Phytophthora ramorum

A hillside in Big Sur, California, devastated by sudden oak death

It was first discovered in California in 1995 when large numbers of tanoaks (Notholithocarpus densiflorus) died mysteriously, and was described as a new species of Phytophthora in 2000. It has subsequently been found in many other areas, including some other U.S. states, Britain, and Germany, either accidentally introduced in nursery stock, or already present undetected.

In tanoaks, the disease is recognized by wilting new shoots, older leaves becoming pale green, and after a period of two to three weeks, foliage turning brown while clinging to the branches. Dark brown sap stains the lower trunk's bark. Bark often splits and exudes gum, with visible discoloration. After the tree dies back, suckers try to sprout the next year, but their tips soon bend and die. Ambrosia beetles (Monarthrum scutellare) will most likely infest a dying tree during midsummer, producing piles of fine white dust near tiny holes. Later, bark beetles (Pseudopityophthorus pubipennis) produce fine, red boring dust. Small black domes, the fruiting bodies of the Hypoxylon fungus, often are present on the bark. Leaf death occurs more than a year after the initial infection and months after the tree has been girdled by beetles.

In coast live oaks and Californian black oaks, the first symptom is a burgundy-red to tar-black thick sap bleeding from the bark surface. These are often referred to as bleeding cankers.

In addition to oaks, many other forest species may be hosts for the disease; in fact, it was observed in the United States that nearly all woody plants in some Californian forests were susceptible to P. ramorum.[4] including rhododendron, madrone (Arbutus menziesii), evergreen huckleberry (Vaccinium ovatum), California bay laurel (Umbellularia californica), buckeye (Aesculus californica), bigleaf maple (Acer macrophyllum), toyon (Heteromeles arbutifolia), manzanita (Arctostaphylos spp.), coast redwood (Sequoia sempervirens), Douglas fir (Pseudotsuga menziesii), coffeeberry (Rhamnus californica), honeysuckle (Lonicera hispidula), and Shreve oak (Quercus parvula). P. ramorum more commonly causes a less severe disease known as ramorum dieback/leaf blight on these hosts. Characteristic symptoms are dark spots on foliage and in some hosts the dieback of the stems and twigs.[5] The disease is capable of killing some hosts, such as rhododendron, but most survive. Disease progression on these species is not well documented. Redwoods symptoms show purple lesions on sprouts and needle discoloration and cankers on small branches, which can lead to sprout mortality.

Leaf death caused by P. ramorum

In Europe, Ramorum blight was first observed on rhododendron and viburnum in the early 1990s,[3] where it was initially found mainly on container-grown plants in nurseries.[6] The principal symptoms were leaf and twig blight.[7] By 2007, it had spread throughout nurseries and retail centers in 16 European countries, and had been detected in gardens, parks, and woodlands in at least eight countries.[3] It has not caused significant harm to European oak species.[7]

In 2009, the pathogen was found to be infecting and killing large numbers of Japanese larch trees (Larix kaempferi) in the United Kingdom at sites in the English counties of Somerset, Devon, and Cornwall.[8] It was the first time in the world that Phytophthora ramorum had been found infecting this species.[9] Since then, it has also been found extensively in larch plantations in Wales[10][11] and in southwest Scotland, which led to harvesting of larch on hundreds of acres/hectares.[6] The UK Forestry Commission noted that eradication of the disease would not be possible, and instead adopted a strategy of containing the disease to reduce its spread.[9] Symptoms of the disease on larch trees include dieback of the tree's crown and branches, and a distinctive yellowing or ginger colour beneath the bark.[9] In August 2010, the disease was found in Japanese larch trees in Counties Waterford and Tipperary in Ireland.[12] It had spread to Japanese larch plantations across the south of the country by February 2014.[13] Coillte, who owned twenty forests where the disease was present, felled 16,000 trees in one of its forests, having already felled 150 hectares to contain the disease.[14]

The closely related Phytophthora kernoviae causes similar symptoms to P. ramorum, but infects the European beech (Fagus sylvatica).[15]

