Insights into the potential host range of Phytophthora foliorum

During a survey for Phytophthora ramorum undertaken in north‐west Scotland in early 2016, Phytophthora foliorum was found infecting foliage of the invasive shrub Rhododendron ponticum. Prior to this, P. foliorum had only been reported from foliage of hybrid azaleas in nurseries in California and Tennessee and from azalea plants in an ornamental nursery in Spain. No other hosts were known, and much of the behaviour of P. foliorum remained enigmatic. The species is classified in Phytophthora Clade 8c, with closest relatives, P. ramorum and Phytophthora lateralis, both of which are highly damaging tree pathogens. To explore the threat that P. foliorum might pose to trees, its growth–temperature responses on agar media and ability to cause lesions in the living bark of various hosts were contrasted with the behaviours of P. ramorum and P. lateralis. Phytophthora foliorum proved faster growing and more tolerant of temperature extremes than the other Phytophthora species. Comparisons of bark colonization initially focussed on R. ponticum and larch species Larix decidua and Larix kaempferi as all three are significant hosts of P. ramorum in the UK. Further experiments included another P. ramorum host, Fagus sylvatica (European beech), and the main host of P. lateralis, Chamaecyparis lawsoniana (Lawson cypress). Findings suggested that as well as being a significant pathogen of R. ponticum, damage caused by P. foliorum to both species of larch and beech was very similar to that of the EU1 lineage of P. ramorum, although growth in host tissue was also influenced by season.     

Redwood/tanoak stand development and response to tanoak mortality caused by Phytophthora ramorum

Coast redwood (Sequoia sempervirens) and tanoak (Lithocarpus densiflorus) form mixed-evergreen forests along the northern California coast. In the mid-1990s, an introduced pathogen (Phytophthora ramorum) began causing extensive mortality of tanoak in these forests. This research reconstructed stand development patterns occurring in stands with and without the pathogen, measured stand responses to tanoak mortality, and developed projections of future stand development and structure in the presence of P. ramorum. Redwood forms an upper canopy layer while tanoak forms a multicohort lower canopy, resulting in distinct vertical stratification patterns. Individual redwood tree response patterns to tanoak mortality included crown expansion, increased basal sprouting, and increased basal area growth. Future stand structures will likely have greater proportions of redwood relative to tanoak.     

Phytophthora ramorum is a generalist plant pathogen with differences in virulence between isolates from infectious and dead‐end hosts

Variation in virulence was examined among isolates of Phytophthora ramorum from epidemiologically important or infectious (non‐oak) and transmissive dead‐end (oak) hosts from North America. Twelve isolates representative of the genetic, geographic and host range of P. ramorum in the western United States were inoculated on leaves of Umbellularia californica (bay laurel or bay) and stems of Quercus agrifolia (coast live oak). In spite of extreme genetic similarity among the isolates employed, and even within the same genotype, significant differences in lesion size were measured, suggesting virulence in this pathogen is also controlled by epigenetic factors. A strong positive correlation between lesion size on bay laurel and coast live oak provides experimental evidence P. ramorum is a generalist pathogen that lacks host specificity. Isolates from non‐transmissive oaks were significantly less pathogenic both on oaks and bays than isolates from infectious hosts. These results are essential to further our understanding of the epidemiology and evolutionary potential of this pathogen. A quantitative differential in virulence of isolates from hosts with different epidemiological roles has been described for many animal diseases, but is a novel report for a plant disease.     

Migratory passerine birds in Britain carry Phytophthora ramorum inoculum on their feathers and “feet” at low frequency

In this study, we investigated whether birds could be vectors facilitating long‐distance spread of Phytophthora ramorum in Britain. Migratory bird species associated with the main sporangium‐producing host plants and most likely to pick up P. ramorum spores were considered. Swabs were taken from the flank and “feet” of 1,014 birds over a 12‐month period (April 2011–March 2012) in the west of Britain and subsequently analyzed for the presence of P. ramorum using nested PCR. Ten positive samples from 10 birds were identified: three in Cornwall, one in Devon, three in Gloucestershire, two in north Wales and one in Merseyside. Phytophthora ramorum was detected on samples from four species of thrushes (Redwing Turdus iliacus, Fieldfare T. pilaris, Blackbird T. merula and Song Thrush T. philomelos) and one species of warbler (Chiffchaff Phylloscopus collybita). All birds that tested positive were sampled in late autumn and winter (October–February), when long‐distance movements (over 100 km) would have stopped. The low incidence of P. ramorum found using PCR suggests that the incidence of inoculum, whether viable or not, on birds was low. The apparently low incidence of inoculum on birds suggests migratory passerine birds can carry P. ramorum inoculum on their feathers and “feet,” albeit at low frequency. The dates of positive samples indicate that birds would not have been moving long distances at the time but further work is needed to estimate the extent of their contribution to the spread of P. ramorum in Britain.     

