Invasive Species Compendium

Detailed coverage of invasive species threatening livelihoods and the environment worldwide

Datasheet

Phytophthora ramorum
(Sudden Oak Death (SOD))

Garbelotto M and Frankel S J, 2020. Phytophthora ramorum (Sudden Oak Death (SOD)). Invasive Species Compendium. Wallingford, UK: CABI. DOI:10.1079/ISC.40991.20210200692

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Datasheet

Phytophthora ramorum (Sudden Oak Death (SOD))

Summary

  • Last modified
  • 16 November 2021
  • Datasheet Type(s)
  • Invasive Species
  • Pest
  • Natural Enemy
  • Preferred Scientific Name
  • Phytophthora ramorum
  • Preferred Common Name
  • Sudden Oak Death (SOD)
  • Taxonomic Tree
  • Domain: Eukaryota
  •   Kingdom: Chromista
  •     Phylum: Oomycota
  •       Class: Oomycetes
  •         Order: Peronosporales
  • Summary of Invasiveness
  • Phytophthora ramorum is considered an invasive species due to its ability to spread, persist, and reproduce in new environments. Its rapid life-cycle, propensity to reproduce asexually and splash dispersal via windblown rain, plus its abi...

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Pictures

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PictureTitleCaptionCopyright
Phytophthora ramorum (sudden oak death (SOD)); aerial stem canker on a tan-oak (Notholithocarpus densiflorus).
TitleSymptoms
CaptionPhytophthora ramorum (sudden oak death (SOD)); aerial stem canker on a tan-oak (Notholithocarpus densiflorus).
Copyright©Matteo Garbelotto/U.C. Berkeley, USA
Phytophthora ramorum (sudden oak death (SOD)); aerial stem canker on a tan-oak (Notholithocarpus densiflorus).
Symptoms Phytophthora ramorum (sudden oak death (SOD)); aerial stem canker on a tan-oak (Notholithocarpus densiflorus).©Matteo Garbelotto/U.C. Berkeley, USA
Phytophthora ramorum (sudden oak death (SOD)); bark seeping on an infected coast live oak (Quercus agrifolia).
TitleSymptoms
CaptionPhytophthora ramorum (sudden oak death (SOD)); bark seeping on an infected coast live oak (Quercus agrifolia).
Copyright©Matteo Garbelotto/U.C. Berkeley, USA
Phytophthora ramorum (sudden oak death (SOD)); bark seeping on an infected coast live oak (Quercus agrifolia).
SymptomsPhytophthora ramorum (sudden oak death (SOD)); bark seeping on an infected coast live oak (Quercus agrifolia).©Matteo Garbelotto/U.C. Berkeley, USA
Phytophthora ramorum (sudden oak death (SOD)); lesions on infected leaves of Umbellularia californica (Oregon myrtle, California bay laurel).
TitleSymptoms
CaptionPhytophthora ramorum (sudden oak death (SOD)); lesions on infected leaves of Umbellularia californica (Oregon myrtle, California bay laurel).
Copyright©Matteo Garbelotto/U.C. Berkeley, USA
Phytophthora ramorum (sudden oak death (SOD)); lesions on infected leaves of Umbellularia californica (Oregon myrtle, California bay laurel).
SymptomsPhytophthora ramorum (sudden oak death (SOD)); lesions on infected leaves of Umbellularia californica (Oregon myrtle, California bay laurel).©Matteo Garbelotto/U.C. Berkeley, USA
Phytophthora ramorum (sudden oak death (SOD)); sporangia  releasing zoospores.
TitleSporangia
CaptionPhytophthora ramorum (sudden oak death (SOD)); sporangia releasing zoospores.
Copyright©David Rizzo/U.C. Davis, USA
Phytophthora ramorum (sudden oak death (SOD)); sporangia  releasing zoospores.
SporangiaPhytophthora ramorum (sudden oak death (SOD)); sporangia releasing zoospores.©David Rizzo/U.C. Davis, USA
Phytophthora ramorum (sudden oak death (SOD)); chlamydospores..
TitleChlamydospores
CaptionPhytophthora ramorum (sudden oak death (SOD)); chlamydospores..
Copyright©David Rizzo/U.C. Davis, USA
Phytophthora ramorum (sudden oak death (SOD)); chlamydospores..
ChlamydosporesPhytophthora ramorum (sudden oak death (SOD)); chlamydospores..©David Rizzo/U.C. Davis, USA

Identity

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Preferred Scientific Name

  • Phytophthora ramorum Werres, De Cock & Man in't Veld

Preferred Common Name

  • Sudden Oak Death (SOD)

International Common Names

  • English: ramorum blight
  • Spanish: muerte repentina de los robles

English acronym

  • SOD

EPPO code

  • PHYTRA

Summary of Invasiveness

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Phytophthora ramorum is considered an invasive species due to its ability to spread, persist, and reproduce in new environments. Its rapid life-cycle, propensity to reproduce asexually and splash dispersal via windblown rain, plus its ability to survive through harsh climatic conditions, are elements favouring this species’ potential invasiveness. Spread potential in forests has been elucidated by several studies in California and Oregon employing population genetics approaches. Results have consistently shown that scale of spread of naturalized endemic pathogen populations in natural ecosystems is limited to a few hundred metres and, occasionally, during extremely wet years, spread may reach a few (3-5) km (Mascheretti et al., 2008; Mascheretti et al., 2009; Eyre et al., 2013). Spread events at scales larger than those reported above appear to be associated either with the movement of infected plant parts, normally from large wild infestations, or with the introduction of infected plants, normally from infested ornamental nursery plant stock (Croucher et al., 2013). Spread scales from the hundreds of metres to the few kilometres apply to pathways that involve only foliar hosts, in particular California bay laurels and tanoaks, and are clearly positively correlated with rainfall (Eyre et al., 2013). However, spread from foliar hosts such as California bay laurels, tanoaks and ornamental rhododendrons to stem hosts such as oaks and tanoaks occur at the much lower scale of 10 to 20 metres and are strongly associated with the occurrence of episodic and above average rainy years (Cobb et al., 2012; Garbelotto et al., 2017). Given the limited spatial scale of dispersal of P. ramorum, its spread is strongly driven by structure and composition of individual forest stands and is projected to increase as the density of infectious foliar hosts increases (Cobb et al., 2010; Meentemeyer et al., 2015). Monocultures of Japanese larch in the UK, stands with high proportion of tanoaks in Oregon and California, and oak woodlands with an abundance of California bay laurels have all been the hardest hit systems. Presence of contiguous forests (Condeso and Meentemeyer, 2007), genetics of host populations (Dodd et al., 2005; Hayden et al., 2011), microclimate (Anacker et al., 2008; DiLeo et al., 2014) and climate (Meentemeyer et al., 2004; Venette and Cohen, 2006; Ireland et al., 2013; Meentemeyer et al., 2015) are all know to drive the spread in ecosystems invaded by P. ramorum. In spite of the theoretical tolerance of the pathogen to both high and low temperatures, models validated by extensive field sampling in regions infested by NA1 populations indicate high maximum temperatures strongly limit the spread of the pathogen (Meentemeyer et al., 2015) and may even cause significant reversion from positive to negative infection status in foliar hosts (Lione et al., 2017). High temperatures have also been shown to make water populations of the pathogen not viable (Eyre et al., 2015). In the lab, exposure of Petri dishes to 55°C for 1 hour, to 45°C for 4 hours or to 40°C for 24 hours has blocked pathogen growth (Swain et al., 2006), but survival of the pathogen has been reported up to 1 week at 55°C for infected California bay laurel leaves (Harnik et al., 2004), and in other trials the pathogen has been shown to survive in pant tissue between 40°C  (2 days) and -20°C (4 days) (Tooley et al., 2008). Finally, the pathogen’s broad host range on popular, nursery grown, ornamental plants, and the non-lethal, nondescript nature of the disease on most of the foliar hosts allows for long-term dispersal.

Disease epidemiology in nursery settings has not been extensively studied, however it has been reported that disease cycles are strongly influenced by seasonality and year-to-year climatic variations (Tjosvold et al., 2008). Propagules have been shown to survive in potting mix for over a year but infested mix only caused infection if leaves were in contact with it (Tjosvold et al., 2009), this is in contrast with shorter survival in natural ecosystems (Fichtner et al., 2007; Eyre et al., 2013). Infection does not seem to be strongly associated with irrigation water, but sprinkler irrigation did favour disease development (Tjosvold et al., 2008). Serrano et al. (2020) have provided a first in depth analysis of an outbreak in a mock nursery setting and have identified leaf to leaf, followed by leaf to soil as the two main pathogen dispersal mechanisms in the absence of flooding. In the same study, dispersal from individual infected potted rhododendrons was always less than 2 metres in a single growing season. Anecdotal evidence suggests outbreaks in nursery settings may be associated with flooding of growing beds (see Serrano et al., 2020). Thus, both in natural ecosystems and in nurseries, foliar infections appear to be driving disease outbreaks and the microevolution (i.e. generation of novel genotypes) of the pathogen (Eyre et al., 2013; Serrano et al., 2020). Soil and water are infested but may often be dead ends, epidemiologically speaking, although water populations have been shown to foster microevolution of the pathogen which could lead to the emergence of adaptive traits (Eyre et al., 2015).