The Primary Incolum (Sporagia) of P. ramorum. develop on the leaves of the primary host, which include tree species like the California Bay Laurel, which is large hardwood that grows on the pacific west coast.[16] These spores are then carried by rain and air currents to the leaves of the new bole canker host, which include broadleaf trees like Tanoaks, where they begin to develop.[17] The Secondary Inoculum infects the inner bark and sapwood, resulting in bleeding cankers on the bark of the new host, which are exacerbated by infected fallen leaves and rain splashing the understory of the canker host which can both serve as sources of inoculum. After the plant matter if infects dies and decomposes, P. ramorum. is transferred to the soil by rainwater, where the final part of its cycle (soil phase) is poorly understood by scientists. However, it is suspected that Chlamydospores play a role in the long term survival of the pathogen, yet the triggers for germination are not known.[18] Additionally, the spores of P. ramorum. only seem to propagate successfully in a temperature range between 65-70 degrees fahrenheit, which is useful to know for protecting nurseries and identifying potential transfer windows for the disease.[19]

Evidence suggests P. ramorum may be an introduced species, and these introductions occurred separately for the European and North American populations, hence why only one mating type exists on each continent – this is called a founder effect.[30] The differences between the two populations are thus caused by adaptation to separate climates. Evidence includes little genetic variability, as P. ramorum has not had time to diversify since being introduced. Existing variability may be explained by multiple introductions with a few individuals adapting best to their respective environments.[31] The behavior of the pathogen in California is also indicative of being introduced; it is assumed that such a high mortality rate of trees would have been noticed sooner if P. ramorum were native.

Where P. ramorum originated remains unclear, but most researchers feel Asia is the most likely, since many of the hosts of P. ramorum originated there.[32] Since certain climates are best suited to P. ramorum, the most likely sources are the southern Himalayas, Tibet, or Yunnan province.[33]

Species of Phytophthora have been shown to have evolved by the interspecific hybridization of two different species from the genus.[34] When a species is introduced into a new environment, it causes episodic selection. The invading species is exposed to other resident taxa, and hybridization may occur to produce a new species. If these hybrids are successful, they may outcompete their parent species.[35] Thus, P. ramorum is possibly a hybrid between two species. Nevertheless, the draft genome of P. ramorum did not show any evidence of recent hybridization.[36]

P. ramorum may be native to the United States. Infection rates could have previously been at a low level, but changes in the environment caused a change to the population structure.[37] Alternatively, the symptoms of P. ramorum may have been mistaken for that of other pathogens. When SOD first appeared in the United States, many other pathogens and conditions were blamed before P. ramorum was found to be the causal agent.[38] With many of the most seriously affected plants being in the forest, the likelihood of seeing diseased trees is also low.

Besides the quick and significant loss of symbiotic host species, since Sudden Oak Death is classified as a stem-girdling disease, which is proven to cause a massive reduction in mycorrhizae soil biomass, the amount of phosphorus and micronutrients that mycorrhizae are able to absorb is greatly reduced in soils occupied by trees with Sudden Oak Death.[44] Another significant environmental impact of P. ramorum. is its tendency to result in large deposits of dry, woody debris in areas prone to forest fires, making them even more difficult to contain.[45] Indeed, hotspots of Sudden Oak Death are “unmanageable” for fire crews, and there is increasing evidence to suggest that Sudden Oak Death plays a large role in a forest’s susceptibility to fire. On the other side of the spectrum are the significant economic impacts of P. ramorum., which are difficult to assess, but the most obvious of which is the reduction to property values of real estate containing Oak Trees, as Oaks in particular tend to raise the property value of the plots they inhabit.[46] Additionally, several U.S industries have suffered dearly due to the spread of Sudden Oak Death, including the ornamental plant, spice, and composting industries, especially in the state of California.[47]

Early detection of P. ramorum is essential for its control. On an individual-tree basis, preventive treatments, which are more effective than therapeutic treatments,[48] depend on knowledge of the pathogen's movement through the landscape to know when it is nearing prized trees. On the landscape level, P. ramorum’s fast and often undetectable movement means that any treatment hoping to slow its spread must happen very early in the development of an infestation. Since P. ramorum’s discovery, researchers have been working on the development of early detection methods on scales ranging from diagnosis in individual infected plants to landscape-level detection efforts involving large numbers of people.