Potential susceptibility of Australian flora to a NA2 isolate of Phytophthora ramorum and pathogen sporulation potential

Phytophthora ramorum is an invasive plant pathogen and the cause of considerable and widespread damage in nurseries, gardens and natural woodland ecosystems of the USA and Europe. It is considered to be a significant plant disease as it could cause biodiversity loss and severe economic losses in plant industries in areas where it is not yet known to exist, such as Australasia. Foliar susceptibility and sporulation potential were tested using detached‐leaf assays for 70 Australian native plant species sourced from established gardens and arboreta in California using a NA2 isolate of P. ramorum. Correa‘Sister Dawn’, Eucalyptus regnans, Isopogon cuneatus, I. formosus, Leptospermum scoparium, L. lanigerum and Melaleuca squamea were identified as potentially highly susceptible host species. Hedycarya angustifolia, Olearia argophylla, Phyllocladus aspleniifolius, Pittosporum undulatum and Podocarpus lawrencei were identified as potentially resistant. All 70 species were able to be infected with P. ramorum, as confirmed by reisolation. Putative sporulating hosts include five members of the Myrtaceae, Agonis flexuosa, Corymbia ficifolia, Eucalyptus haemastoma, E. delegatensis and E. viminalis. As a part of a precautionary strategy, the potentially highly susceptible species found in this study are suitable candidates for targeted surveillance programmes in high‐risk incursion areas of Australia and within the global horticultural trade.     

Phytophthora ramorum in England and Wales: which environmental variables predict county disease incidence?

Phytophthora ramorum is the oomycete pathogen responsible for Sudden Oak Death on the West Coast of the USA and Sudden Larch Death in the British Isles. It also causes twig dieback and leaf blight on a series of ornamental hosts (e.g. Rhododendron, Viburnum, Pieris and Camellia) commonly grown in plant nurseries, traded by garden centres and cultivated in public and private gardens. The role of the plant trade in the dispersal of P. ramorum has been well documented, but there is a need for regional analyses of which environmental variables can predict disease expression in the trade and in the wild, so as to be able to better predict the further development of this worldwide plant health issue. In this study, we analyse data on the incidence of P. ramorum (2002–2009, thus before the reports in Japanese larch plantations) in counties in England and Wales as a function of environmental variables such as temperature and rainfall, controlling for confounding factors such as county area, human population and spatial autocorrelation. While P. ramorum county incidence in nurseries and retail centres was positively related to county area and human population density, county incidence in gardens and the wild did not show such correlations, declined significantly towards the East and was positively correlated with disease incidence in the trade. The latter finding, although not conclusively proving causation, suggests a role of the trade in the dispersal of this pathogen across English and Welsh landscapes. Combined together, P. ramorum county incidence in the trade and in the semi‐natural environment increased with increasing precipitation and with declining latitude. This study shows the importance of environmental variables in shaping regional plant epidemics, but also yields results that are suggestive of a role of people in spreading plant diseases across entire countries.     

Multispecies Phytophthora disease patterns in declining beech stands

Decline diseases are typically caused by complex abiotic and biotic interactions and characterized by a suite of symptoms indicative of low plant vigour. Diseased trees are frequently infected by Phytophthora, but the complex interactions between pathogen, host and the heterogeneous forest environment mask a comprehensive understanding of the aetiology. In the present study, we surveyed European beech (Fagus sylvatica) stands in Swiss forests with recent increases in bleeding lesions for the presence of Phytophthora. We used a combined approach of analysing soil and bark samples from trees displaying bleeding lesions and trees free from bleeding lesions. Soil baiting revealed a higher prevalence of Phytophthora spp. around trees with bleeding lesions than around trees without bleeding lesions. For the bark samples from bleeding lesions, we used several detection methods. Phytophthora spp. were detected in 74% of the trees by an immunological on‐site diagnostic kit, in 64% by a specific PCR assay, and 38% by isolation on selective media. All samples tested were negative for P. ramorum using qPCR. Overall, nine Phytophthora species were identified by ITS sequencing, the most common of which were P. plurivora, P. gonapodyides, P. × cambivora and P. syringae. We identified distinct species in bleeding lesions and the rhizosphere of the same host tree which suggests a multispecies Phytophthora disease patterns in these declining beech. Among the recovered species, P. × cambivora and P. × serendipita were identified as hybrid genotypes with the former abundant in bleeding lesions.     

