We conducted four experiments to evaluate the effectiveness of phosphonate application to tanoak (Notholithocarpus densiflorus (Hook. & Arn.) Manos, Cannon & S.H.Oh) forests in south-western Oregon: (1) aerial application to forest stands; (2) trunk injection; (3) foliar spray of potted seedlings; and (4) foliar spray of stump sprouts. We compared aerial spray treatments: (1) no treatment (unsprayed); (2) low-dose (17.35 kg a.i. ha^-1); and (3) high dose (34.5 kg a.i. ha^-1), applied by helicopter in a carrier volume of 188 L ha^-1 to 4-ha treatment plots. Treatments were applied in November 2007, in May 2008, and in December 2008 and May 2009 (double treatment). At the same time as the aerial application we injected phosphonate into the trunk of nearby mature tanoak trees at the standard label rates of 0.43 g a.i. cm-dbh^-1. We used three different biological assays to measure uptake of phosphonate: (1) canopy twig dip in zoospore suspension; (2) in situ bole inoculation with Phytophthora gonapodyides (Petersen) Buisman; and (3) laboratory inoculation of log bolts with Phytophthora ramorum S. Werres, A.W.A.M. de Cock & W.A. Man in ‘t Veld and P. gonapodyides. We also simulated an aerial spray of potted seedlings, comparing an untreated control, a low dose (2.9 kg a.i. ha^-1 applied in 935 L spray solution ha^-1), and a high dose (17.35 kg a.i. ha^-1applied in 187 L spray solution ha^-1).

Aerial spray with phosphonate consistently resulted in smaller bole lesions on trees challenge inoculated with Phytophthora gonapodyides in situ and in logs inoculated with P. ramorum. This effect persisted for 18 months post treatment. Results from detached canopy twig assays were variable and showed only small treatment effects. Trunk injection consistently reduced bole lesion size in trees and logs, but gave inconsistent results in the canopy twig assay, possibly due to the twig assay methodology. In the spring and autumn trunk injection treatment, canopy twig lesion length was reduced by 32 percent compared to untreated controls, indicating that trunk-injected phosphonate was mobilised to the outer twigs of the tree crown. Trunk injection with phosphonate resulted in a greater reduction in bole lesion area than aerial spray. Spray application of phosphonate to tanoak seedlings did not protect them from infection when exposed to artificial or natural inoculum of P. ramorum. Foliar application of phosphonate to stump sprouts reduced lesion length by 44% of control in a shoot-dip assay three months post-treatment.

Sudden Oak Death (SOD) disease caused by Phytophthora ramorum Werres, de Cock & Man in ‘t Veld was first discovered in Oregon forests in July 2001. Since then, an interagency team has been attempting to eradicate the pathogen though a programme of early detection (aerial and ground surveys, stream baiting) and destruction (herbicide treatment, felling and burning) of infected and nearby host plants, which has evolved over time. Post-treatment monitoring has indicated that although the disease has been eliminated from many of the sites and spread of inoculum may have been reduced, the disease continues to spread slowly. The quarantine area has expanded from 23 km² in 2001 to 420 km² in 2009. We attribute continued spread of the disease to the slow development of recognisable symptoms and to delays in treatment application associated with inconsistencies in funding.

Recent findings, policy, regulation, and management relating to tree disease caused by Phytophthora species in wildlands and nurseries of North and South America are reviewed. These include the isolation of Phytophthora alni uniformis Brasier & S.A.Kirk in Alaska, and detection of population shifts in NA1, NA2 and EU1 clonal lineages of Phytophthora ramorum Werres, de Cock, & In’t Veld. The dissemination of Phytophthora ramorum from infested nurseries in water run-off presents a challenge for forest and plantation management. In the United States, forest Phytophthoras are viewed as a biosecurity threat and are monitored by the United States Department of Agriculture, Animal and Plant Health Inspection Service but tools designed to protect forests and nurseries need refinement. In Mexico, Phytophthora cinnamomi Rand is recognised as a threat to Quercus forests. The Phytophthora pinifolia Alv. Durán, Gryzenh. & M.J.Wingf. epidemic in Chilean Pinus radiata D.Don plantations has receded. Work with Phytophthora austrocedrae Gresl. & E.M.Hansen continues in Argentina.

Phytophthora cinnamomi Rands is involved in the decline and mortality of Quercus suber L. and Quercus ilex L. in Southern Europe, in particular in Portugal and Spain. The presence and spread of P. cinnamomi in these regions is a severe threat to these oak ecosystems leading to expectable severe consequences for the production of cork and acorns in the near future.

