Invasive Species Compendium

Detailed coverage of invasive species threatening livelihoods and the environment worldwide


Phytophthora lateralis
(Port-Orford-cedar root disease)



Phytophthora lateralis (Port-Orford-cedar root disease)


  • Last modified
  • 06 November 2018
  • Datasheet Type(s)
  • Invasive Species
  • Pest
  • Preferred Scientific Name
  • Phytophthora lateralis
  • Preferred Common Name
  • Port-Orford-cedar root disease
  • Taxonomic Tree
  • Domain: Eukaryota
  •   Kingdom: Chromista
  •     Phylum: Oomycota
  •       Class: Oomycetes
  •         Order: Peronosporales
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Preferred Scientific Name

  • Phytophthora lateralis Tucker & Milbrath 1942

Preferred Common Name

  • Port-Orford-cedar root disease

International Common Names

  • English: Lawson's cypress root disease

Local Common Names

  • Germany: Wurzelfaeule: Scheinzypresse

EPPO code

  • PHYTLA (Phytophthora lateralis)

Taxonomic Tree

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  • Domain: Eukaryota
  •     Kingdom: Chromista
  •         Phylum: Oomycota
  •             Class: Oomycetes
  •                 Order: Peronosporales
  •                     Family: Peronosporaceae
  •                         Genus: Phytophthora
  •                             Species: Phytophthora lateralis

Notes on Taxonomy and Nomenclature

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P. lateralis is a rather uniform and distinctive species and its taxonomy has not been complicated by name changes. Unrelated species have been misidentified as P. lateralis, however, especially in work completed before molecular diagnostics were available (Hansen et al., 1999; 2000).


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In culture, P. lateralis is rather nondescript, distinguished primarily by its relatively slow growth and formation of characteristic lateral (hence the name lateralis) chlamydospores on the hyphae. Although it was classified in morphological group V by Stamps et al. (1990), because it was reported to be homothallic with oogonia and paragynous antheridia as well as nonpapillate proliferating sporangia, oogonia are seldom seen and sporangia are seen to form only in water.

Sporangia (average 26–60 µm by 12–20 µm) are ovoid, obovoid or obpyriform. They are non-papillate on simple sympodial sporangiophores with internal proliferation. Sporangia may be persistant or caducous, depending on the isolate and cultural conditions, which are poorly understood. Hyphal swellings are absent but chlamydospores (20–77 µm, average 40 µm) are formed in broth or agar. They are usually sub-globose to globose, terminal or intercalary, and often positioned laterally or are sessile on hyphae (Tucker and Milbrath, 1942; Hansen, 2011).


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The known distribution of P. lateralis outside of Taiwan is confined to the native range of the Port Orford cedar (Chamaecyparis lawsoniana) and places where this tree is grown horticulturally. Scattered reports from other areas or unrelated hosts have proven to be misidentifications, or cannot be confirmed. Its distribution in Taiwan is still very incompletely known (Brasier et al., 2010; Webber et al., 2012), and other Asian countries where species of Chamaecyparis grow have not been explored.

Distribution Table

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The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.

Continent/Country/RegionDistributionLast ReportedOriginFirst ReportedInvasiveReferenceNotes


TaiwanLocalisedNative Not invasive Brasier et al., 2010; EPPO, 2011; CABI/EPPO, 2012; Webber et al., 2012; EPPO, 2014

North America

CanadaRestricted distributionCABI/EPPO, 2012; EPPO, 2014
-British ColumbiaLocalisedIntroduced Invasive Atkinson, 1965; CABI/EPPO, 2012; EPPO, 2014
USARestricted distributionCABI/EPPO, 2012; EPPO, 2014
-CaliforniaLocalisedIntroduced Invasive Kliejunas and Adams, 1981; CABI/EPPO, 2012; EPPO, 2014According to Hansen et al. (2000), northern coastal California (adjacent to Oregon)
-FloridaPresentCABI/EPPO, 2012
-OhioAbsent, invalid recordCABI/EPPO, 2012
-OregonLocalisedIntroduced Invasive Tucker and Milbrath, 1942; CABI/EPPO, 2012; EPPO, 2014According to Hansen et al. (2000), southwestern coastal Oregon
-PennsylvaniaAbsent, invalid recordCABI/EPPO, 2012
-VirginiaAbsent, unreliable recordCABI/EPPO, 2012
-WashingtonLocalisedIntroduced Invasive Tucker and Milbrath, 1942; CABI/EPPO, 2012; EPPO, 2014