Occurrence of Phytophthora cinnamomi in cork oak forests in Italy

An increasing decline and mortality of cork oak trees have been recently observed in central Italy and Sardinia Island. Following surveys conducted in three declining cork oak forests, a Phytophthora species was consistently isolated from soil samples collected from trees displaying different level of decline. Based on morphological features, growth rates at different temperatures and analysis of DNA sequences of the ITS region, all isolates were identified as Phytophthora cinnamomi Rands. This pathogen caused large brownish lesions on inoculated freshly cut branches of cork oak. It was re‐isolated from all infected tissues. These findings represent the first report of P. cinnamomi on cork oak trees in Italy.     

Susceptibility of common tree species in Sweden to Phytophthora cactorum,P. cambivora and P. plurivora

In Sweden, invasive Phytophthora pathogens have been recognized as a growing threat to urban and production forests, calling for an urgent update of regeneration strategies for infested areas. Stem inoculation tests were performed to test the relative susceptibility of common conifer and broadleaved tree species Pinus sylvestris, Picea abies, Larix x eurolepis, Betula pendula, Quercus robur, Fagus sylvatica, Populus trichocarpa and Tilia cordata to the root pathogens Phytophthora cactorum, P. cambivora and P. plurivora commonly isolated from Swedish soils. Results indicate that all the species tested were susceptible and formed lesions following stem inoculation with all three Phytophthora species, but to varying degrees. Of particular interest are the high levels of susceptibility in P. trichocarpa to all three Phytophthora species compared to other tested tree species.     

Efficacy of local eradication treatments against the sudden oak death epidemic in Oregon tanoak forests

Phytophthora ramorum, cause of sudden oak death, has been distributed widely across the United States in horticultural situations, but is not established in forests outside of California and Oregon. Here, it has triggered widespread concern and, especially in Oregon, an intensive disease management programme. Now, we provide the first systematic evaluation of the efficacy of that effort. This paper evaluates four measures of the efficacy of Sudden Oak Death (SOD) local eradication treatments: inoculum availability; inoculum from tree species other than tanoak; disease spread from treated areas; and cumulative infested area with and without treatment. We conclude that local treatments demonstrably reduce local inoculum levels. Eradication of SOD from infested sites is difficult but not impossible. The disease usually does not persist after cutting infected trees but spread on the landscape continues because the pathogen may be present on undetected new infections for a year or two before whole tree symptoms are visible. This limits early detection and coupled with delays in completing eradication treatments, prolongs the chances for long‐distance aerial dispersal.     

Relationship between field resistance to Phytophthora ramorum and constitutive phenolic chemistry of coast live oak

Sudden oak death, caused by Phytophthora ramorum, has resulted in high levels of coast live oak (CLO) mortality. However, some CLO survive in areas with high disease pressure and may thus be resistant. We tested the hypothesis that such field‐resistant trees contain constitutively higher levels of phenolics than susceptible trees. Phloem was sampled from the trunks of two groups of trees (one previously inoculated, one naturally infected with P. ramorum) categorized over the course of several years as putatively resistant (PR, no symptoms), in remission (IR, showed symptoms but then recovered) and symptomatic (S). Individual and total soluble phenolics from these trees were quantified. There were no significant differences in individual or total soluble phenolics between groups of naturally infected trees. However, inoculated PR and IR trees were characterized by higher constitutive levels of ellagic acid, a tyrosol derivative, and an unidentified phenolic than S trees. Ellagic acid and tyrosol‐like compounds in CLO phloem are promising resistance biomarker candidates.     