Genetic analysis shows P. ramorum infection initially reported only in Europe and parts of North America is also present in northern Vietnam, at the border with China (Jung et al., 2020); three clonal lineages were originally identified: EU1 (Europe and North America, nurseries and forests), NA1 (North America, nurseries and forests) and NA2 (North American nurseries) (Ivors et al., 2006). Information to date indicates that divergence of these lineages occurred ages ago, P. ramorum originated from isolated populations and has migrated at least four times to North America and Europe (Grünwald et al., 2011). In 2012, Van Poucke et al. (2012) identified a fourth lineage, named EU2, in the UK, and additional lineages may be present in Vietnam (Jung et al., 2020). Whole genome resequencing has identified genomic differences among the lineages (Dale et al., 2019), but lineages also differ phenotypically from one another both in vitro and in planta, although estimates of their relative invasiveness may often be biased by the choice of hosts tested (Elliott et al., 2011; Eyre et al., 2014; Franceschini et al., 2014; O’Hanlon et al., 2017). One mating type only is found in each of the four widely studied lineages: EU1 and EU2 are A1 and NA1 and NA2 are A2 (Grunwald et al., 2019). Lineages with different mating types are co-mingled in some North American nurseries (Grünwald et al., 2012) and in a few forest stands of southwestern Oregon (Grünwald et al., 2016) raising the issue of potential sexual reproduction between lineages carrying different mating types. In vitro tests have indicated mating is possible (Boutet et al., 2010), however ‘hybrid’ progeny appears to be unstable possibly due to aberrant aneuploid genome size (Vercauteren et al., 2011). In spite of the clonal nature of populations within lineages, significant intralineage phenotypic variation has been observed; such variation is mostly due to the presence of non-wild types (nwt) that are often less virulent and are caused by genomic alterations, including locus copy number variation and chromosomal rearrangements (Kasuga et al., 2012; Kasuga et al., 2016). The emergence of nwt is strongly associated with host species (stem hosts) or habitat (plants grown in nursery settings) (Elliott et al., 2018) and may be a trait identifying pathogen populations from ‘dead end’ hosts.

Taxonomic Tree

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

Notes on Taxonomy and Nomenclature

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On the basis of DNA data and morphological traits such as sporangia and chlamydospores, P. ramorum belongs to clade 8c, closely related to Phytophthora lateralis, a pathogen of Chamaecyparis lawsoniana (Port Orford cedar) and Phytophthora hibernalis, a pathogen of many orchard tree species. This clade is more distantly related to a group of species including Phytophthora syringae, P. cryptogea, P. trifolii and P. drechsleri, among others. Draft whole-genome (Tyler et al., 2006; Malar et al., 2019) and mitochondrial (Martin et al., 2007) sequences of P. ramorum are available.

Description

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Sporangia are hyaline, ellipsoid or elongated-ovoid (length x width = 25-97 x 14-34 µm, mean 46-65 x 21-28 µm), sympodial, semipapillate, and deciduous, carried on a short stalk. They are produced readily on most media if plant material is included. They are also produced on V8 agar plates, although not consistently. Chlamydospores are large, round, hyaline or yellow-cinnamon depending on substrate. They can be terminal and intercalary or more rarely lateral, and are a good diagnostic feature, especially because of their size (20-91 µm, mean 46-60 µm). P. ramorum is a heterothallic, amphigynous species, and both mating types are known in nature but do not readily form sexual spores when artificially crossed. Measurements of mature gametangia are as follows: oogonial diameter, mean 30.5 µm, range 25-35 µm; oospore diameter, mean 25.5 µm, range 22.5-27.5 µm; antheridial width, mean 17.3 µm; antheridial length, mean 15.0 µm. Growth is optimal at 18-20°C: a relatively slow grower. Hyphae are often extremely knobbly, although they lack swellings, and abundant septation can be observed, especially when producing chlamydospores. Mycelium is appressed, forming concentric growth rings more or less pronounced based on the type of media (Werres et al., 2001).

Distribution

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In Europe, P. ramorum has been reported in commercial nurseries in over 20 countries, and is under regulatory control (Sansford et al., 2009). Outside of nurseries, in the UK infection appeared to be at first limited to ornamental plants in gardens and scattered woodland trees and shrubs until 2009 (Grünwald et al., 2012), when extensive mortality and infection erupted in the UK, on Japanese larch (Larix kaempferi) in timber plantations (Brasier and Webber, 2010). Limited outbreaks have been reported in France (Schenck et al., 2018) and the Netherlands (De Gruyter et al., 2006).

P. ramorum was first identified in 2001 in the USA in the California coastal forests around Marin County (Kliejunas, 2010; COMTF, 2017b; COMTF, 2017c). When P. ramorum was subsequently found infecting woody ornamentals in a California nursery surrounded by P. ramorum-infected Umbellularia californica trees, federal regulations were imposed on the US nursery industry to prevent the spread of the pathogen. In 2004, two large wholesale nurseries in California and Oregon mistakenly sent P. ramorum-infested host plant material to numerous states across the USA. As a result, multiple surveys conducted in all affected states in 2004 detected P. ramorum in nurseries in 41 states (R Bulluck, National Science Director, USDA-APHIS-PPQ, personal correspondence, 2018). These detections were considered episodic and the infested plants were destroyed. P. ramorum has been detected on infected plants outside of nursery or managed landscape perimeters in only two states, California and Oregon. P. ramorum remains under official control in the USA (NAPPO, 2016) and surveys continue on an annual basis in forest habitats, regulated nurseries and nearby areas (USDA-APHIS, 2016).

In California and Oregon, P. ramorum has killed over 50 million trees in coastal regions (Meentemeyer et al., 2011Frankel and Palmieri, 2014; Cunniffe et al., 2016, Suddenakdeath.org). Tanoak (Notholithocarpus densiflorus) is the most susceptible species in California and Oregon (Frankel and Palmieri, 2014; Swiecki and Bernhardt, 2017). Sudden oak death was first reported in California on the coast near San Francisco (Rizzo et al., 2002b) and it spread along the coast into northern counties. In California, P. ramorum is quarantined in 15 counties, Humboldt, Mendocino, Napa, Lake, Sonoma, Solano, Contra Costa, Alameda, Marin, San Mateo, Santa Clara, Santa Cruz, San Francisco, Monterey and Trinity (USDA-APHIS-PPQ, 2016). The infestation is patchy and limited to areas within 80 km of the Pacific Ocean (Frankel and Palmieri, 2014).

In 2001, sudden oak disease was discovered in Oregon, in the south-western part of the state, Curry County (Goheen et al., 2002). Mitigation efforts in Curry Co. were implemented soon after detection, continued for many years and then discontinued in 2015 when it was apparent that although measures slowed down the spread of disease, they did not eliminate it (Frankel and Palmieri, 2014; Goheen et al., 2017). After treatments were used in the cooperative programme, the pathogen could no longer be detected in 39% of the treated forest plots (Goheen et al., 2013). P. ramorum is quarantined in a forested area of Curry Co., Oregon (NAPPO, 2016).

In Washington State, P. ramorum has been found infecting individual plants along streams passing through plant nurseries and significant outbreaks on planted ornamentals have been identified in a preserve (Strenge et al., 2017).

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.