FranceLocalisedIntroduced Invasive Hansen, 1999; EPPO, 2011; CABI/EPPO, 2012; EPPO, 2014Bretagne region (Finistère) on Chamaecyparis lawsoniana
-France (mainland)Restricted distributionCABI/EPPO, 2012
IrelandPresent, few occurrencesEPPO, 2011; CABI/EPPO, 2012; EPPO, 2014
NetherlandsPresent, few occurrencesIntroduced Invasive NPPO of the Netherlands, 2013; IPPC, 2006; Meffert, 2007; EPPO, 2011; CABI/EPPO, 2012; EPPO, 2014; IPPC, 2014Present, at low prevalence.
UKLocalisedIntroduced Invasive IPPC, 2011a; IPPC, 2011b; EPPO, 2011; CABI/EPPO, 2012; Forestry Comission, 2013; Green et al., 2013; EPPO, 2014; Schlenzig et al., 2014England, Scotland and Northern Ireland
-England and WalesPresent, few occurrencesCABI/EPPO, 2012; EPPO, 2014
-Northern IrelandPresent, few occurrencesCABI/EPPO, 2012; EPPO, 2014
-ScotlandPresentSchlenzig et al., 2011; CABI/EPPO, 2012; EPPO, 2014


New ZealandAbsent, invalid recordCABI/EPPO, 2012

History of Introduction and Spread

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The first reports of Port Orford cedea (POC) root disease, caused by P. lateralis, are documented in pathology records from western Washington State, USA, from the 1920s. Horticultural nurseries growing ornamental Port Orford cedar (Chamaecyparis lawsoniana) cultivars in Washington, Oregon and British Columbia reported rapidly increasing losses to root rot (Torgeson et al., 1954; Zobel et al., 1985). The first reports of P. lateralis's introduction into the native range of C. lawsoniana in southwest Oregon are from the 1950s (Roth et al., 1957). The pathogen was reported in California in 1981 (Kleijunas and Adams, 1981), and is now present in all parts of the POC range in these two states (USDA, 2003).

The first reports from Europe, in the 1990s, were from horticultural nurseries in France (Hansen et al., 1999) and the Netherlands (Meffert, 2007). P. lateralis was first diagnosed in landscape plantings in France in 2009 (Robin et al., 2011) and in Britain in 2010 (Green et al., 2013), although it was evident that initial infections predated these diagnoses by several years.


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Introduced toIntroduced fromYearReasonIntroduced byEstablished in wild throughReferencesNotes
Natural reproductionContinuous restocking
British Columbia Horticulture (pathway cause) Yes Present where host grown horticulturally
France Horticulture (pathway cause) Yes Present where host grown horticulturally
UK 1990s Horticulture (pathway cause) Yes Green et al. (2013) Present where host grown horticulturally
USA 1920s Horticulture (pathway cause) Yes Zobel et al. (1985) Present where host grows, either horticulturally or in native forest

Risk of Introduction

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The risk of introduction must be considered high wherever its main host Port Orford cedar (Chamaecyparis lawsoniana) is grown. There is a chance of soil or water movement from infested areas. In Europe especially, transport of nursery stock is the most likely pathway of spread (Meffert 2007Brasier et al., 2012; Green et al., 2013).