Swiss stone pine trees and spruce stumps represent an important habitat for Heterobasidion spp. in subalpine forests

In the Western Italian Alps (WIA), the three European species of the forest pathogen Heterobasidion spp. can coexist in the same area. Heterobasidion parviporum Niemelä & Korhonen and Heterobasidion abietinum Niemelä & Korhonen are normally found in areas with a significant presence of their respective primary hosts, spruce (Picea spp.) and fir (Abies spp.). The host/niche occupied by Heterobasidion annosum (Fr.) Bref. in the region still remains unclear. Although Scots pine (Pinus sylvestris), a major host for this fungal species in other parts of Europe, is abundant in the region, little or no evidence of disease caused by H. annosum is visible in this tree species. Two different, but not mutually exclusive, hypotheses can explain the presence of H. annosum: (1) Scots pines are infected but largely asymptomatic and (2) H. annosum has adapted to different hosts. An analysis of Heterobasidion species was performed in two natural, mixed‐conifer forests using traditional isolation techniques and novel direct molecular diagnosis from wood. In a subalpine stand of mixed spruce (Picea abies), larch (Larix spp.), and Swiss stone pine (Pinus cembra), 18 naturally infected spruces and larches only yielded H. parviporum. A Swiss stone pine in the same stand was extensively colonized by both H. parviporum and H. annosum. In a second subalpine stand, an analysis of 18 spruce stumps and nine Swiss stone pine stumps yielded both H. parviporum and H. annosum isolates. Pine stumps had been mostly colonized by H. parviporum prior to tree felling, suggesting that this species may be secondarily infected by the locally predominant Heterobasidion species (i.e. H. parviporum). Results of our analysis also indicated that primary colonization of spruce stumps (e.g. through basidiospores) was caused by both H. parviporum and H. annosum, while secondary infection of such stumps was mostly because of H. parviporum.     

Forest stand dynamics and sudden oak death: Mortality in mixed-evergreen forests dominated by coast live oak

Sudden oak death (SOD), caused by the recently discovered non-native invasive pathogen, Phytophthora ramorum, has already killed tens of thousands of native coast live oak and tanoak trees in California. Little is known of potential short and long term impacts of this novel plant–pathogen interaction on forest structure and composition. Coast live oak (Quercus agrifolia) and bay laurel (Umbellularia californica) form mixed-evergreen forests along the northern California coast. This study measured tree mortality over a gradient of disease in three time periods. Direct measurements of current mortality were taken during 2004, representing a point-in-time estimate of present and ongoing mortality. Past stand conditions, c. 1994, were estimated using a stand reconstruction technique. Future stand conditions, c. 2014, were calculated by assuming that, given a lack of host resistance, live trees showing signs of the disease in 2004 would die. Results indicate that coast live oaks died at a rate of 4.4–5.5% year⁻¹ between 1994 and 2004 in highly impacted sites, compared with a background rate of 0.49% year⁻¹, a ten-fold increase in mortality. From 2004 to 2014, mortality rates in the same sites were 0.8–2.6% year⁻¹. Over the entire period, in highly impacted sites, a 59–70% loss of coast live oak basal area was predicted, and coast live oak decreased from 60% to 40% of total stand basal area, while bay laurel increased from 22% to 37%. Future stand structures will likely have greater proportions of bay laurel relative to coast live oak.     

An improved method for qPCR detection of three Phytophthora spp. in forest and woodland soils in northern Britain

Using TaqMan qPCR assays, DNA of P. ramorum, P. kernoviae and P. austrocedri was detected in 500 g soil samples collected from twelve infected forest and woodland sites in northern Britain. Phytophthora DNA was also amplified in soil adhering to boots after walking transects along footpaths or animal trails. At two sites, Phytophthora DNA was detected in soil over a 4‐year period following removal of infected hosts. This new method enabling assessment of larger quantities of soil demonstrates the contamination risk of these pathogens in soil at infected sites and improves our understanding of the mechanisms of persistence and spread.     