Last updated: 20 May 2022
Continent/Country/Region Distribution Last Reported Origin First Reported Invasive Reference Notes

Asia

IndiaAbsent, Invalid presence record(s)
-KeralaAbsent, Invalid presence record(s)
JapanPresent
-KyushuPresentNative
-ShikokuPresentNative
VietnamPresent

Europe

AustriaAbsent, Confirmed absent by survey
BelgiumPresent, Localized
CroatiaPresent, Localized
CzechiaAbsent, Eradicated
DenmarkPresent, Few occurrences
EstoniaAbsent, Confirmed absent by survey
FinlandPresent, Transient under eradication
FrancePresent, Localized
GermanyPresent, Few occurrences
GreecePresent, Few occurrences
IrelandPresent, Localized
ItalyAbsent, Eradicated
LatviaAbsent, Intercepted only
LithuaniaAbsent, Intercepted only
LuxembourgPresent
NetherlandsPresent, Localized
NorwayPresentIntroduced1989
PolandPresentIntroduced2000
PortugalPresent, Few occurrences
SerbiaPresent, Few occurrences
SlovakiaAbsent, Eradicated
SloveniaPresent, Localized
SpainAbsent, Eradicated
-Balearic IslandsAbsent, Eradicated
SwedenAbsent, Eradicated
SwitzerlandPresent, Few occurrences
United KingdomPresent, Localized
-Channel IslandsPresent, Few occurrences
-EnglandPresent, Localized
-ScotlandPresent, Localized
-WalesPresent

North America

CanadaPresent, Transient under eradication
-British ColumbiaPresent, Transient under eradicationPersonal communication to CABI from CFIA, 2 February 2022.
United StatesPresent, LocalizedPresent in some areas, under official control
-AlabamaPresent, Transient under surveillance
-ArizonaAbsent, Confirmed absent by survey
-ArkansasPresent, Few occurrences
-CaliforniaPresent, Localized
-ColoradoPresent, Few occurrences
-ConnecticutAbsent, EradicatedP. ramorum has not been found in Connecticut outside of prior regulatory instances.
-FloridaAbsent, Confirmed absent by survey
-GeorgiaAbsent, Confirmed absent by survey
-IllinoisPresent, Few occurrences
-IndianaPresent, Few occurrences
-IowaPresent, Few occurrences
-KansasPresent, Few occurrences
-LouisianaPresent, Transient under surveillance
-MarylandAbsent, EradicatedIn 2019, P. ramorum was detected in Rhododendron plants from one nursery. All infected plants were destroyed. Annual surveys conducted by the Maryland Department of Agriculture targeting P. ramorum in state forests, streams and nurseries did not detect the pathogen in 2019, 2020 and 2021.
-MassachusettsAbsent, Confirmed absent by survey
-MississippiPresent, Transient under surveillance
-MissouriAbsent, Confirmed absent by surveyP. ramorum has never been detected in Missouri in the environment or in planted plants.
-NebraskaPresent, Few occurrences
-New JerseyAbsent, EradicatedP. ramorum has not been found in New Jersey outside of prior regulatory instances.
-New MexicoPresent, Few occurrences
-New YorkAbsent, EradicatedNew York State Department of Agriculture & Markets has actively been surveying for P. ramorum since 2004 and has been unable to detect P. ramorum established in the natural habitats of New York State.
-North CarolinaPresent, Transient under surveillance
-OklahomaPresent, Few occurrences
-OregonPresent, Localized
-PennsylvaniaAbsent, EradicatedAnnual specific surveys conducted by the Pennsylvania Department of Agriculture have shown that P. ramorum is not present in the natural environment of Pennsylvania. Any shipments received into Pennsylvania which test positive for P. ramorum are always subject to eradication measures.
-South CarolinaAbsent, Confirmed absent by survey
-TennesseeAbsent, Confirmed absent by survey
-TexasAbsent, Confirmed absent by survey
-VirginiaAbsent, EradicatedNo positive sites were found during the statewide survey in 2019, 2020 and 2021.
-WashingtonAbsent, EradicatedP. ramorum has not been detected in the native environment in Washington State since surveys began in 2003. All detections of this pathogen have occurred at nurseries, or closely allied settings, and in each case, a mandatory eradication has taken place to USDA standards.

South America

ArgentinaPresent

Risk of Introduction

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P. ramorum is a quarantined pest in California and southern Oregon, USA (other than nurseries, the rest of Oregon is not quarantined due to wildland eradication and containment efforts). All host plants are regulated in nurseries shipping out of state in Washington, Oregon and California.

The USA, the EU, Canada, New Zealand, Australia, the Czech Republic, Mexico, Taiwan, South Korea and other countries have identified P. ramorum as a quarantine pest (Frankel, 2008). The European Union Pest Risk Analysis for P. ramorum (Sansford et al., 2009) lists 68 countries that mention P. ramorum in their regulations (Kliejunas, 2010). The European Plant Protection Organization (EPPO) includes P. ramorum in the A2 list of quarantined organisms (www.eppo.int/ACTIVITIES/plant_quarantine/A2_list).

Habitat

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Natural habitats that have become endemic for the pathogen include a few types of coastal California forests. The first is the mixed evergreen forest characterized by coast live oak (Quercus agrifolia), California bay laurel (Umbellularia californica) and Pacific madrone (Arbutus menziesii). The second is the tanoak-redwood forest, characterized by redwood (Sequoia sempervirens) dominance, with a significant tanoak (Notholithocarpus densiflorus), California bay laurel and Douglas fir (Pseudotsuga menziesii) component (Rizzo et al., 2002a; Rizzo et al., 2002b; Garbelotto et al., 2003). In 2009, the pathogen was found infecting tanoak in a bishop pine (Pinus muricata) forest along the Mendocino Coast (Frankel and Hansen, 2011). In Oregon impacted forests are dominated by tanoak and Douglas fir with red alder (Alnus rubra) and Oregon myrtlewood (California bay laurel) (Umbellularia californica) (Hansen et al., 2008).

In Europe, and particulary in the UK, the pathogen is present in woodlands characterized by the presence of hosts such as native beech (Fagus sylvatica), Quercus cerris and Q. falcata, horse chestnut (Aesculus hippocastanum) and naturalized Rhododendron ponticum, all hosts on which infection of the stem and branches can be observed. In these environments, it may also infect the leaves of holm oak (Quercus ilex), sweet chestnuts (Castanea sativa) and ash (Fraxinus excelsior) (Slawson et al., 2006).

It is also established and an aggressive lethal pathogen of Japanese larch in plantations of the UK (Webber et al., 2010), Ireland (McCracken, 2013) and northern France (Schenck et al., 2018).

In northern Vietnam, P. ramorum has been found in subalpine and montane Rhododendron scrub and forests, and in montane broadleaved forests.

Habitat List

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CategorySub-CategoryHabitatPresenceStatus
Terrestrial

Hosts/Species Affected

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Quercus rubra, Q. palustris, Pittosporum undulatum and many other species are regarded as potential hosts: for these species, inoculation experiments have been completed, confirming susceptibility, but no natural infection has been recorded to date (2003). A database of species tested for susceptibility is available at the Risk Analysis for Phytophthora ramorum website (http://rapra.csl.gov.uk/). More information on host range is given in the following references: Werres et al. (2001); Davidson et al. (2002a); Hansen and Sutton (2002); Linderman et al. (2002); Maloney et al. (2002); Parke et al. (2002); Rizzo et al. (2002a); Rizzo et al. (2002b)Tooley and Englander (2002); Garbelotto et al. (2003)Hüberli et al. (2003) and  Kliejunas (2010). A host list is maintained by the USDA Animal and Plant Health Inspection Service (http://www.aphis.usda.gov/plant_health/plant_pest_info/pram/downloads/pdf_files/usdaprlist.pdf. To date (2012) there are over 120 species listed. The California Oak Mortality Task Force (www.suddenoakdeath.org) also maintains a host list with photos of symptoms. In 2017 the abundant rainfall associated with el nino, combined with the extensive distribution of the disease in the San Francisco Bay Area, resulted in the infection of eight Arctostaphylos species commonly known as Manzanita spp., of which at least six are considered rare or endangered (Rooney-Latham et al., 2017; Garbelotto et al., 2020).

In 2009, P. ramorum was confirmed as the cause of extensive dieback and mortality in mature and juvenile Japanese larch (Larix kaempferi) at a number of sites in south-west England. In 2010, P. ramorum was isolated from larch plantations displaying similar symptoms in south Wales. Overall, 2400 ha or ca. 0.6 million mature larch were affected (Webber et al., 2010) in addition to a large area of juvenile larch. This is the first widespread and lethal damage caused by P. ramorum to a commercially important conifer species anywhere in the world. In 2013, it was reported in Japanese larch plantations of Ireland (McCracken, 2013) and in 2018 in France (Schenck et al., 2018). Secondary infection of Fagus sylvatica, Nothofagus obliqua, Castanea sativa, Betula pendula, Rhododendron ponticum, Tsuga heterophylla and Pseudotsuga menziesii was found adjacent to some affected larch sites in south-west England.

Yaupon (Ilex vomitoria), sweetbay magnolia (Magnolia virginiana) and baldcypress (Taxodium distichum) were reported as hosts of P. ramorum when several plant species native to the Gulf Coast and south-eastern US forests were tested for reaction to P. ramorum (Preuett et al., 2013).