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The known distribution of P. lateralis outside of Taiwan is confined to the native range of the Port Orford cedar (Chamaecyparis lawsoniana) and places where this tree is grown horticulturally. In forests, Port Orford cedars (POC) killed by P. lateralis are most often found next to roads or infested streams that are downstream from previously established infections. In Europe especially, trees in wet and windy environments may be infected through foliage by inoculum carried in wind driven rain (Robin et al., 2011; Green et al., 2013).

In Taiwan, the pathogen has been recovered in old-growth forest areas from wet soil in drainage ways and from foliage of young trees of Chamaecyparis obtusa (Brasier et al., 2010; Webber et al., 2012).

Habitat List

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Terrestrial – ManagedManaged forests, plantations and orchards Principal habitat Harmful (pest or invasive)
Urban / peri-urban areas Principal habitat Harmful (pest or invasive)
Terrestrial ‑ Natural / Semi-naturalNatural forests Principal habitat Harmful (pest or invasive)
Riverbanks Principal habitat Harmful (pest or invasive)
Lakes Principal habitat Harmful (pest or invasive)
Rivers / streams Principal habitat Harmful (pest or invasive)

Hosts/Species Affected

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Although yews (DeNitto and Kleijunis, 1991; Murray and Hansen, 1997) and other cedar species (Green et al., 2013) are sometimes killed by P. lateralis, such deaths are usually associated with especially high inoculum levels around dead and dying C. lawsoniana.

Growth Stages

Top of page Flowering stage, Seedling stage, Vegetative growing stage


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The crown colour of trees dying from root infection changes uniformly from healthy green to red and brown in one or two years (Trione, 1959). P. lateralis colonizes and kills the inner bark (phloem) tissues of roots and stems of infected trees. Healthy inner bark of C. lawsoniana is white whereas necrotic tissues are red-brown, and there is usually a distinct demarcation between healthy and diseased tissues (Betlejewski et al., 2011; Hansen, 2011). Outer bark is red-brown on both healthy and diseased trees, and wood remains white regardless of infection. On some seedlings and saplings healthy outer bark may still be green, and diseased outer bark may turn red-brown. Necrotic inner bark is not a specific symptom for P. lateralis, however, and P. cinnamomi and P. cambivora cause similar symptoms in Port Orford cedar (C. lawsoniana).

List of Symptoms/Signs

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SignLife StagesType
Leaves / abnormal colours
Leaves / necrotic areas
Leaves / yellowed or dead
Roots / cortex with lesions
Roots / reduced root system
Stems / discoloration of bark
Stems / necrosis
Whole plant / discoloration
Whole plant / early senescence
Whole plant / plant dead; dieback
Whole plant / seedling blight
Whole plant / wilt

Biology and Ecology

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P. lateralis is in phylogenetic clade 8, defined by ITS DNA sequence (Cooke et al., 2000). The most closely related species is P. ramorum, the sudden oak death pathogen (Ivors et al., 2004). Other related species include P. hibernalis and P. foliorum; these species sometimes cross-react in molecular diagnostic tests designed for P. lateralis (Winton and Hansen, 2001).

In much of Europe and in western North America, P. lateralis populations are very uniform in appearance and genetics, as expected of a clonally spread pathogen that is inbreeding or functionally asexual (McWilliams, 1999; 2000). In Taiwan and one small area of Britain, however, morphologically and genetically distinctive populations are established (Brasier et al., 2012). Isolates from Taiwan are especially variable, as might be expected of an ancestral, endemic population. In total, four distinct lineages have been described, including two from Taiwan, the invasive population found in North America and Europe, and the small UK population.

Reproductive Biology

P. lateralis reproduces primarily through asexual zoospores and chlamydospores (Trione, 1974; Englander and Roth, 1980). Zoospores are released from sporangia, sack-like hyphal appendages that hold about 20 zoospores each. Sporangia form in water or in saturated soil, on hyphae that grow from colonies in tree roots (or in agar).