Detecting the presence of Phytophthora species requires laboratory confirmation. The traditional method of culturing is on a growth medium selective against fungi (and, in some cases, against other oomycetes such as Pythium species). Host material is removed from the leading edge of a plant tissue canker caused by the pathogen; resulting growth is examined under a microscope to confirm the unique morphology of P. ramorum. Successful isolation of the pathogen often depends on the type of host tissue and the time of year that detection is attempted.[49]

Because of these difficulties, researchers have developed some other approaches for identifying P. ramorum. The enzyme-linked immunosorbent assay test can be the first step in nonculture methods of identifying P. ramorum, but it can only be a first step, because it detects the presence of proteins that are produced by all Phytophthora species. In other words, it can identify to the genus level, but not to the species level. ELISA tests can process large numbers of samples at once, so researchers often use it to screen out likely positive samples from those that are not when the total number of samples is very large.[49] Some manufacturers produce small-scale ELISA “field kits” that the homeowner can use to determine if plant tissue is infected by Phytophthora.

Researchers have also developed numerous molecular techniques for P. ramorum identification. These include amplifying DNA sequences in the internal transcribed spacer region of the P. ramorum genome (ITS polymerase chain reaction, or ITS PCR); real-time PCR, in which DNA abundance is measured in real time during the PCR reaction, using dyes or probes such as SBYR-Green or TaqMan; multiplex PCR, which amplifies more than one region of DNA at the same time; and single strand conformation polymorphism (SSCP), which uses the ITS DNA sequence amplified by the PCR reaction to differentiate Phytophthora species according to their differential movement through a gel.[49]

Additionally, researchers have begun using features of the DNA sequence of P. ramorum to pinpoint the minuscule differences of separate P. ramorum isolates from each other. Two techniques for doing this are amplified fragment length polymorphism, which through comparing differences between various fragments in the sequence has enabled researchers to differentiate correctly between EU and U.S. isolates,[49] and the examination of microsatellites, which are areas on the sequence featuring repeating base pairs. When P. ramorum propagules arrive in a new geographic location and establish colonies, these microsatellites begin to display mutation in a relatively short time, and they mutate in a stepwise fashion. Based on this, researchers in California have been able to construct trees, based on microsatellite analyses of isolates collected from around the state, that trace the movement of P. ramorum from two likely initial points of establishment in Marin and Santa Cruz Counties and out to subsequent points.[50]

Early detection of P. ramorum on a landscape scale begins with the observation of symptoms on individual plants (and/or detecting P. ramorum propagules in watercourses; see below). Systematic ground-based monitoring has been difficult within the range of P. ramorum because most infected trees stand on a complex mosaic of lands with various ownerships. In some areas, targeted ground-based surveys have been conducted in areas of heavy recreation or visitor use such as parks, trailheads, and boat ramps. In California, when conducting ground-based detection, looking for symptoms on bay laurel is the most effective strategy, since P. ramorum infection of true oaks and tanoaks is almost always highly associated with bay laurel, the main epidemiological springboard for the pathogen.[51][52][53] Moreover, on many sites in California (though not all), P. ramorum can typically be detected from infected bay laurel tissues via culturing techniques year-round; this is not the case for most other hosts, nor is it the case in Oregon, where tanoak is the most reliable host.[54]

As part of a nationwide USDA program, a ground-based detection survey was implemented from 2003 to 2006 in 39 U.S. states to determine whether the pathogen was established outside the West Coast areas already known to be infested. Sampling areas were stratified by environmental variables likely to be conducive to pathogen growth and by proximity to possible points of inoculum introduction such as nurseries. Samples were collected along transects established in potentially susceptible forests or outside the perimeters of nurseries. The only positive samples were collected in California, confirming that P. ramorum was not yet established in the environment outside the West Coast.[55]

Aerial surveying has proven useful for detection of P. ramorum infestations across large landscapes, although it is not as “early” a technique as some others because it depends on spotting dead tanoak crowns from fixed-wing aircraft. Sophisticated GPS and sketch-mapping technology enable spotters to mark the locations of dead trees so that ground crews can return to the area to sample from nearby vegetation.[56]

