http://www.suddenoakdeath.org/ http://www.suddenoakdeath.org]
* [http://www.newscientist.com/news/news.jsp?id=ns99996257 New Scientist news]
* [http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7498.html University of California SOD pest notes]
* [http://californiaagriculture.ucop.edu/0301JFM/pdfs/SOD.pdf California Agriculture]
* [http://www.defra.gov.uk/planth/pestnote/newram.pdf UK Department of Environment, Food and Rural Affairs - notes on SOD]
* [http://www.puyallup.wsu.edu/ppo/resources.html Washington State University SOD Resources]
* [http://nature.berkeley.edu/comtf/html/sanitation___reducing_spread.htm Reducing spread of SOD]
* [http://www.pocketdiagnostic.com/products/ornament/Trees.html Pocket Diagnostic Test Kits]
==References==
<references/>
[[Category:Plant pathogens and diseases]]
[[Category:Tree diseases]]
[[Category:Water moulds]]
[[es:Phytophthora ramorum]]