Phytophthora ramorum

Overview

P. ramorum Werres, De Cock, & Man in’t Veld (2001) appears to be an exotic species introduced from an unknown origin to Europe and western N. America in the mid-1990s, likely on nursery plants (Mascheretti et al. 2008).  In California and southwest Oregon, the pathogen spread to native oak and tanoak forests, where it causes lethal stem cankers (“sudden oak death”).  More than a million trees have been killed (Meentemeyer et al. 2011).  In Europe, where nurseries in over 20 countries reported P. ramorum, the disease on trees was limited to isolated stands of beech and oak associated with infected rhododendrons.  In 2009, the pathogen began causing widespread mortality on Japanese larch timber plantations in the UK (Brasier & Webber 2010).  International, national, state and county quarantines are in place to prevent the further spread of disease with nursery plants and to contain the infestation in forests. P. ramorum is unusual among forest Phytophthora species for its wide host range and infection of aerial plant parts.  

Morphology

Sporangia (average 46–65 µm by 21–28 µm) formed on agar and in water, ellipsoid, spindle-shaped, or elongate-ovoid, semi-papillate, caducous with short pedicel (< 5 µm), produced singly or in clusters of 2–12 on sympodially branched sporangiophores.  Chlamydospores (20–91 µm; average 46-60 µm) formed abundantly in agar, globose, mostly thin-walled, terminal or intercalary. Heterothallic, oogonia not seen in single culture (Werres et al. 2001).

Genetics

P. ramorum is placed in clade 8c, with P. lateralis as its closest relative (Blair et al., 2008). Populations in North America and Europe are clonal and belong to three lineages according to several molecular markers (Grünwald et al., 2008).  

*Most EU1 isolates are mating type A1 but several isolates from Belgium are mating type A2

Evidence suggests that clonal lineages descended from a sexually recombining population but have been reproductively isolated for a long time (Goss et al. 2009).  Oospores have not been observed under natural conditions despite the concurrent presence of both mating types in nurseries.   A 7X draft whole-genome sequence (Tyler et al. 2006) and mitochondrial sequence (Martin et al., 2007) of P. ramorum are available.

Growth

Temperature optimum ca. 20°C, min. ca. 2°C, max. ca. 26–30°C.  Growth at 20°C on V8 agar 2.2 mm/d, on cornmeal agar-β sitosterol 1.9 mm/d, on cornmeal agar with pimaricin, ampicillin, and rifampicin (PAR) 0.7 mm/d, on potato dextrose agar 1.2 mm/d.

Distinguishing characteristics for identification

Colony morphology and hyphal characteristics on isolation plates are distinctive.  The abundance of large chlamydospores in combination with clusters of semi-papillate, caducous, ellipsoid sporangia formed on the surface of the agar may be considered definitive.  Be aware that these features may be seen independently in other Phytophthora species growing on isolation plates.

The searchable web-based database Phytophthora-ID is useful for rapid identification of Phytophthora species based on sequencing of the ITS or Cox spacer regions, followed by BLAST searching the database. Phytophthora-ID maintains a database of sequences that is selective for sequence accessions that come from trusted sources including published, peer-reviewed studies whenever possible.

Disease History

Increasing mortality of coast live oak and tanoak was noted in rural and suburban areas around the San Francisco Bay area beginning about 1995. Public awareness and research attention grew rapidly, and the syndrome was named “sudden oak death,” but a causal agent was not proved until 2000, when a new Phytophthora species was isolated from stem cankers on dying trees (Rizzo et al. 2002). The California species proved to be identical to another unnamed Phytophthora detected in 1993 in Germany from declining rhododendron, and was ultimately named P. ramorum. Nurseries in California were first implicated as a disease source in 2001, and in 2003 it became evident that P. ramorum was in the nursery trade along the entire west coast of N. America, and was also widespread in European nurseries.  Tree infections in Europe were limited to beech and non-native oak in southwest England and the Netherlands, often associated with infected rhododendrons. By 2009 the pathogen began to cause widespread damage to forest plantations of Japanese larch in the UK (Brasier & Webber 2010). International, national, and regional quarantines have been imposed to stop further spread of the pathogen.  Efforts are focused on eradicating the pathogen from nurseries and containing the disease in forests. The pathogen is considered to have been introduced separately and relatively recently to both the US and Europe, although its native home is unknown (Mascheretti et al. 2008, Goss et al. 2009).

Impacts in the Forest

P. ramorum threatens the survival of tanoak in western parts of the tree’s range and is reducing populations of coast live oak dramatically.  More than a million trees in California and Oregon have been killed, and an epidemic is occurring on Japanese larch in the UK where more than 1500 ha of infested timber plantations have already been felled.  Large areas of U.S. southeastern forests are considered at risk (summarized in Kliejunas, 2010) as are heathlands, oak and beech forests, evergreen oak woodlands and laurel forests in Europe (Sansford et al. 2009). P. ramorum  spreads locally through rain splash of sporangia that form abundantly on the foliage of certain hosts, for example, California bay laurel, Japanese larch, Rhododendron ponticum, or on twigs of tanoak.  Long distance aerial dispersal of sporangia is believed to occur infrequently during storms (Hansen et al. 2008). Although the pathogen does not appear to infect roots, soilborne chlamydospores can survive long periods (Fichtner et al. 2009) and give rise to new sporangia that are splashed or carried to infect above-ground parts of plants. P. ramorum has also been baited from forest streams in California and Oregon and from waterways near several infested nurseries.