Molecular mechanisms underlying oomycete-host interactions are poorly understood. As a first step to identify transcripts involved in the Quercus suber - Phytophthora cinnamomi interaction, we applied complementary deoxyribonucleic acid-amplified fragment length polymorphism (cDNA-AFLP) methodology to cork oak seedlings infected with zoospores or mycelium of P. cinnamomi.

Forty-four Quercus suber genes that were differentially expressed when exposed to Phytophthora cinnamomi were selected and sequenced. Several of these genes were fully sequenced and the deduced aminoacid sequences showed consistent homology with proteins involved in the defence mechanism of other plant species. These findings led to the design of a simplified hypothetical model that illustrates the initial events of the interaction between Q. suber and P. cinnamomi.

Since the discovery of Phytophthora ramorum Werres, De Cock & Man In’t Veld in south-western Oregon forests in 2001, newly infected areas are detected each year. Yet, there are still gaps in our knowledge about how the pathogen spreads or where new infections come from. Our study aims to track the spread of P. ramorum in Oregon forests and within individual trees using DNA fingerprinting. We examined the genetic diversity of 1589 samples collected from 2001 to 2008 on several temporal and spatial scales. We identified 60 novel multilocus genotypes (MGs) with 9 to 44 MGs found in each year. While the majority of MGs were present in very low numbers (< 1%) one MG was dominant in all years representing 39 to 73% of isolates. The dominance of one MG was not attributable to higher fitness by any measure examined. Frequency of the dominant MG declined with time. This supports the hypothesis that it represents the founder genotype, and is being progressively diluted by new genotypes that arise through mutation. Our data also demonstrate that P. ramorum populations in Oregon forest are genetically distinct from those in nurseries and in California forests.

The Fitzgerald River National Park (FRNP) is an International Biosphere Reserve on the south coast of Western Australia. The National Park is recognised for its high biodiversity with over 2000 plant species (including many endemics), threatened ecological communities and rare fauna. In contrast with many other areas of high biodiversity value in the region, the FRNP remains largely free of the introduced plant pathogen, Phytophthora cinnamomi Rands, with less than 0.1% of the Park currently infested.

Phytophthora plurivora T.Jung & T.I.Burgess and P. pseudosyringae T.Jung & Delatour exhibit different potential to colonise host plants. In order to clarify whether P. plurivora is more aggressive than P. pseudosyringae simply because of its faster growth and sporulation, a method for root infection was developed ensuring equal growth and sporulation for both pathogens during infection of F. sylvatica L. seedlings. Infection with P. plurivora strongly reduced CO₂ uptake of seedlings and five out of eight seedlings died by the end of the experiment. In contrast, P. pseudosyringae did not alter physiology of infected plants and no mortality was recorded. The DNA contents of roots infected by either pathogen were similar at the end of the experiment, which indicated that a similar amount of fungal material was present for each species. This indicates that the greater aggressiveness of P. plurivora in comparison to P. pseudosyringae cannot be explained by its faster growth compared to P. pseudosyringae at a given temperature.

Phytophthora ramorum Werres, De Cock & Man in‘t Veld, causal agent of sudden oak death (SOD) and ramorum blight, has been detected in container-grown plants, soil and irrigation ponds in various United States’ nurseries. Phytophthora ramorum has also been detected in runoff water from some nurseries and adjoining streams. Despite emergency regulatory actions, there is concern that P. ramorum infected nursery stock may further spread the disease in the United States of America (USA), particularly to previously unaffected wildlands. If established in the south-eastern USA, it could cause damage similar to that occurring in the coastal forests of California and Oregon. To develop solutions for nurseries that trade plants susceptible to P. ramorum, a quarantine nursery was established in Marin County, California, to investigate pathogen eradication and disease management. More than four years of collaborative efforts between the California Department of Food and Agriculture, California county Agriculture Commissioners, the California Oak Mortality Task Force, US National Plant Board, United States Department of Agriculture Animal and Plant Health Inspection Service Plant Protection and Quarantine and nursery industry resulted in locating a suitable site for developing the National Ornamentals Research Site (NORS) at the Dominican University of California (DUC). Funding to set-up and run the research nursery was awarded in 2008 through congressionally approved, Farm-bill (Section 10201) funding. The site is designed to perform research on quarantine pests and pathogens while safeguarding plant health and the surrounding natural environment. Research initiatives on P. ramorum have commenced at the NORS-DUC. Research grants are awarded to undertake research at the NORS-DUC and proposals can be submitted through www.dominican.edu/norsduc.