Host Plants and Other Plants Affected

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Plant nameFamilyContextReferences
Abies alba (silver fir)PinaceaeUnknown
Abies concolor (Rocky Mountain white fir)PinaceaeUnknown
Abies grandis (grand fir)PinaceaeOther
Abies magnifica (red fir)PinaceaeOther
Abies procera (noble fir)PinaceaeUnknown
Acer circinatumAceraceaeOther
Acer laevigatumAceraceaeUnknown
Acer macrophyllum (broadleaf maple)AceraceaeOther
Acer pseudoplatanus (sycamore)AceraceaeOther
Adiantum aleuticumPteridaceaeOther
Vettraino et al. (2006); Vettraino et al. (2006)
Adiantum jordanii (California maidenhair fern)PteridaceaeOther
Elliott et al. (2018); Vettraino et al. (2006); Vettraino et al. (2010); Vettraino et al. (2006)
Aesculus californica (California buckeye)HippocastanaceaeOther
Aesculus hippocastanum (horse chestnut)HippocastanaceaeOther
Alnus cordata (Italian alder)BetulaceaeUnknown
Alnus glutinosa (European alder)BetulaceaeUnknown
Alnus incana (grey alder)BetulaceaeUnknown
ArbutusEricaceaeUnknown
Arbutus menziesii (Pacific madrone)EricaceaeMain
Arbutus unedo (arbutus)EricaceaeUnknown
Arctostaphylos (bearberry)EricaceaeOther
Arctostaphylos canescensUnknown
Rooney-Latham et al. (2017)
Arctostaphylos columbiana (Hairy manzanita)EricaceaeOther
Arctostaphylos glandulosaEricaceaeUnknown
Rooney-Latham et al. (2017)
Arctostaphylos manzanitaOther
Arctostaphylos manzanitaUnknown
Arctostaphylos nummulariaUnknown
Rooney-Latham et al. (2017)
Arctostaphylos virgataUnknown
Rooney-Latham et al. (2017)
Artemisia tridentata (big sagebrush)AsteraceaeUnknown
AzaleasUnknown
Betula pendula (common silver birch)BetulaceaeUnknown
Calluna vulgaris (heather)EricaceaeOther
CamelliaTheaceaeOther
Camellia japonica (camellia)TheaceaeOther
Camellia japonica (camellia)TheaceaeOther
Camellia sasanqua (Sasanqua)TheaceaeOther
Camellia sinensis (tea)TheaceaeUnknown
Castanea sativa (chestnut)FagaceaeOther
Denman et al. (2005); King et al. (2015); O'Hanlon et al. (2016); O'Hanlon et al. (2017); Denman et al. (2005)
Castanopsis chrysophylla (Golden chestnut)FagaceaeUnknown
Rooney-Latham et al. (2017)
Castanopsis orthacanthaUnknown
Ceanothus thyrsiflorus (Blueblossom ceanothus)RhamnaceaeOther
Chamaecyparis lawsoniana (Port Orford cedar)CupressaceaeOther
ChoisyaRutaceaeUnknown
Choisya ternata (mexican orange-blossom)RutaceaeUnknown
Cinnamomum camphora (camphor laurel)LauraceaeOther
Corylus cornuta (beaked hazel)BetulaceaeOther
Corylus cornuta var. californica (California hazel)BetulaceaeOther
Cryptantha torreyanaUnknown
CryptomeriaTaxodiaceaeUnknown
Cytisus scoparius (Scotch broom)FabaceaeUnknown
Eucalyptus haemastomaLithomyrtusUnknown
Fagus sylvatica (common beech)FagaceaeOther
Frangula californicaRhamnaceaeOther
Fraxinus excelsior (ash)OleaceaeOther
GaultheriaUnknown
Gaultheria procumbens (Aromatic wintergreen)EricaceaeOther
Gaultheria shallon (salal)EricaceaeUnknown
Griselinia littoralisCornaceaeOther
Hamamelis (witchhazel)HamamelidaceaeUnknown
Hamamelis virginiana (Virginian witch-hazel)HamamelidaceaeOther
Heteromeles salicifolia (toyon)RosaceaeMain
Kalmia (laurel)EricaceaeOther
Kalmia latifolia (Mountain laurel)EricaceaeUnknown
Larix (larches)PinaceaeOther
Larix decidua (common larch)PinaceaeUnknown
Larix kaempferi (Japanese larch)PinaceaeOther
Larix marschlinsii (hybrid larch)PinaceaeUnknown
Laurus nobilis (sweet bay)LauraceaeOther
Leucothoe walteriEricaceaeUnknown
Lilium spp.LiliaceaeUnknown
Lithocarpus (stone oaks)FagaceaeWild host
Lonicera hispidulaCaprifoliaceaeOther
Loropetalum chinenseHamamelidaceaeOther
MagnoliaMagnoliaceaeUnknown
Magnolia stellata (Star magnolia)MagnoliaceaeOther
Magnolia x loebneriMagnoliaceaeOther
Maianthemum racemosumAsparagaceaeOther
Marah fabaceaUnknown
MicheliaMagnoliaceaeUnknown
Michelia doltsopa (champ)MagnoliaceaeOther
Myristica fragrans (nutmeg)MyristicaceaeUnknown
Nerium oleander (oleander)ApocynaceaeUnknown
NothofagusNothofagaceaeUnknown
Nothofagus obliqua (roble)NothofagaceaeUnknown
Notholithocarpus densiflorus (Tanoak)FagaceaeMain
OsmanthusOleaceaeUnknown
Osmanthus delavayiUnknown
Osmanthus fragransOleaceaeUnknown
Osmanthus heterophyllus (holly olive)OleaceaeUnknown
Osmorhiza berteroiUnknown
Oxalis (wood sorrels)OxalidaceaeUnknown
Parrotia persica (persian ironwood)HamamelidaceaeOther
Phoradendron serotinumViscaceaeUnknown
Photinia fraseriRosaceaeOther
Picea sitchensis (Sitka spruce)PinaceaeOther
Pickeringia montanaUnknown
Rooney-Latham et al. (2017)
Pieris (Ericaceae)EricaceaeUnknown
Pieris formosaUnknown
Pieris japonica (Lily-of-the-valley shrub)PieridaeOther
Pittosporum undulatum (Australian cheesewood)PittosporaceaeUnknown
Populus deltoides (poplar)SalicaceaeUnknown
Prunus (stone fruit)RosaceaeUnknown
Prunus lusitanicaRosaceaeUnknown
Pseudotsuga menziesii (Douglas-fir)PinaceaeMain
PterisPteridaceaeUnknown
Pyracantha koidzumiiUnknown
Quercus (oaks)FagaceaeUnknown
Quercus acuta (japanese evergreen oak)FagaceaeUnknown
Quercus agrifolia (California live oak)FagaceaeMain
Quercus cerris (European Turkey oak)FagaceaeOther
Quercus chrysolepis (Canyon live oak)FagaceaeMain
Quercus falcata (red oak)FagaceaeMain
Quercus ilex (holm oak)FagaceaeOther
Denman et al. (2005); Denman et al. (2009); King et al. (2015); Denman et al. (2005)
Quercus kelloggii (California black oak)FagaceaeMain
Quercus parvulaUnknown
Quercus parvula var. shreveiFagaceaeMain
Quercus petraea (durmast oak)FagaceaeUnknown
Quercus phillyraeoides (ubame oak)FagaceaeUnknown
Quercus robur (common oak)FagaceaeUnknown
Quercus rubra (northern red oak)FagaceaeUnknown
Rhamnus cathartica (buckthorn)RhamnaceaeUnknown
Rhamnus purshiana (Cascara buckthorn)RhamnaceaeOther
Hansen et al. (2005); Vettraino et al. (2006); Vettraino et al. (2010); Vettraino et al. (2006)
Rhododendron (Azalea)EricaceaeOther
Rhododendron arboreumEricaceaeUnknown
Rhododendron catawbienseEricaceaeOther
Rhododendron hirsutumUnknown
Rhododendron impeditumUnknown
Rhododendron macrophyllum (Pacific rhododendron)EricaceaeUnknown
Rhododendron ponticum (rhododendron)EricaceaeUnknown
Rhus diversiloba (Pacific poisonoak)AnacardiaceaeUnknown
Rosa (roses)RosaceaeUnknown
Rosa californica (California rose)RosaceaeUnknown
Rosa gymnocarpaOther
Rosa gymnocarpaUnknown
Rubus spectabilis (salmonberry)RosaceaeUnknown
Rubus ursinus (boysenberry)RosaceaeUnknown
Rooney-Latham et al. (2017); Vettraino et al. (2010)
Salix caprea (pussy willow)SalicaceaeOther
Sambucus nigra (elder)CaprifoliaceaeOther
SarcococcaUnknown
Schima wallichii (Chinese guger tree)TheaceaeUnknown
Sequoia sempervirens (coast redwood)TaxodiaceaeOther
Symphoricarpos (snowberry)CaprifoliaceaeUnknown
Syringa vulgaris (lilac)OleaceaeOther
Beales et al. (2004); Schlenzig et al. (2015)
Taxus baccata (English yew)TaxaceaeOther
Lane et al. (2004)
Toxicodendron diversilobumAnacardiaceaeOther
Trientalis latifoliaPrimulaceaeOther
Tsuga heterophylla (western hemlock)PinaceaeUnknown
Umbellularia californica (California laurel)LauraceaeOther
Vaccinium (blueberries)EricaceaeWild host
Vaccinium arboreum (Tree huckleberry)EricaceaeUnknown
Vaccinium myrtillus (blueberry)EricaceaeOther
Vaccinium ovatum (Box blueberry)EricaceaeMain
ViburnumCaprifoliaceaeMain
Viburnum bodnantenseCaprifoliaceaeOther
Viburnum davidiiCaprifoliaceaeUnknown
Viburnum farreriUnknown
Viburnum plicatumCaprifoliaceaeUnknown
Viburnum plicatum var. tomentosum (Dubble-file viburnum)CaprifoliaceaeUnknown
Viburnum tinusCaprifoliaceaeOther
Vinca (periwinkle)ApocynaceaeUnknown

Growth Stages

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Flowering stage, Fruiting stage, Post-harvest, Seedling stage, Vegetative growing stage

Symptoms

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P. ramorum causes three distinct types of disease with corresponding symptoms.