Detection of P. ramorum in watercourses has emerged as the earliest of early detection methods. This technique employs pear or rhododendron baits suspended in the watercourse using ropes, buckets, mesh bags, or other similar devices. If plants in the watershed are infected with P. ramorum, zoospores of the pathogen (as well as other Phytophthora spp.) are likely present in adjacent waterways. Under conducive weather conditions, the zoospores are attracted to the baits and infect them, causing lesions that can be isolated to culture the pathogen or analyzed via PCR assay.[57][58] This method has detected P. ramorum at scales ranging from small, intermittent seasonal drainages to the Garcia, Chetco, and South Fork Eel Rivers in California and Oregon (144, 352, and 689 mi2 drainage areas, respectively). It can detect the existence of infected plants in watersheds before any mortality from the infections becomes evident. Of course, it cannot detect the exact locations of those infected plants: at the first sign of P. ramorum propagules in the stream, crews must scour the watershed using all available means to find symptomatic vegetation.

A less technical means of detecting P. ramorum at the landscape level involves engaging local landowners across the landscape in the search. Many local county agriculture departments and University of California Cooperative Extension offices in California have been able to keep track of the distribution of the pathogen in their regions through reports and samples brought to them by the public. In 2008, the Garbelotto Laboratory at University of California, Berkeley, along with local collaborators, hosted a series of educational events, called "SOD Blitzes", designed to give local landowners basic information about P. ramorum and how to identify its symptoms; each participant was provided with a sampling kit, sampled a certain number of trees on his or her property, and returned the samples to the lab for analysis. This kind of citizen science hopefully can help generate an improved map of P. ramorum distribution in the areas where the workshops are held.

Since regulators in 2004 discovered that P. ramorum. had spread nationwide to a number of hosts, proactive inspections of agricultural shipments have been shown to help reduce the risk of infestations of Sudden Oak Death in the U.S.[59] Moreover, the USDA’s APHIS specifically plans to stop the spread of Sudden Oak Death by continuing their public outreach program and by passing regulations on the transfer of agricultural products that might be a disease vector for P. ramorum. Indeed, In Oregon and California, the USDA has successfully regulated the stock of potential host plants at nurseries to “starve” the disease of potential plant hosts.[60] Furthermore, When managing P. ramorum. In nurseries, it is important to consider that nursery personnel are often required to visit sites in the field such as greenhouses, fields, and other nurseries.[61] Therefore, a number of biosecurity measures must be taken to ensure that Sudden Oak Death is not unintentionally transferred to one’s nursery including driving vehicles only on paved, concrete, or gravel areas at inspection sites in order avoid contact with soil organic matter that could pose a potential threat as a disease vector.[62]

The course that P. ramorum management should take depends on a number of factors, including the scale of the landscape upon which one hopes to manage it. Management of P. ramorum has been undertaken at the landscape/ regional level in Oregon in the form of a campaign to completely eradicate the pathogen from the forests in which it has been found (mostly private, but also USDA Forest Service and USDI Bureau of Land Management ownership).[63][64][65] The eradication campaign involves vigorous early detection by airplane and watercourse monitoring, a U.S. Department of Agriculture Animal and Plant Health Inspection Service (USDA APHIS) and Oregon Department of Agriculture-led quarantine to prevent movement of host materials out of the area where infected trees are found, and immediate removal of P. ramorum host vegetation, symptomatic or not, within a 300-foot (91 m) buffer around each infected tree.

The Oregon eradication effort, which began near the town of Brookings in southwest Oregon in 2001, has adapted its management efforts over the years in response to new information about P. ramorum. For example, after inoculation trials of various tree species more clearly delineated which hosts are susceptible, the Oregon cooperators began leaving nonhost species such as Douglas fir and red alder on site. In another example, after finding that a small percentage of tanoak stumps that were resprouting on the host removal sites were infected with the pathogen—whether these infections were systemic or reached the sprouts from the surrounding environment is unknown—the cooperators began pretreating trees with very small, targeted amounts of herbicide to kill the root systems of infected tanoaks before cutting them down. The effort has been successful in that while it has not yet completely eradicated the pathogen from Oregon forests, the epidemic in Oregon has not taken the explosive course that it has in California forests.[66]

California, though, faces significant obstacles that preclude it from mounting the same kind of eradication effort. For one thing, the organism was too well established in forests in the Santa Cruz and San Francisco Bay areas by the time the cause of sudden oak death was discovered to enable any eradication effort to succeed. Even in still relatively uninfested areas of the north coast and southern Big Sur, regionally coordinated efforts to manage the pathogen face huge challenges of leadership, coordination, and funding. Nevertheless, land managers are still working to coordinate efforts between states, counties, and agencies to provide P. ramorum management in a more comprehensive manner.