Using various microscopic techniques, Giesbrecht et al. (pp. S89-S100) show that nearly all tanoak bark tissues are capable of being colonised by Phytophthora ramorum that this host responds to infection with callose deposition, tissue discoloration, and cell collapse; and that elicitins are present in cell walls of hyphae in infected bark tissues.

Colonisation of Notholithocarpus densiflorus (Hook. and Arn.) Rehder tissues by Phytophthora ramorum Werres, De Cock & Man in’t Veld is not well understood. The pathogen is able to colonise nearly all tissues of this host but it is unclear how a tree is ultimately killed. In this research, P. ramorum infected N. densiflorus bark tissues were examined using various microscopic techniques to better understand the role of bark infection in killing a tree. Host responses to infection were detected by histological methods in conjunction with examining P. ramorum colonisation. Results of this work indicate that the pathogen can colonise nearly all N. densiflorus bark tissues but that phellogen and parenchyma of the inner bark are the most frequently and densely colonised. Pathogen specific elicitin labelling of P. ramorum-infected N. densiflorus sprouts caused hyphal cell walls to fluoresce in plant tissues, allowing specific identification of hyphae. Findings of this research show that nearly all bark tissues are capable of being colonised, that this host responds to infection with callose deposition, tissue discoloration, and cell collapse; and that elicitins are present in cell walls of hyphae in infected bark tissues

To evaluate the number of stream sample sites needed to effectively survey a given stream network for species of Phytophthora, two stream networks, Davidson River and Cathey’s Creek, in western North Carolina (USA) were studied. One-litre water samples were collected from the terminal drainage points and most of the tributaries in each stream network and filtered through polycarbonate membrane filters with 3-µm pores. Ten taxa of Phytophthora were detected in the two stream networks: six species-P. cinnamomi, P. citricola, P. citrophthora, P. gonapodyides, P. heveae, and P. pseudosyringae-and four distinct groups of isolates based on morphological and molecular characters. A total of nine taxa were detected in the Davidson River network on two sample dates in 2007, and five of these taxa eventually were found downstream at the drainage point. In the Cathey’s Creek network, a total of seven taxa were found on two sample dates in 2008, and five of these taxa eventually were found at the drainage point. Even though all the taxa found within a stream network were not detected at the terminal drainage point, all of the taxa in the network that represented at least 10% of the total population were detected at the drainage point. More intensive sampling throughout a stream network may be necessary to detect a species with a low population density.

Issues concerning Phytophthora diseases in woody plants and Phytophthora diversity were overlooked in the Czech Republic until 2000. The investigation of a number of important problems concerning Phytophthora diseases of woody plants was initiated in the past decade, including problems related to alder decline caused by Phytophthora alni, the identification of the spectrum of Phytophthora species affecting forest and amenity trees, and Phytophthora spp. diversity in ericaceous plants (especially rhododendron) as an infection reservoir in nurseries and ornamental greenery.

Between 2006 and 2010, parasitic oomycetes were isolated from more than 20 host taxa, particularly from Rhododendron spp., Alnus spp., Fagus sylvatica, Fraxinus excelsior, Quercus spp., Acer spp., and Tilia cordata. In total, more than 360 isolates of pythiaceous oomycetes have been acquired and deposited in our culture collection. Sixteen Phytophthora species have been found thus far: P. alni, P. cactorum, P. cambivora, P. cinnamomi, P. citrophthora, P. gallica, P. gregata, P. gonapodyides, P. megasperma, P. multivora, P. taxon oaksoil, P. plurivora, P. polonica, P. ramorum, P. taxon raspberry, and P. taxon salixsoil. Phytophthora alni, P. plurivora, and P. cactorum are the most frequently detected species in the country.

Phytophthora-induced alder decline is the most important of the problems caused by Phytophthora species in the Czech Republic, and it has become a devastating epidemic. To date, this disease has been detected in approximately 300 sites throughout the Czech Republic. The severity of the disease and its impact on riparian alder stands in the western part of the Czech Republic is comparable to the situations in Great Britain, northeastern France and Bavaria.