Zoospores are encased only in a membrane and are vulnerable to drying. In water they are motile with flagellae. They are chemotactic, following chemical gradients or root exudates. They remain active for about 24 hours. Zoospores encyst on root surfaces or after agitation, such as in a fast moving stream, and germinate with hyphae that may penetrate a host root and establish a new infection (Oh and Hansen, 2007).

Chlamydospores are thick-walled asexual resting spores that form on hyphae in roots, foliage or in culture. They allow the pathogen to survive drying conditions, germinating with hyphae and sporangia when cool moist conditions resume (Trione, 1959).

Oospores are sexual spores formed from the fusion of antheridia and oogonia. They are thick-walled and presumably function as resting spores in roots and foliage, as well as providing the population with genetic variation. They are very seldom seen in culture, however.


P. lateralis survives for up to 10 years in infected root fragments in the soil (Hansen and Hamm, 1996).

Population Size and Structure

Populations of propagules free in the soil are small (Tsao et al., 1995).

Environmental Requirements

P. lateralis dies quickly from surface horizons exposed to sun as it cannot survive warm dry conditions (Hansen and Hamm, 1996). P. lateralis is active during cool, wet times of the year (Trione, 1959).


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C - Temperate/Mesothermal climate Preferred Average temp. of coldest month > 0°C and < 18°C, mean warmest month > 10°C
Cf - Warm temperate climate, wet all year Preferred Warm average temp. > 10°C, Cold average temp. > 0°C, wet all year
Cs - Warm temperate climate with dry summer Preferred Warm average temp. > 10°C, Cold average temp. > 0°C, dry summers

Means of Movement and Dispersal

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P. lateralis moves as zoospores and perhaps sporangia in water, and is transported via root fragments in soil, especially mud. In forests in western USA, it moves upslope in mud carried on vehicles, logging equipment, road maintenance machinery and in mud attached to animals. Inoculum is carried in runoff water along roadside ditches and into streams at road crossings (Hansen et al., 2000; Jules et al., 2002).

Transport in the horticultural landscape is similar in both mud and water. Potted plants transplanted to new sites can transmit the pathogen, and both hosts and non-hosts growing in infested soil can carry it. Infested nursery stock is evidently an important contributor to the epidemic in Europe (Brasier et al., 2012). Infested bark mulch has also been implicated. After initial introduction of the pathogen it spreads tree-to-tree where root systems overlap (Gordon and Roth, 1976).

P. lateralis can also spread aerially under some conditions. In Europe especially (Green et al., 2013), and apparently in Taiwan (Webber et al., 2012), but only rarely in the USA (Trione and Roth, 1957; Trione, 1959), foliage is infected by wind-blown sporangia or zoospores. Initial infection arises via splash dispersal of propagules from soil to low-lying branches. Subsequent aerial spread is from sporangia and zoospores which have formed on the foliage (Trione, 1974) in cool wet conditions. New infections can spread into branches and the main stem, killing trees from the top down.

Plant Trade

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Plant parts liable to carry the pest in trade/transportPest stagesBorne internallyBorne externallyVisibility of pest or symptoms
Bark hyphae Yes Pest or symptoms usually invisible
Growing medium accompanying plants hyphae; spores Yes Pest or symptoms usually invisible
Roots hyphae Yes Pest or symptoms usually visible to the naked eye
Seedlings/Micropropagated plants hyphae Yes Pest or symptoms usually visible to the naked eye
Stems (above ground)/Shoots/Trunks/Branches hyphae Yes Pest or symptoms usually visible to the naked eye

Impact Summary

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Cultural/amenity Negative
Economic/livelihood Negative
Environment (generally) Negative

Economic Impact

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In Oregon and California, where Port Orford cedar (Chamaecyparis lawsoniana) grows as a forest tree, the economic impact of the introduction of P. lateralis has been great. Port Orford cedar (POC) was the most valuable tree in western forests. Its wood, especially in large old trees, is fine and straight grained, white and decay resistant. It was milled domestically for specialty products that utilized the fine grain and decay resistance, but its main value was for export to Asia where the wood is used as a substitute for C. obtusa and other Asian Chamaecyparis species that are in short supply. It was used in temple reconstructions and interior screens and other domestic construction (Zobel et al., 1985). Because of the disease, large POC trees are now in very limited supply for the export market. Most trees came from US National Forests and their export is now prohibited (USDA, 2003).