Stem Cankers (Rizzo et al., 2002a).

The cankers resemble those caused by other Phytophthora species. Discoloration can be seen in the inner bark, the cambium and within the first few sapwood rings, in some hosts causing blockage or disruption of the vascular system (Parke et al., 2007). Discoloration is always associated with the cankers, but its intensity is extremely variable, ranging from dark-brown, almost black, lesions to slight discoloration of the infected tree tissue. Black zone lines are often, but not always, present at the edge of the cankers. Smaller tanoaks (Notholithocarpus densiflorus) tend not to have any zone lines. Most notably, P. ramorum cankers stop abruptly at the soil line, and there are few reports of root infection in tanoak. Viburnum is the only host in which root collar infection is common (Werres et al., 2001) and root infection is only reported for Rhododendrons (Parke and Lewis, 2007). Typical bleeding symptoms can be seen on the outside of the cankers. Bleeding is not necessarily associated with cracks or wounds, and tends to be rather viscous in consistency. A distinct fermentation smell (or alcoholic smell) emanates from bark seeps. Intensity and viscosity of bleeding changes with time. Older cankers may display a thin, brown-amber crust where seeps were originally present. Crown symptoms are often associated with expansion rate of cankers. Rapidly expanding cankers rapidly girdle the tree. In this case, there is no real crown decline, but once the tree has exhausted the resources accumulated in its aerial part, the whole crown browns. Normally this browning occurs 1-4 years post infection. The entire foliage turns orange-brown and then becomes grey with time. The name 'sudden oak death' was coined because of the high frequency of rapidly declining trees. In the phase between girdling and apparent death of the crown, secondary processes are initiated. These include growth and fruiting of Annulohypoxylon thouarsianum, syn. Hypoxylon thouarsianum. A. thouarsianum will cause a mottled decay of portions of the sapwood and will fruit abundantly on the bark. Other secondary processes include attacks by bark and ambrosia beetles and acceleration of decay processes, at times with basidiocarps produced on trees which are still green.

When cankers are slow-growing, typical decline symptoms can be seen in the crown and include: chlorosis of the foliage, premature leaf abscission resulting in sparse crowns, and sometimes dieback of branches corresponding to portions of the stem affected by the canker. Epicormic shoots are often associated with both types of cankers (slow and fast). On oak species, most cankers are found within 1 m of the root collar, but cankers higher up on the stem and on major branches are not uncommon, especially if oak branches are intertwined with California bay laurel branches. Oak leaves, twigs, and juvenile plants are rarely infected. Tanoak cankers tend to be present throughout the vertical length of the tree and most trees have multiple cankers on them. Plants of all ages can be infected and killed. Leaves and twigs can also be infected. Foliar infection can precede or follow twig infection and it results in leaf spotting and a characteristic blackening of the main rib of the leaf, with lesions continuing into the petiole.

Leaf Blight and Branch Dieback (Rizzo et al., 2002b; Garbelotto et al., 2003).

Leaves develop lesions often associated with twig dieback. The primary infection court can be either in the twig or in the leaf. Cankers develop on branches. Symptoms on leaves develop rather rapidly, with watersoaked lesions developing mainly along the mid-vein of the leaf, and may result in death of the leaf. Rhododendron spp., Pieris spp. and Rhamnus spp. display these symptoms. In ericaceous hosts with small leaves (e.g. Vaccinium ovatum and Arctostaphylos spp.), foliar symptoms are not as pronounced. Leaf abscission and cane cankers are more common, resulting in the death of clumps of branches. Symptoms on coniferous hosts such as Douglas fir (Pseudotsuga menziesii) and Grand fir (Abies grandis) fall into this general category. In these two hosts, branch tips are typically affected. Branch tips, especially the last year's growth, are girdled and will wilt. Needles hang from the infected branch at first and then will drop, leaving a barren branch tip appearing similar to browse injury.

Leaf Spots, Blotches, and Scorches (Rizzo et al., 2002b; Garbelotto et al., 2003).

In some hosts, the disease affects leaves but not the twigs or branches. Lesions are normally associated with the accumulation of water on the leaf. These symptoms are in general rather nondescript. Lesions on Umbellularia californica are generally dark in colour, often at the leaf tip where water accumulates, but can also be on the blade or near the petiole depending how the leaf is carried and on where water accumulates. Lesions are generally demarcated by an irregular margin, often followed by a chlorotic halo. Premature chlorosis of the entire leaf, followed by its abscission, is common in drier areas. Infection in Aesculus californica starts as light circular spots, coalescing into large blotches often affecting the whole leaf, and at times the petiole. In Acer macrophyllum, symptoms appear as a marginal leaf scorch. The scorch does not, at least initially, affect the whole leaf contour, and scorched portions are interrupted by healthy areas.

On redwood the disease affects mostly needles of the lower branches. Needles appear to be infected individually, and often partially infected needles will display a black demarcation line between healthy and diseased tissue. Such lines can be developed either longitudinally following the length of the needle, or transversally across the width of the needle. Eventually, most needles in a portion of the branch may be infected and die. In general, dead needles remain attached to the branches. Basal sprouts of redwood can sometimes be girdled, cankers will appear as a dark lesion, and the entire portion of the sprout above the lesion will desiccate.

List of Symptoms/Signs

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SignLife StagesType
Growing point / dieback
Growing point / discoloration
Growing point / lesions
Growing point / wilt
Leaves / abnormal colours
Leaves / abnormal leaf fall
Leaves / necrotic areas
Leaves / wilting
Leaves / yellowed or dead
Stems / canker on woody stem
Stems / dieback
Stems / discoloration
Stems / discoloration of bark
Stems / gummosis or resinosis
Stems / internal discoloration
Stems / necrosis
Stems / odour
Stems / visible frass
Whole plant / discoloration
Whole plant / frass visible
Whole plant / plant dead; dieback
Whole plant / uprooted or toppled

Biology and Ecology

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Our understanding of P. ramorum is still limited and subject to be modified in the future. For reviews on current knowledge, refer to Rizzo et al. (2005), Sansford et al. (2009), Kliejunas (2010)Grünwald et al. (2011); Garbelotto and Hayden (2012)Grünwald et al. (2019).

The coastal distribution of the disease caused by P. ramorum (Rizzo et al., 2005) in California, USA, suggests that the pathogen is favoured by moist and moderate climates. Moisture in the infested region is provided both by precipitation and fog, and temperature fluctuations are relatively small when compared with the interior of California. Within the coastal region, eastern and southern slopes are significantly drier. A comparison of inoculum viability between these drier areas and the more mesic areas within the same region has indicated a much more pronounced seasonal pattern in drier climates (Davidson et al., 2002c). These observations, combined, indicate that maximum disease progression is to be expected in mesic areas with moderate climate. Meentemeyer et al. (2015) and Lione et al. (2017) have both identified high temperature in spring-summer-autumn as a limiting factor in the distribution and/or persistence of the pathogen in the interior of California. In the latter study, a yearly mean temperature of 20-25°C and a maximum temperature of 35-30°C were extremely limiting for P. ramorum.

Experimental evidence has indicated that infection of bay (Umbellularia californica) leaves is optimal when a film of free water remains on the leaf surface for at least 9-12 hours and temperatures are approximately 18°C (Garbelotto et al., 2003). These conditions are frequently met in the fog-drenched coastal region of California, where over 200 days of fog per year have been recorded. However, Garbelotto et al. (2017) have shown in a 9-year-long study that sporangia production is exclusively limited to the rainy season and is basically nil in other periods, including those when the marine layer brings in significant moisture to coastal forest. In the same study, Garbelotto et al. (2017) were also able to determine that oak infection is extremely episodic and as of 2020 may only have occurred a few times since the arrival or the pathogen in California forests (2000-2001, 2006-2007, 2010-2011, 2016-2017). The reason for the episodic nature of oak infection relies in the requirements necessary for infection namely: inoculum density needs to reach 5 x 10^4 zoospores per mL, temperature needs to be around 20°C multiple hours per day for a week before infection starts to be successful, and rainfall must have surpassed the 250 ml threshold in the 3-6 weeks prior to infection. Interestingly, the overwhelming majority of oak infections were also modelled to occur when oaks are within 5 m from more than one bay laurel (Garbelotto et al., 2017).