Various Phytophthora species were recovered from tanoak trees, tanoak canopy drip, soils, and streams, which were sampled as part of a larger survey and management effort aimed at limiting the spread of Phytophthora ramorum Werres, De Cock & Man in’t Veld (the causal agent of sudden oak death) in an epidemic area encompassing native forest and urbanised forest areas in south-western Oregon. Environmental samples were analysed by baiting with either green pear fruits or rhododendron and tanoak leaves. Tanoak bark samples and baits from environmental samples were plated on media semi-selective for the isolation of Phytophthora spp. After incidence of P. ramorum growing on isolation plates was recorded, other Phytophthora species growing on the isolation plates were sub-cultured for identification. DNA sequencing was used to identify the unknown Phytophthora species. A total of seventeen Phytophthora species and one Halophytophthora species were identified across all substrates. Over an 8-year period, P. ramorum was detected in cultures from 41% of samples from over 1600 diseased tanoak trees, while other Phytophthora species were detected in 14% of these samples. Of 5189 tanoak canopy drip samples collected over a 4-year period, Phytophthora species other than P. ramorum were detected in 106 samples (2.0%). Of 5967 soil samples baited over an 8-year period, Phytophthora species other than P. ramorum were detected in 71 samples (1.2%). Phytophthora ramorum was detected in ca. 10% of 642 stream samples over a 3-year period, and other Phytophthora species were detected in ca. 86% of these stream samples.

In spring, 2009, the Oregon State University Plant Clinic received reports of severe defoliation of California wax-myrtle plants (Morella californica (Cham. & Schlecht.) Wilbur) on the north-central coast of Oregon, in western North America. Isolations from necrotic leaf tissue yielded an organism which, from morphological characteristics and a genus-specific enzyme-linked immunosorbent assay, was identified as a species of the genus Phytophthora. Total DNA was extracted from hyphal tip-derived cultures from leaf or twig tissue and subjected to a polymerase chain reaction process aimed at species identification. Sequencing techniques revealed a ≥99.7% match with P. syringae although our isolates differed from published descriptions of this species in some respects. Inoculation of healthy plants with cultured mycelium resulted in symptoms similar to those originally observed in the field, and reisolations produced colonies of the same organism. This is the first report of a species of Phytophthora causing disease in M. californica. Leaf blight of California wax-myrtle is now widespread on the north-central coast of Oregon. This disease is serious and is adversely affecting the health of this native understory species which is frequently used for amenity plantings.

Phosphite (PO₃^3-) is well known for its ability to induce pathogen and host-mediated resistance to Phytophthora spp. in a wide range of plants. This review addresses how phosphite moves in citrus trees, fate of phosphite when applied to soil, how phosphite controls Phytophthora infection of citrus tissues, phosphites as fungicides for control of Phytophthora diseases in citrus and their applicability for management of Phytophthora species on forest trees. Experimental data from citrus is presented to illustrate these properties. As an example, phosphite is rapidly taken up by leaves and highly systemic enabling phosphite applied to the tree canopy to move to fruit and provide protection against citrus brown rot of fruit caused by Phytophthora palmivora (Butler) Butler for several months after application. Foliar-applied phosphite also moves readily to the trunk and roots for control of collar and root rot caused by P. nicotianae Breda de Haan for several weeks after application. Soil application of phosphite is more effective for control of root rot than foliar applications due to higher concentrations of phosphite in roots, but soil-applied phosphite may be oxidised to phosphate by soil bacteria before root uptake. Because phosphite moves readily to metabolically active root-, shoot- and reproductive tissues, foliar-, stem- or soil applications are highly effective for the long-term, above- and below-ground protection of trees against Phytophthora infection.

Inconsistent recovery of Phytophthora cinnamomi Rands from forest soils has been documented in climates with seasonally wet and dry periods. Phytophthora cinnamomi can be recovered when soils are moist or wet but can be difficult to recover from dry soil. Recovery may be complicated further by the physical location of P. cinnamomi in soil. Our objectives were: (1) to investigate factors that might affect recovery of P. cinnamomi from dry soil-i.e. length of time remoistened soil was stored, storage temperature, and presence of host tissue; and (2) to determine the spatial distribution of this organism in forest soil. Recovery of P. cinnamomi from soil samples that had been dried and then remoistened was very rare (1/90 samples); therefore, additional studies are needed to better understand the factors that affect recovery of P. cinnamomi from, and the viability of propagules present in, dry soil. Spatial distribution of P. cinnamomi was examined using three grids at each of three forest sites. Horizontal distribution was determined at 30-cm intervals along the soil surface of each grid. Phytophthora cinnamomi was found in soil samples in seven of the nine grids and was recovered in 14 to 97% of the samples from those grids. Vertical distribution at standard depths (0, 6, 23, 40, 57, and 74 cm) was studied in 13 soil cores collected at the three forest sites. Phytophthora cinnamomi was present in 85% of vertical cores, occurred more frequently near the soil surface than at any other depth, was detected up to 74 cm below the surface, and often was not contiguous in a core.