POC was also a very valuable tree in the horticultural trade in woody ornamental plants. Many cultivars were named and vegetatively propagated, taking advantage of the tree’s variation in form and colour (Zobel et al., 1985). Following the introduction and spread of P. lateralis within the trade in Oregon and Washington, most nurseries switched production to substitute species, such as other cedars and junipers.

Environmental Impact

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In the forests of Oregon and California where Port Orford cedar (POC) grows, the progressive loss of the tree has had significant negative environmental impacts. In much of its range, POC grows as scattered individuals in a mixed species conifer forest. The death of cedars reduces diversity and allows resistant species, most often Douglas-fir (Pseudotsuga menziesii) to quickly expand their crowns and replace the cedars in the forest canopy (Hansen, 1999).

Along waterways and on ultramafic soil, however, impacts are more severe (Hansen et al., 2000). POC grows most abundantly close to streams and in other locations where water is readily available. Death of trees adjacent to streams reduces shading, leading to increased water temperatures which are deleterious to salmon and aquatic invertebrates. Bank stability is reduced, increasing landslides into streams with consequent silting. Within its limited range POC is an important component of the ‘coarse woody debris’ input to streams, important because its large size and decay resistance make it an important structural component of the stream channel. Although disease may lead to a short term increase in logs in the stream, in the long term the loss of these inputs may limit stream productivity.

On ultramafic soils, the environmental impacts are compounded. These soils contain high heavy metal contents, including serpentine, and support a distinctive flora with a high level of endemic species. POC is one of only a few conifers that grow on these relatively toxic soils. Loss of cedar on serpentine landscapes may therefore amount to loss of the total tree cover, which may in turn may reduce the entire shading overstory above streams (Kleijunas, 1994).

Social Impact

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Port Orford cedar is valued in the landscape as a large shapely tree of varied form and colour. It can also be pruned into dense hedges. In some areas such as Brittany it has been widely planted as a windbreak. These amenity values are quickly lost when the disease is introduced. Direct costs include tree removal and replacement.

Risk and Impact Factors

Top of page Invasiveness
  • Proved invasive outside its native range
  • Highly mobile locally
  • Long lived
  • Has high reproductive potential
  • Has propagules that can remain viable for more than one year
  • Reproduces asexually
Impact outcomes
  • Altered trophic level
  • Changed gene pool/ selective loss of genotypes
  • Damaged ecosystem services
  • Ecosystem change/ habitat alteration
  • Host damage
  • Modification of hydrology
  • Negatively impacts cultural/traditional practices
  • Negatively impacts forestry
  • Negatively impacts livelihoods
  • Reduced amenity values
  • Reduced native biodiversity
  • Threat to/ loss of native species
Impact mechanisms
  • Pathogenic
Likelihood of entry/control
  • Highly likely to be transported internationally accidentally
  • Difficult/costly to control


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Specific diagnosis requires either culture of the pathogen and examination of its distinctive growth and morphology, or the use of molecular tools. It is generally impossible to recover the pathogen by soil dilution, although concentrating the organic fraction, including cedar roots (Ostrofsky et al., 1977; Hamm and Hansen, 1984), and baiting (Trione, 1959) improves recovery. Diagnosis is more successful when diseased tissue is taken from immediately behind a distinct margin between healthy white and recently infected red-brown inner bark tissue. Culturing is usually successful with the use of Phytophthora-selective agar media and careful selection of the diseased tissues to be plated. P. lateralis grows slowly and distinctive hyphal features may not be visible for seven days. Reference cultures at a similar growth stage are useful for identification.