In the absence of free water, plant infection is significantly reduced. It has been presumed that zoospores are the main source of infection, and micrographs have shown zoospores producing infection pegs that enter the host through bark lenticels or leaf stomata. The most important feature of this forest Phytophthora is that all plant parts affected by the disease are aerial. Is this an aerial Phytophthora? There is no doubt that an aerial phase has to be invoked to explain the epidemiology of Sudden Oak Death (SOD), but current knowledge on dispersal, that is when sporangia are produced and dispersal scale strongly indicate P. ramorum to be an aerial splash dispersed species, such as P. capsici (Ristaino et al., 2000). Sporangia are extremely deciduous and are produced in abundance on leaves and sometimes twigs of hosts such as bay laurel (U. californica) and tanoak (Notholithocarpus densiflorus). Infections can vary greatly from extremely abundant on almost every leaf in the lower and mid crown of bay trees in infested areas to just a few leaves, and sporangia can be recovered from rainwater. The extent of the aerial spread of P. ramorum is becoming clearer as extensive population genetics studies in colonized forests have shown that leaf pathogen populations rather than soil, water or even populations from stem hosts are epidemiologically relevant and drive the genotypic composition and the microevolution of this pathogen (Hüberli and Garbelotto, 2012; Eyre et al., 2013; Eyre et al., 2015). It is unclear whether in nature, soil and water populations may be epidemiologically significant, possibly because selection pressure leading to adaptation to live in soil and water may not be beneficial to the aerial lifestyle of the pathogen.

In California the disease is almost always correlated to the presence of two species, bay laurel and tanoaks (although a few exceptions are known). In areas where tanoaks and California bay laurel coexist, infection of the latter tree species precedes and overwhelmingly surpasses infection of tanoaks, suggesting a key role is being played by this species in the epidemiology of the disease. In a study where position of tanoaks and bay laurels was studied at the landscape level, it was shown that tanoaks and bays are ecologically overlapping in terms of physiological requirements, but are not epidemiologically identical when studying SOD, as only bay laurels are strongly associated with oak infection (Garbelotto et al., 2017).

After the first few rains, P. ramorum can be detected in the environment (soil, rain traps) (Davidson et al., 2002c). Leaf infection will occur in a few hours, and infected leaves may persist on the branch for more than a year, providing a persistent source of inoculum on some trees. When relative humidity is high, and temperatures are warm, sporangia and chlamydospores will be produced on the infected leaves and will be splashed by rain onto other leaves and into the soil, and may become airborne in rain driven winds. In hosts like tanoaks, foliar infections are often associated with twig infection: the outcome is twig and branch dieback. Oaks and the main stems of tanoaks are probably infected in a final stage of the disease. Sporangia are the most effective transmission propagule, but the motile zoospores that they contain are the most effective infection propagules. Girdling cankers will proceed at varying rates, probably mostly affected by the genetic makeup of the individual infected tree (Dodd et al., 2005). Variations in susceptibility to the pathogen have been noticed in both bay laurel (Hüberli et al., 2012) and oaks (Dodd et al., 2005). Susceptible individuals will potentially be girdled in just a few weeks. Girdled trees will look apparently healthy for several more months, until all resources have been depleted and the trees undergo a rapid decline. Disease tolerance has been identified in tanoaks (Hayden et al., 2011; Hayden et al., 2013) and oaks (Conrad et al., 2017).

Soil, water and leaves may be infected/infested and infectious but only infected plants have been shown to be highly contagious (Serrano et al., 2020): it is possible to experimentally infect leaves by placing them over infested soil, and it is possible to infect wood by placing it under infected leaves. Conversely, it is extremely difficult to infect leaves by placing them near or on infected wood. These results indicate that leaves (including twigs) and not wood play a crucial role in the epidemiology of the disease. Because oak leaves are not generally infected by P. ramorum, oaks may not effectively spread the disease. In support of this hypothesis, surveys have indicated that oaks are much more likely to become infected by P. ramorum if they grow in proximity to bay laurel trees (Kelly and Meentemeyer, 2002; Swiecki and Bernhardt, 2002). While the epidemiological role of soil in infested areas is probably minor (Eyre et al., 2013), the epidemiological role of water remains an open question, and there is anecdotal evidence that occasionally floods may lead to infection. Serrano et al. (2020) and Tjosvold et al. (2008) have indicated that water used for irrigation is not a good infection pathway, however they have not tested flooding. On the other hand, Eyre et al. (2015) have shown that genotypic composition in water is different to that of leaf populations, so it remains to be seen whether genotypes that thrive in water may be able to start aerial outbreaks.

Sporangia can survive for several weeks even if dried, whereas the survival period for chlamydospores is still unknown. Chlamydospores are found in abundance in soil, streams and embedded in leaves. Chlamydospores embedded in bay leaves are quite resilient and will survive for a week with a constant temperature of 55°C (Harnik et al., 2004) and for multiple days at -20°C (Tooley et al., 2008). Both of these propagule types are potential means of long-range spread of the disease.

Notes on Natural Enemies

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Both Phytophthora nemorosa and Phytophthora pseudosyringae have been found to be competitors of P. ramorum. Their interaction has been studied in the laboratory and in the field: in dry weather the competitors are more fit than P. ramorum, and vice versa in rainy years (Kozanitas et al., 2017).

Means of Movement and Dispersal

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Natural dispersal of P. ramorum is by drifting plant material, possibly by waterborne and soilborne chlamydospores: most spread occurs via wind-blown rain containing sporangia.

There are no known vectors of the disease other than man but any animal that can move soil is potentially a vector. P. ramorum has been proven to be effectively moved through the trade of ornamental plants and green waste. There is compelling evidence that mature compost will not be infectious (Swain et al., 2006).

Pathway Vectors

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VectorNotesLong DistanceLocalReferences
Clothing, footwear and possessionsSoil on clothing and equipment
Land vehiclesAll if soil moved.
Machinery and equipment
Soil, sand and gravel Yes

Plant Trade

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Plant parts liable to carry the pest in trade/transportPest stagesBorne internallyBorne externallyVisibility of pest or symptoms
Bark fungi/hyphae Yes Yes Pest or symptoms usually visible to the naked eye
Growing medium accompanying plants fungi/sporangia; fungi/spores Pest or symptoms usually invisible
Leaves fungi/fruiting bodies; fungi/hyphae; fungi/plasmodia; fungi/sclerotia; fungi/sporangia; fungi/spores Yes Yes Pest or symptoms usually visible to the naked eye
Seedlings/Micropropagated plants fungi/fruiting bodies; fungi/hyphae; fungi/plasmodia; fungi/sclerotia; fungi/sporangia; fungi/spores Yes Yes Pest or symptoms usually visible to the naked eye
Stems (above ground)/Shoots/Trunks/Branches fungi/fruiting bodies; fungi/hyphae; fungi/plasmodia; fungi/sclerotia; fungi/sporangia; fungi/spores Yes Yes Pest or symptoms usually visible to the naked eye
Wood fungi/hyphae Yes Pest or symptoms usually visible to the naked eye
Plant parts not known to carry the pest in trade/transport
Bulbs/Tubers/Corms/Rhizomes
Flowers/Inflorescences/Cones/Calyx
Fruits (inc. pods)
Roots

Wood Packaging

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Wood Packaging liable to carry the pest in trade/transportTimber typeUsed as packing
Loose wood packing material No
Solid wood packing material with bark No
Solid wood packing material without bark No
Wood Packaging not known to carry the pest in trade/transport
Non-wood
Processed or treated wood

Economic Impact

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The overall economic impact of P. ramorum in California, USA, is hard to assess. Californian oaks (Quercus spp.) and tanoaks (Notholithocarpus densiflorus) affected by the pathogen are not commercial timber species but add to property values and offer a swath of ecosystem services ranging from protection from erosion, water quality, quality of pastures, recreation and biodiversity, all of difficult economic evaluation. The addition of redwood (Sequoia sempervirens), Douglas fir (Pseudotsuga menziesii), grand fir, red fir, western hemlock and other conifers as reported hosts poses a potential economic cost to the timber industry. Symptoms and impact on these hosts are somewhat limited, mortality is only reported from small (less than 1 inch diameter) trees. Dead tree counts have surpassed 15 million as of 2020, but the extreme patchiness and wide extent of the disease makes any assessment of tree mortality extremely difficult (Kelly and Meentemeyer, 2002; Meentemeyer et al., 2008). Oaks in particular are known to increase the real estate value of property: large numbers of landscape oaks have been infected, with significant financial repercussions for property owners. A decrease in property values has been shown in areas near infected forests in Marin County (Kovacs et al., 2011a; Kovacs et al., 2011b). Mortality of oaks is likely to have doubled and that of tanoaks quadrupled (Swiecki and Bernhardt, 2002; McPherson et al., 2002) due to this disease. Costs of removal of dead trees and of disposal of the infectious green waste are also significant. The need for precautionary and sanitary practices is an added indirect cost to arborists and other tree professionals.