Phytophthora spp. de Bary are being increasingly recognised as pathogens that cause tree death, without necessarily having any clear understanding of how this happens. Suggested mechanisms include:

extensive fine-root necrosis especially on wet or drought prone sites, leading to reduced water uptake, crown decline and death, e.g. Phytophthora quercina T. Jung infection of European oaks;
root and stem cankers resulting from phloem invasion and cambial death, leading to death of basal buds and carbon starvation of the root system, e.g. Phytophthora alni Brasier & S.A. Kirk infection of alders;
xylem invasion, leading to reduced conduction, hydraulic failure and death, e.g. Phytophthora ramorum Werres, De Cock & Man in 't Veld infection of tanoaks; and
hormonal imbalance and/or damage from toxins, e.g. Phytophthora cinnamomi Rands infection of eucalypts.

These possible mechanisms are reviewed, together with different hypotheses of why trees die, and the predisposing environmental stresses that contribute to tree death. Extensive xylem invasion provides a mechanistic explanation of how death occurs, but is the least frequently reported symptom of Phytophthora infection.

Phytophthora kernoviae Brasier, Beales & Kirk, recently found in the UK and New Zealand, is a pathogen of more than 30 host species. It is not known to produce chlamydospores, but is homothallic and produces abundant oospores and sporangia.

This study was conducted to examine long-term survival of oospores, sporangia, and mycelium buried in sand at different temperatures. Viability of oospores buried in sand kept at 4, 10, 20 or 30 ^oC was assessed by staining with tetrazolium bromide solution. After 1 year at these temperatures, 82, 81, 79, and 58% of oospores of a New Zealand isolate respectively had survived. Corresponding values for an English isolate were 86, 75, 82, and 78%. Necrosis was observed on Rhododendron L. leaf discs exposed to oospores that had been buried for 1 year at temperatures below 30 ^oC. Oospores exposed for 1 and 6 h at 50 ^oC and 24 h at 40 and 50 ^oC were less viable than controls and did not germinate.Sporangia or mycelium of two New Zealand and two English isolates introduced to moist sand and kept at different temperatures showed a population decline within 1 week. Numbers of colony-forming units then remained at a low but steady level over time. Sporangia and oospores were formed at 4, 10 and 20 ^oC but not at 30 ^oC. The ability of P. kernoviae to persist in sand for long periods of time at different temperatures is likely to be one of the factors determining the rate of spread of this pathogen.

With the rapidly growing international trade in plants and ongoing impacts of climate change, impacts of plant pathogens in the genus Phytophthora are increasing, threatening the biodiversity and sustainability of European forest ecosystems. Through the European Cooperation in Science and Technology (COST) framework Action FP0801, scientists and disease-control experts are working on phytophthora in forest ecosystems with the overall aim of increasing understanding of the biology and ecology of Phytophthora species with potential to cause damage to European forestry. This knowledge will be used in the development of effective control and management protocols for the problems caused. Outcomes of the Action will be promoted in an effort to increase knowledge and awareness of the problem by disseminating information to end-users and authorities in the forestry sector, and to the general public. Four interrelated working groups have been established to (i) examine the ways in which Phytophthora species spread into and within Europe; (ii) determine how phytophthoras kill woody plants and elucidate mechanisms for host resistance; (iii) disseminate state-of-the-art rapid molecular diagnostic techniques, and (iv) seek sustainable protocols for management and control of the diseases. The project is expected to increase understanding of threats to forest ecosystems by phytophthora, improve the ability to rapidly detect phytophthora in environmental samples, and provide sustainable management solutions to the diseases caused by these destructive organisms.

Many algicides are registered for use in potable water, irrigation ponds, and swimming pools. Because oomycetes, including species of Phytophthora de Bary, are closely related to brown algae, algicides also may prove to be effective at eliminating propagules of Phytophthora spp. from water. In laboratory studies, we have shown that two copper-based algicides were lethal to zoospores, sporangia, and chlamydospores of several species of Phytophthora. These algicides also were lethal to propagules of species of Phytophthora naturally occurring in six streams in the northwest region of South Carolina, USA. Recently, we have investigated the effects of season and temperature on the efficacy of an algicide containing copper hydroxide (Cu[OH]₂) as the active ingredient on natural populations of Phytophthora spp. in two streams. Water samples of 10 L were collected monthly in February, April, June, August, and November 2009 and were maintained at 5, 10, or 22 ^oC during treatment. Propagules usually were not detected at 2 h after treatment and never were detected at 4 h after treatment. The copper hydroxide algicide was effective in all five months and over a range of temperatures; therefore, it may provide an effective management strategy for species of Phytophthora present in some waterways.