Colony morphology on V8 agar is nearly patternless and appressed, with no aerial hyphae. In combination with the lower maximum temperature for growth (18-20°C), this feature is especially useful in distinguishing P. lateralis from taxon ‘Pg chlamydo’, which also forms lateral chlamydospores. On cornmeal agar hyphae are usually wavy and irregularly branched and growth is about 1.3 mm/day at 20°C (Hansen, 2011).

Molecular identification can be made on mycelium from agar cultures, or directly from infected inner bark pieces. ELISA (enzyme linked immuno sorbent assay) tests produce a colour reaction based on antibody binding with cell proteins. Commercially available kits detect Phytophthora but do not distinguish among the species. ELISA tests are most useful as a first screen to focus more time consuming tests on the samples most likely to be positive.

Several DNA-based diagnostic procedures have been described. Most will work from either cultures or infected tissues. They require chemical extraction of the DNA, followed by some means of distinguishing P. lateralis DNA among the mixed DNA from host tissues and other microorganisms. The most widely used protocol uses PCR (polymerase chain reaction) to specifically amplify P. lateralis DNA and gel electrophoresis to separately visualize pathogen and host DNA (Winton and Hansen, 2001). All molecular tests should include positive and negative control samples. There is danger of cross-reaction with closely related Phytophthora species such as P. ramorum, but this risk is minimized by careful sample selection from symptomatic tissues.

Detection and Inspection

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P. lateralis is most readily detected through the distinctive symptoms that it induces on its principal hosts. Dead or dying trees are the most evident symptoms, but a number of other agents can cause mortality. Crown colour of trees dying from root infection changes uniformly from healthy green to red and brown across one or two years (Trione, 1959). Diagnosis is easiest on trees exhibiting the early stages of crown discoloration. P. lateralis colonizes and kills the inner bark (phloem) tissues of roots and stems of infected trees. Healthy inner bark of C. lawsoniana is white. Necrotic tissues are red-brown, and there is usually a distinct demarcation between healthy and diseased tissues (Betlejewski et al., 2011; Hansen, 2011). Once trees have been dead for several months, or after secondary bark beetles have attacked, all inner bark tissues, colonized or not, turn tan or brown and margins between healthy and diseased tissues are much less evident. Care must be taken, when diagnosing based on the necrotic lesion, to expose the inner bark without cutting into the wood (xylem) itself. Outer bark is red-brown on both healthy and diseased trees, and wood remains white regardless of infection. On some seedlings and saplings healthy outer bark may still be green, and diseased outer bark may turn red-brown, allowing identification of the necrotic margin without scrapping away the outer bark. Necrotic inner bark is not a specific symptom for P. lateralis, however. P. cinnamomi and P. cambivora cause similar symptoms in Port Orford cedar (C. lawsoniana).

In areas where P. lateralis is spreading aerially, trees may be killed one branch at a time, or from the top down. Distinctive margins between healthy and diseased phloem will still be evident when the outer bark is scraped away. Necrotic areas are continuous with dead foliage on branches and do not extend upwards from the roots. Such branch infections can be confused with cankers caused by Seiridium or other pathogens. Usually Seiridium causes bark distortion in the cankered area, whereas P. lateralis does not. Other foliar pathogens of C. lawsoniana may also be confused with P. lateralis in the early stages of aerial infection, before the Phytophthora has grown into the stems. Foliar pathogens such as Stigmina usually kill scattered individual leaflets on a foliar branch, in contrast to the more uniform foliar necrosis caused by P. lateralis.

Similarities to Other Species/Conditions

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The most closely related species is P. ramorum, the sudden oak death pathogen (Ivors et al., 2004). Other related species include P. hibernalis and P. foliorum.

Prevention and Control

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Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.


As with most invasive pathogens, the key to disease control is to prevent establishment in the first place (Greenup, 1998; Hansen, 2008). For soil/root borne Phytophthoras, this means identifying sources of inoculum and blocking the transport of potentially infested materials from those sources. With P. lateralis in landscape situations, this requires inspection and regulation of nursery stock before sale and transport. This tree is readily propagated locally from vegetative material or seeds and so there is no need for its international trade, which risks further introductions of P. lateralis.