In 2009, P. ramorum was confirmed as the cause of extensive dieback and mortality in mature and juvenile Japanese larch (Larix kaempferi) at a number of sites in south-west England. In 2010, P. ramorum was isolated from larch plantations displaying similar symptoms in south Wales. Overall, 2400 ha or ca. 0.6 million mature larch were affected (Webber et al., 2010) in addition to a large area of juvenile larch. This is the first widespread and lethal damage caused by P. ramorum to a commercially important conifer species anywhere in the world. Additional highly valued specimen rhododendron plants in historic gardens have been removed to maintain phytosanitary conditions.

Some industries have been particularly affected by P. ramorum, including the ornamental plants industry (Linderman et al., 2002; Parke et al., 2002), the composting industry (Garbelotto, 2003) and the spice industry (Harnik et al., 2004). It is not only Californian growers that are affected, but also those outside California due to the decreased availability of Californian-grown propagative material. Edible mycorrhizal mushrooms are reported to have decreased in numbers in areas highly affected by the disease (Bergemann et al., 2006; Bergemann et al., 2013). Economic impacts in Oregon were estimated by Hall and Albers (2009).

Death of large numbers of trees in popular parks and recreation areas, and the partial closure of some infested areas during the rainy season, are having an impact on the recreational value of open spaces. The cost of tree removal and disposal for agencies, public or private, running large open spaces can be extremely high. The need to lower the risk of spread in industries affected by Sudden Oak Death will translate into management practices with a significant price tag (e.g. the washing of logging trucks, the closure of some roads and accesses, the need to process bay leaves to ensure the pathogen is dead, the need to ensure mature compost does not become contaminated by the green waste in fresh compost piles, and so forth). These expenditures can be considered indirect economic costs of P. ramorum. Particular mention should be made of First Nations’ tribal lands threatened by Sudden Oak Death and of the costly efforts to protect them (Cobb et al., 2013; Cobb et al., 2019).

The relationship between fire and SOD has an obvious economic component.

Kliejunas (2010) reviewed P. ramorum economic and environmental impacts. Sansford et al. (2009) reviews economic impacts in Europe.

Environmental Impact

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Oaks and tanoaks are extremely susceptible to the disease. Tanoak (Notholithocarpus densiflorus) is the most susceptible species in California, USA. Some local populations, e.g. some canyons in the Big Sur area (Monterey County), have lost all of their adult trees because of Sudden Oak Death (SOD). The disease has the potential to cause a significant genetic bottleneck in this tree species. The demise of entire tanoak populations is changing the nature of those forests, in general moving towards pure redwood (Sequoia sempervirens) stands with marginal presence of oaks and Douglas firs. Wildlife depending on tanoaks and oaks is likely to be locally affected, and several studies already document that for some smaller vertebrates and birds (Monahan and Koenig, 2006; Swei et al., 2011).

Tanoak is also the major ectomycorrhizal host of most of these stands (Bergemann et al., 2006) and its disappearance is likely to have a significant impact on ectomycorrhizal populations.  Besides loss of the symbiotic host, it has been shown that a stem-girdling disease such as SOD will cause a significant shift in mycorrhzial function. Bergemann et al. (2013) provide evidence that girdling causes a reduction in the mycorrhizal soil biomass, a key element in the relationship between mycorrhizae and pant hosts. This reduction is expected to greatly reduce the amount of P and micronutrients mycorrhizae will be able to absorb and transfer to their symbionts.

Abundance of dead and fallen woody material constitutes a significant fire hazard (Cobb et al., 2011); it is likely that the intensity of these fires may be much higher than the norm because of the large accumulation of woody debris (Forrestel et al., 2015). The arrangement of fuels is altered, posing serious challenges to firefighter response in infested stands (Valachovic et al., 2011). However, maybe the most surprising effect of SOD on fire behaviour is the creation of previously unreported hotspots in conjunction with dead standing oaks and tanoaks. These hotspots were unmanageable by fire crews, leaf to multiple fire jumps adding to unpredictability of fire spread and behaviour and caused unreported mortality of adult redwoods growing next to the dead trees and scorched by the intense heat (Metz et al., 2013).

The presence and incidence of coast live oak (Quercus agrifolia) is also bound to change, and some local strong shifts towards Douglas fir forests may be observed. The extremely adaptable and drought-resistant woodlands dominated by coast live oaks, are much more resilient than Douglas fir stands to prolonged periods of drought. While almost 100% of tanoaks have died in the worst-hit localized areas (Davis et al., 2010), rates of mortality for coast live oak are much lower, not exceeding 40% (McPherson et al., 2002; McPherson et al., 2010). However, yearly oak estimates were done prior to the understanding that oak infection is episodic and occurs approximately once every 5 years (Garbelotto et al., 2017). In spite of this, the amount of tree cover has decreased significantly in some areas affected by SOD. This reduced cover enhances the potential for erosion and for the establishment of undesirable plant species, including non-natives. Finally, P. ramorum may have a detrimental, but more subtle, effect on hosts other than oaks and tanoaks. Infected trees may be more susceptible to decline in the event of the onset of unfavourable climatic or ecological conditions. P. ramorum may also significantly affect the regeneration of some host species: by reducing overall seed productivity (killing mature plants, causing branch dieback) and by directly killing seedlings. One recent discovery is the ability of the pathogen to infect several rare and endangered manzanitas (Garbelotto et al., 2020) during particularly wet years, something that occurred for the first time in 2017, and not in previous years, in spite of comparable precipitation values. This finding suggests that P. ramorum has truly become endemic in the greater San Francisco Bay area where it was presumably first introduced as early as 1987 (Croucher et al., 2013).

Diagnosis

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P. ramorum will grow on most media used for other Phytophthora species such as cornmeal agar, PDA (not optimal), V8 and carrot agar (Werres et al., 2001). It grows relatively well between 15 and 22°C, but slows down significantly when temperatures go over 25°C. The most widely used selective medium to isolate P. ramorum from infected plants is corn meal based PARP (Rizzo et al., 2002a). While colonies will generally emerge from plated plant material within 6 days, at times up to 3 weeks has been necessary to obtain cultures. Cultures can be obtained from the following substrates: canker margins from oaks (Quercus spp.) and tanoaks (Nothorlithocarpus densiflorus), rhododendron leaves and stems, bay (Umbellularia californica). It is harder to isolate the pathogen from Rhamnus, Pseudotsuga, Arbutus or Heteromeles. From hosts such as Arctostaphylos, Acer, Aesculus and Lonicera, isolations can be extremely difficult. In California, a state characterized by an extremely marked Mediterranean climate, isolation success of P. ramorum follows a seasonal pattern, a trait not uncommon among Phytophthora spp. Isolation success tends to be best during late winter and spring, while it progressively declines as the dry California summer progresses. Almost every host has some specific requirements to maximise isolation success. The length of time that has passed between sample collection and processing has been shown to strongly affect the sensitivity of culture-based diagnostic methods (Vettraino et al., 2010).

For some foliar hosts, such as Umbellularia and Rhododendron, incubation in a moist chamber for 48 to 64 hours results in abundant sporulation. Sporangia and chlamydospores can then be analysed.

It is possible to bait P. ramorum from soil, streams, infected leaves, and to a limited extent from wood (Werres et al., 2001; Davidson et al., 2002c). The most common baits used for P. ramorum are pears, and 'Cunningham White' rhododendron leaves. Baiting is normally done for 5-7 days, and appears to be facilitated by temperatures between 12 and 15°C, at least for some substrates such as wood. Baits are collected at the end of the baiting period, left to dry for a couple of days, and then plated on selective PARP medium and incubated at 18°C. From mid-summer on, baiting from soil becomes unsuccessful. It is unclear whether this corresponds to a temporary dormancy period of the pathogen, or whether viability of the inoculum may be permanently lost. In infested areas it is possible to bait P. ramorum directly from the air by using healthy susceptible foliar hosts such as rhododendrons. Baiting from streams is strongly affected by air temperatures and season: in San Mateo County, Eyre et al. (2015) could not bait the pathogen out of almost 20 baiting stations starting in July and ending in November. Water baiting from plant production facilities may also be influenced by temperature, oxygen levels and by the presence of other microorganisms: Serrano et al. (2020) report that baiting from tanks of recycled irrigation water was only successful when baiting from the top layer of the water tanks and was never successful from the bottom layers. Success of baiting from top layers was also negatively affected by warmer temperatures and by the presence in the water of Phytophthora chlamydospora.

There is a wide selection of DNA PCR-based protocols to identify the pathogen both from cultures and directly from infected plant material (Garbelotto et al., 2003). The first government sanctioned protocol was that of Hayden et al. (2004). A nested PCR approach is needed to obtain a signal from most foliar hosts with the exception of bay and rhododendron leaves. Because of the difficulty in isolating the pathogen from most foliar hosts, it is recommended to back up diagnosis based on isolations, with PCR-based data. Vettraino et al. (2010) concluded that a combination of either culturing and molecular diagnosis, or of two molecular assays was the most successful approach to identifying P. ramorum. The best comparative analysis of various PCR methods by ring trials is that reported by Martin et al. (2009).