Forest and Wildland Hosts and Symptoms

Symptoms differ on different host species. The pathogen causes sudden oak death (SOD), characterized by lethal stem cankers, on some Fagaceae (oaks), shoot dieback on  some Ericaceae and conifers, and foliar blight on  a diverse group of hosts.  P. ramorum has an extremely broad host range that includes more than 100 species in 37 families, although only a few forest species are highly susceptible. Many nursery hosts, such as rhododendron, camellia, and viburnum are important in long distance dispersal of the pathogen. P. ramorum is established in oak and tanoak forests in 14 coastal California counties, and in Curry County, Oregon, as well as in isolated forest estates in Europe.   Timber plantations of Japanese larch in the UK are also infested.

The table of susceptible hosts below includes only the most ecologically and economically important host species.  For a more complete list of the many other host species, download a pdf from APHIS (Animal and Plant Health Inspection Service of the U.S. Department of Agriculture) or FERA (Food and Environmental Research Agency of the U.K.).  Our Disease Finder also includes more host species than this table.

Management and Educational Materials

View additional educational and management resources on our Phytophthora Web Resources page. A comprehensive website on sudden oak death is available at www.suddenoakdeath.org.

Management of P. ramorum–associated diseases in forests, woodlands, and urban areas has taken a multiscale approach ranging from individual trees to landscapes to international quarantines (Alexander and Lee, 2010; Frankel, 2008; Rizzo et al., 2005). Disease prevention and mitigation at the individual plant level or urban–wildland interface in California has been focused on chemical control or other programs designed to maintain health of plants. Some fungicides have been developed that act as protectants (e.g., phosphonates) against infection, but few chemicals have been developed that work once the plant is infected (Garbelotto and Schmidt, 2009; Garbelotto et al., 2009). Removal of inoculum-producing plants, such as bay laurel or rhododendron, has also been important at smaller scales to protect high-value oaks (Swiecki and Bernhardt, 2008). Education and involvement of local communities has been critical at the urban–wildland interface to the implementation of management programs (Alexander and Lee, 2010).

At larger landscape scales in wildland forest communities, management strategies for P. ramorum have included prevention, eradication, treatment, and restoration (Rizzo et al., 2005). Eradication has been attempted in some cases, most notably with tanoak forests in Oregon (Kanaskie et al., 2010) and larch plantations in the United Kingdom (Brasier and Webber, 2010), but has met with mixed success. Important successes have been balanced by continuing tree mortality in many areas (Kanaskie et al., 2010). Difficulties have been encountered in detecting the pathogen at an early enough stage for eradication to be completely effective at a landscape scale. Cryptic infection (i.e., with minimal or no symptoms) of foliage during the initial invasion of a site by P. ramorum has allowed the pathogen to stay one step ahead of detection efforts in many cases. The development of management strategies, beyond eradication, for forest lands following invasion by P. ramorum is still in the early stages (Rizzo et al., 2005; Valachovic et al., 2008, 2010). Decision making requires the ability to fit disease management into the context of other management goals (e.g., fire prevention, wildlife) within the broader forest landscape (Rizzo et al., 2005). Examples of approaches that are being tested include forest stand thinning to remove inoculum-producing hosts and use of prescribed fire (Valachovic et al., 2008, 2010).

The broadest scale for disease management, regional to international, is driven by regulations and management practices designed to prevent further spread of Phytophthora (Brasier, 2008; Frankel, 2008; Rizzo et al., 2005). In recent years, broadening of national and international quarantines designed to prevent pathogen movement has led to an increased effort to manage all Phytophthora diseases in nursery settings (Osterbauer et al., 2004; Parke et al., 2010a,b). While dozens of plant species have been found infected in nurseries, the majority of infections have been associated with the genera Rhododendron, Camellia, Pieris, Kalmia, and Viburnum (Osterbauer et al., 2004; Parke et al., 2010a). These plant species have become the focal point for development of best management practices and pathogen detection strategies (Parke et al., 2010a). The need for pathogen detection in nursery plants as part of quarantine inspections has resulted in the development of a number of PCR-based molecular tests (e.g., Hayden et al., 2004; Vettraino et al., 2010).

• Consultation on future management of risks from Phytophthora ramorum and Phytophthora kernoviae
• Phytophthora ramorum - A threat to our woodlands, heathlands and historic gardens - Plant Disease Factsheet
• Phytophthora ramorum - A practical guide for the Nursery Stock and Garden Centre Industry
• Phytophthora ramorum A Practical Guide for Established Parks & Gardens, Amenity Landscape and Woodland Areas
• Nursery guide for diseases caused by Phytophthora ramorum on ornamentals: diagnosis and management
• Protecting trees from sudden oak death before infection
• Muerte subita de Robles (Sudden oak death)
• Nursery industry best management practices for Phytophthora ramorum
• Life Cycle of Phytophthora ramorum As it relates to soil and water
• Risk Analysis for Phytophthora ramorum Werres, de Cock & Man in't Veld, Causal Agent of Sudden Oak Death, Ramorum Leaf Blight, and Ramorum Dieback
• Sudden oak death and Phytophthora ramorum: a summary of the literature
• Phytophthora ramorum - Identifying symptomatic larch