Several options exist for landowners who want to limit the impacts of SOD death on their properties. None of these options is foolproof, guaranteed to eradicate P. ramorum, or guaranteed to prevent a tree from becoming infected. Some are still in the initial stage of testing. Nevertheless, when used thoughtfully and thoroughly, some of the treatments do improve the likelihood of either slowing the spread of the pathogen or of limiting its impacts on trees or stands of trees. Assuming that the landowner has correctly identified the host tree(s) and symptom(s), has submitted a sample to a local authority to send to an approved laboratory for testing, and has received confirmation that the tree(s) are indeed infected with P. ramorum—or, alternatively, assuming that the landowner knows that P. ramorum-infected trees are nearby and wants to protect the resources on his or her property—he or she can attempt control by cultural (individual-tree), chemical, or silvicultural (stand-level) means.

The best evidence that cultural techniques might help protect trees against P. ramorum comes from research that has established a correlation between disease risk in coast live oak trees and the trees’ proximity to bay laurel.[23] In particular, this research found that bay laurel trees growing within 5 m of the trunk of an oak tree were the best predictors of disease risk. This implies that strategic removal of bay laurel trees near coast live oaks might decrease the risk of oak infection. Wholesale removal of bay laurel trees would not be warranted, since the bay laurels close to the oak trees appear to provide the greatest risk factor. Whether the same pattern is true for other oaks or tanoaks has yet to be established. Research on this subject has been started for tanoak, but any eventual cultural recommendations will be more complicated, because tanoak twigs also serve as sources of P. ramorum inoculum.

An initially promising treatment for preventing infection of individual oak and tanoak trees—not for curing an already established infection—is a phosphonate fungicide marketed under the trade name Agri-fos. Phosphonate is a neutralized form of phosphorous acid that works not by direct antagonism of Phytophthora, but by stimulating various kinds of immune responses on the part of the tree.[48] It is mostly environmentally benign if not applied to nontarget plants and can be applied either as an injection into the tree stem or as a spray to the bole. When applying Agri-fos as a spray, it must be combined with an organosilicate surfactant, Pentra-bark, to enable the product to adhere to the tree trunk long enough to be absorbed by the tree. Agri-fos has been very effective in preventing tree infections, but it must be applied when visible symptoms of P. ramorum on other trees in the immediate neighborhood are still relatively distant; otherwise, the tree to be treated likely is already infected, but visible symptoms have not yet developed (especially true for tanoak). However, later extensive field trials showed that phosphonate (also called phosphite) had little if any beneficial effect.[67]

Trials of silvicultural methods for treating P. ramorum began in Humboldt County in northwest coastal California in 2006. The trials have taken place on a variety of infested properties both private and public and have generally focused on varying levels and kinds of host removal. The largest (50 acres (200,000 m²)) and most replicated trials have involved removal of tanoak and bay laurel by chainsaw throughout the infested stand, both with and without subsequent underburning designed to eliminate small seedlings and infested leaf litter.[68] Other treatments included host removal in a modified “shaded fuelbreak” design in which all bay laurel is removed, but not all tanoaks; bay and tanoak removal using herbicides; and removal of bay laurel alone. The results of these treatments are still being monitored, but repeated sampling has so far detected only very small amounts of P. ramorum in the soil or on vegetation in the treated sites.

Research and development in managing P. ramorum in nursery settings extends from P. ramorum in the individual plant, to P. ramorum in the nursery environment, to the pathogen's movement across state and national borders in infected plants.