Tomlinson et al. (2005) developed a sensitive and specific single-round real-time PCR (TaqMan) assay using a portable real-time PCR platform. This has the advantage of being able to diagnose P. ramorum entirely in the field, independent of laboratory facilities.

Diagnostic methods are reviewed in Kliejunas (2010). For the USA, diagnostic and sampling protocols for official regulatory confirmations are posted at the USDA Animal and Plant Health Inspection Service (APHIS) P. ramorum website http://www.aphis.usda.gov/plant_health/plant_pest_info/pram/protocols.shtml#diag. A duplexPCR detection method, based on the internal transcribed spacer (ITS) regions of the ribosomal DNA, was developed by Schlenzig (2011) to enable sensitive detection of P. ramorum and to distinguish it from the similar P. kernoviae. Chimento et al. (2012) developed a real-time RT-PCR which is able to clearly distinguish between dead and viable P. ramorum pathogens.

Detection and Inspection

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P. ramorum is difficult to diagnose from dead trees, even if death has occurred recently. Looking around the dead tree for trees with obvious symptoms (see Symptoms section) that are still green is recommended. Bleeding can be an easily visible symptom, but it can be due to a myriad of causes including mechanical wounding, cracking, insect attack and wood decay. Using a blade, cut the outer part of the bark and check for discoloration under it. A P. ramorum lesion may or may not have a clear zone line, but it will almost never have a regular shape. Borders are in general angular, and shape is often irregular. It is essential to uncover the border of the canker in order to diagnose a tree as infected by P. ramorum. Cultures should also come from the margin of the lesion (see Diagnosis).

Bleeding is not always associated with cankers, and is normally absent in small tanoak (Notholithocarpus densiflorus) branches. In these cases, cankers will be visible as slight depressions in the bark, often appearing slightly water-soaked. A dark-brown or black lesion, without zone lines, will be present under the bark.

For all other hosts, inspection for leaf and branch symptoms is the first step. In rhododendron, look for both leaf blight and branch cankers. The two are often connected, so it will be possible to follow a lesion from the leaf into the bearing stem, and vice-versa. Lesions on rhododendrons often have diffuse margins, or conversely, dark lines in a concentric pattern highlighting growth patterns of the pathogen. Visible branch dieback is normally seen as a second stage of infection.

In the case of Umbellularia foliage, there is a good correspondence between the side of the leaf carried downwards and location of the P. ramorum lesion. This is due to the fact that swimming zoospores (requiring free water) are the main infection propagule responsible for foliar infections. If leaves are carried sideways, lesions will develop on the lower blade; if they are carried with their tip downwards, lesions will develop on the leaf tips, and so forth.

In California, USA, the most valuable diagnostic approach is to focus not on one tree but on an entire area. The diagnosis is strengthened by identifying a few different hosts, each displaying their characteristic symptoms. The use of the database SODmap both on the web (www.sodmap.org) and through the Application SODmap Mobile for smartphones and tables (www.sodmapmobile.org) allows to determine whether the pathogen is present in any given locality (Garbelotto et al., 2014). A new database called Calinvasives and part of the Calflora database will be accessible in 2020 to geolocate all proven infestations by SOD and other emergent pathogens in California (calflora.org).

Similarities to Other Species/Conditions

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Symptoms on plants are fairly generic and can be easily confused with those caused by other Phytophthora species, or with other fungal diseases. Culturing or molecular diagnostics need to be used to confirm pathogen presence.

P. ramorum cankers are in general above ground and do not develop in the roots: this feature can be helpful, but at times other Phytophthora species also cause aerial cankers. Lesions on Umbellularia resemble symptoms caused by bay anthracnose and by other Phytophthora species, in particular P. nemorosa and P. pseudosyringae, which are very common on the West Coast of the USA (Davidson et al., 2002b). The lack of fungal reproductive structures can assist in differentiating P. ramorum symptoms from other symptoms caused by fungi. Also, while fungi are facilitated by high relative humidity, infection by Phytophthora spp. requires a film of water, so correspondence between lesion location on a leaf and where water would accumulate can be a helpful diagnostic feature to differentiate fungal from Phytophthora infections. On Rhododendron spp., symptoms are identical to those caused by species such as Phytophthora cactorum, P. citricola, P. citrophthora and P. nicotianae for twig symptoms, and P. syringae for foliar symptoms (Werres et al., 2001).

On some hosts, foliar infection by P. ramorum leads to early leaf abscission. This is particularly noticeable in Camellia species, but other plant species, at times characterized by smaller leaves, and even California Bay laurels growing in warmers sites may develop the same symptoms. Some foliar hosts both in North America and in Europe are known to bear viable infections that are not visible as necrotic lesions, blotches or spots, and can only be detected by isolation, PCR or baiting (Denman et al., 2009).

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.

Best management practices for P. ramorum are available at: http://www.suddenoakdeath.org/diagnosis-and-management/best-management-practices/ and at www.matteolab.org

-Removal of California bay laurels (Garbelotto et al., 2017), tanoaks (Cobb et al., 2017) and Japanese larch (O’Hanlon et al., 2018) have all been shown to be effective.

-Employment of disease tolerant plants (Hayden et al., 2013; Conrad and Bonello, 2016; Cobb et al., 2019) holds significant promise and some results have been produced on higher survival of tanoak recruitment using disease tolerance.

-Planting non-susceptible species and other strategies are being developed to prevent or manage P. ramorum in wildlands (Lee et al., 2010; Swain and Alexander, 2010; Cobb et al. 2019).

-It has been shown that fresh wounds will be optimal infection courts; pruning of large branches or of stems should occur in the autumn, months in California before the rain-driven spread of P. ramorum occurs, or 4 months before the infectious period in other parts of the world.

-In restoration projects, avoid bay laurel (Umbellularia californica) if possible, especially in areas where oaks may be growing. Eradication has been attempted in southern Oregon, USA, via the burn-and-slash technique (Goheen et al., 2002b).

- Kiln drying: 55°C for 30 minutes was insufficient to kill the pathogen. At least 1 hour is required but only for substrates without abundant chlamydospores such as wood. If substrate supports chlamydospores 2 weeks at 55°C were necessary to kill the pathogen. However, vacuum plus exposure at 55°C, together killed the pathogen after 22 hours (Harnik et al., 2004).

- Composting following EPA guidelines for California is completely successful in eliminating the pathogen (Swain et al., 2006), however mature compost can be re-infected by the pathogen if exposed to large amounts of inoculum (Swain and Garbelotto, 2015).

- P. ramorum is susceptible to label-dosages of copper sulfates and copper hydroxides (Garbelotto et al., 2009). In different formulations it is moderately susceptible to mancozeb. The pathogen is sensitive to phosphites or phosphonates. Phosphite injections are effective in oaks for 2 years (Garbelotto and Schmidt, 2009) and also temporarily in tanoaks (Notholithocarpus densiflorus). Phosphite foliar sprays are not effective on oaks and tanoaks. The pathogen is extremely sensitive to metalaxyl, but drenches and foliar sprays are ineffective in oaks (Garbelotto et al., 2007; Garbelotto et al., 2009).

- Water and moisture management are extremely important, especially when temperatures are between 15 and 20°C. Infection on bay (Umbellularia californica) leaves requires 9-12 hours of leaf wetness. Oak infection requires 250 mm of rain in for 3-6 weeks and at least 1 week when max temperature daily reaches 20°C (Garbelotto et al., 2017).

- Natural contagion from oaks is estimated to be low: susceptible oaks should not be planted near foliar hosts like bays and rhododendrons.

- Early infection can be detected on foliar hosts: new infection on bay leaves are good signs of inoculum level.

- Sites, soil and streams can be monitored by baiting with rhododendron leaves (Davidson et al., 2002c).

- Removal of California bays 10 and 20 m around oaks has been proven highly effective in reducing possible infection events and remains the main prescription to preventatively protect oaks from becoming infected (Garbelotto et al., 2017).

- Sanitation of small tools such as axes, hand saws or chain saws is best done by removing all organic debris, followed, if desired,by treatment with lysol, 70% ethanol, or a 10% dilution of commercial bleach.

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Links to Websites

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WebsiteURLComment
Global register of Introduced and Invasive species (GRIIS)http://griis.org/Data source for updated system data added to species habitat list.

Contributors

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11/05/2020 Updated by:

Matteo Garbelotto, Department of Environmental Science, Policy & Management, University of California, Berkeley, California, USA.


09/03/12 Updated by:

Susan J. Frankel, Sudden Oak Death Research, USDA Forest Service, Pacific Southwest Research Station, Berkeley, California, USA

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