In regions where the disease is already locally established, efforts must be concentrated on containment of the pathogen, blocking transport, via soil or on contaminated equipment, to uninfested areas. Early detection of new infestations is important for containment success. Once established, there is little chance for eradication; eradication efforts also risk aggravating the situation by moving infested soil. With time in host-free conditions (about ten years), the pathogen will die out (Hansen and Hamm, 1996).

In vulnerable areas of forests in western United States where P. lateralis is already broadly established, management of Port Orford cedar (POC) root disease is focussed on stopping further spread and providing special protection for the remaining uninfested stands of POC. This has led to permanent or seasonal closure of many roads which might provide pathways for vehicular transport of the pathogen, as well as systematic efforts to reduce the population of P. lateralis in infested areas as a means to lower the probability of its further transport (USDA, 2004). Young cedars which have naturally regenerated next to roads are at great risk. Their systematic removal, locally termed ‘sanitation’, reduces the risk of further disease increase should the pathogen be introduced to the area (Goheen et al., 2012). P. lateralis is a poor saprophytic competitor and dies quickly in the surface soil in the absence of host roots.

In addition to road closures and roadside sanitation, equipment washing requirements are invoked as appropriate to protect healthy areas. High pressure hoses effectively remove mud from vehicles. Water alone is sufficient so long as soil is removed; steam cleaning or the use of biocides is not necessary. Care must be taken to insure that contaminated wash water flows off the road and back into already diseased areas or into host-free areas (Goheen et al., 2012).

Physical Control

When disease is established in an area, the challenge is then to protect nearby cedars from further spread of the pathogen. The main strategy is to identify trees that can be saved and prevent the transport of P. lateralis to them via either soil or water. Trees growing with roots intertwined with an infected tree, or growing downslope along a waterway, may already be infected and probably cannot be saved. Cutting the most vulnerable cedars to create a gap between infected trees and those to be saved may be effective if care is taken to prevent soil disturbance. P. lateralis is a poor saprophytic competitor and will not spread through a dead root system. Mulching cut trees and using the chips in landscaping risks spreading POC root disease; unless the chips are properly composted at elevated temperatures, P. lateralis will survive in contaminated mulch and spread.

In windy areas where wind spread and foliar infection by P. lateralis are evident, the challenge is still greater. In some situations it may be possible to cut trees with foliar infection, removing the above ground source of aerial inoculum. Pruning low hanging branches may prevent initial foliar infection from soil splash.

Chemical Control

Fungicides with specific action against oomycetes may help to protect especially valuable trees (Pscheidt and Ocamb, 2012). There is limited experience with chemical control of P. lateralis, but it can be effective if trees are treated before infection, or at least before symptoms are visible. Once the crown starts to fade it is not possible to save the tree. Application by bole injection is generally more effective than soil drench.

Biological Control

Biological control using a soil bacterium has been reported to be effective in keeping POC alive in limited tests (Utkhede et al., 1997). It has apparently not achieved wide use.

Host Resistance

A few resistant C. lawsoniana trees from throughout the tree’s natural range have been identified and propagated in Oregon (Oh et al., 2006; Hansen et al., 1989; Hansen et al., 2012). A resistance selection and breeding program is maintained by the USDA Forest Service, with seed orchards producing seed with improved resistance suitable for most areas where cedar grows wild (Sniezko et al., 2012). In another strategy exploiting resistance, scions from susceptible horticultural cultivars were grafted onto resistant rootstock (Hunt and O’Reilly, 1984).


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Forest Phytophthoras of the World
USDA Forest Service, Dorena Genetic Resource Center
USDA Forest Service, SW Oregon Forest Health Protection


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16/01/13 Original text by:

EM Hansen, consultant, USA

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