An array of studies have tested the curative and protective effects of various chemical compounds against P. ramorum in plants valued as ornamentals or Christmas trees. Many studies have focused on the four main ornamental hosts of P. ramorum (Rhododendron, Camellia, Viburnum, and Pieris). Several effective compounds have been found; some of the most effective include mefenoxam, metalaxyl, dimethomorph, and fenamidone. Many of these studies have converged upon the following conclusions: chemical compounds are, in general, more effective as preventives than as curatives; when used preventively, chemical compounds must be reapplied at various intervals; and chemical compounds can mask the symptoms of P. ramorum infection in the host plant, potentially interfering with inspections for quarantine efforts. In general, these compounds suppress but do not eradicate the pathogen, and some researchers are concerned that with repeated use the pathogen may become resistant to them. These studies and conclusions are summarized by Kliejunas.[69]

Another area of research and evolving practice deals with eliminating P. ramorum from nursery environments in which it is established to prevent human-mediated pathogen movement within the ornamental plant trade. One way of approaching this is through a robust quarantine and inspection program, which the various federal and state regulatory agencies have implemented. Under the federal P. ramorum quarantine program implemented by USDA APHIS, nurseries in California, Oregon, and Washington are regulated and must participate in an annual inspection regimen; nurseries in the 14 infested counties in coastal California, plus the limited infested area in Curry County, Oregon, must participate in a more stringent inspection schedule when shipping out of this area.[70]

Much of the research into disinfesting nurseries has focused on the voluntary best management practices (BMPs) that nurseries can implement to prevent P. ramorum’s introduction into the nursery and movement from plant to plant. In 2008, a group of nursery industry organizations issued a list of BMPs that includes subsections on pest prevention/management, training, internal/external monitoring/audits, records/traceability, and documentation. The document includes such specific recommendations as “Avoid overhead irrigation of high-risk plants”; “After every crop rotation, disinfect propagation mist beds, sorting area, cutting benches, machines and tools to minimize the spread or introduction of pathogens”; and “Nursery personnel should attend one or more P. ramorum trainings conducted by qualified personnel or document self-training”.[71][72]

Research on control of P. ramorum in nurseries has also focused on disinfesting irrigation water containing P. ramorum inoculum. Irrigation water can become infested from bay trees in the forest (if the irrigation source is a stream), from bay trees overhanging irrigation ponds, from runoff from infested forests,[73] or from recirculated irrigation water.[74] Experiments in Germany with three types of filters—slow sand filters, lava filters, and constructed wetlands—showed that the first two removed P. ramorum from the irrigation water completely, while 37% of the post-treatment water samples from the constructed wetland still contained P. ramorum.[75]

Since P. ramorum can persist for an undetermined period of time within the soil profile, management programs in nurseries should also deal with delineating the pathogen's distribution in nursery soil and eliminating it from infested areas. A variety of chemical options has been tested for soil disinfestation, including such chemicals as chloropicrin, metham sodium, iodomethane, and dazomet. Lab tests indicated that all of these chemicals were effective when applied to infested soil in glass jars. Additionally, tests on volunteer nurseries with infested soil demonstrated that dazomet (trade name Basamid) fumigation followed by a 14-day tarping period successfully removed P. ramorum from the soil profile.[76] Other soil disinfestation practices under investigation, or in which interest has been expressed, include steam sterilization, solarization, and paving of infested areas.

One of the most important aspects of P. ramorum control involves interrupting the human-mediated movement of the pathogen by ensuring that infested materials do not move from location to location. While enforceable quarantines perform part of this function, basic cleanliness when working or recreating in infested areas is also important. In most cases, cleanliness practices involve ridding potentially infested surfaces—such as shoes, vehicles, and pets—of foliage and mud before leaving the infested area. The demands of implementing these practices become more complex when large numbers of people are working in infested areas, as in construction, timber harvesting, or wildfire suppression. The California Department of Forestry and Fire Protection and USDA Forest Service have implemented guidelines and mitigation requirements for the latter two situations; basic information about cleanliness in P. ramorum-infested areas can be found at the California Oak Mortality Task Force web site (www.suddenoakdeath.org) under the “Treatment and Management” section (subsection “Sanitation and Reducing Spread”).