Pre-impact forest composition and ongoing tree mortality associated with sudden oak death in the Big Sur region; California

Mixed-evergreen forests of central coastal California are being severely impacted by the recently introduced plant pathogen, Phytophthora ramorum. We collected forest plot data using a multi-scale sampling design to characterize pre-infestation forest composition and ongoing tree mortality along environmental and time-since-fire gradients. Vegetation pattern was described using trend surface analysis, spatial autocorrelation analysis and redundancy analysis. Species-environment associations were modeled using non-parametric multiplicative regression (NPMR). Tanoak (Lithocarpusdensiflorus) mortality was analyzed with respect to environmental and biotic factors using trend surface analysis and multivariate regression. Mixed-evergreen forest occurs throughout the Big Sur region but is most widespread in the north, on north facing slopes, at mid-elevations near the coast. Relative basal area of the dominant tree species changes fairly predictably from north to south and from coast to interior in relation to mapped patterns of precipitation, temperature factors and soil characteristics. Most dominant tree species sprout vigorously after fire. The forests experience a mixed-fire regime in this region ranging from low severity understory burns to high severity crown fires, with the latter increasing above the marine inversion layer and at more interior locations. Ceanothus spp. can dominate mixed-evergreen sites for several decades after severe fires. All of the dominant broadleaf evergreen tree species are hosts of P. ramorum, although not all will die from infection. Tanoak mortality decreases from northwest to southeast and is significantly correlated with climate, especially growing degree days and mean annual precipitation, and with basal area of the foliar host bay laurel (Umbellularia californica) in a 0.5–1 ha neighborhood. Adaptive management of mixed-evergreen forest to mitigate P. ramorum impacts in the region will need to consider large local and regional variation in forest composition and the potentially strong interactions between climate, fire, forest composition and disease severity.     

Geographic distribution,disease symptoms and pathogenicity of Phytophthora nemorosa and Phytophthora pseudosyringae in California,USA

During the course of surveys for Phytophthora ramorum in coastal forests of California and Oregon, P. nemorosa and P. pseudosyringae were frequently isolated from foliage and stems of the same hosts as P. ramorum. Both species ranged from central California to Oregon within 50 km of the Pacific Ocean. Both were also found in the Sierra Nevada Mountains. Phytophthora nemorosa was primarily found infecting trees in coast redwood forests and was most often isolated from bay laurel leaves (Umbellularia californica), bleeding cankers on the main bole of tanoak (Lithocarpus densiflorus), and leaf and small stem tissue of redwood (Sequoia sempervirens). Phytophthora pseudosyringae was primarily isolated from hosts found in coast live oak woodlands. Bay laurel was the most common host while infection of coast live oak (Quercus agrifolia) stems was less frequent. Inoculation studies confirmed the pathogenicity of P. nemorosa and P. pseudosyringae on their most common hosts.     

The destructive invasive pathogen Phytophthora lateralis found on Chamaecyparis lawsoniana across the UK

In 2010–2011, Phytophthora lateralis was isolated from diseased Chamaecyparis lawsoniana exhibiting dieback and mortality at eight geographically separate forest, parkland and shelterbelt locations in England, Scotland and Northern Ireland. In 2011, P. lateralis was also isolated from young symptomatic nursery plants of C. lawsoniana and Thuja occidentalis recently imported into Scotland from mainland Europe. These are the first findings of P. lateralis in the UK. At six of the field sites, only collar and root lesions were observed. However, at two sites, large stem and branch lesions unconnected to the collar region were also observed. Phytophthora lateralis was readily isolated from both aerial and basal lesions. In artificial inoculation experiments, two Scottish isolates of the pathogen caused lesions on C. lawsoniana shoots and were readily reisolated from the lesions, their pathogenicity being comparable to that of P. lateralis isolates originating from outside the UK. Isolates from six field sites and the two nursery interceptions exhibited ITS and cox II sequences identical to published sequences of French and North American isolates. However, the isolates from two field sites shared an ITS sequence with Taiwanese isolates and differed from North American, French and Taiwanese isolates by a single‐base substitution in cox II, suggesting a separate evolutionary history. It is clear that P. lateralis now presents a significant threat to C. lawsoniana in Britain. The main source of the outbreaks is likely to be imported infested nursery stock.     

Fine root dynamics of oak saplings in response to Phytophthora cinnamomi infection under different temperatures and durations

The belowground effects of Phytophthora cinnamomi on 1‐year‐old saplings of two common oak species in mid‐Atlantic US forests, white (Quercus alba) and red oak (Q. rubra), were examined after incubation in pathogen‐infested soilless potting mix. Fine root lengths (0–1.5 mm in diameter) of both oak species were quantified after incubation at successive 30‐day intervals up to 300 days, for a total of 10 incubation periods. In addition, colony‐forming units (CFU) of P. cinnamomi were quantified after white oak saplings were incubated in infested soilless potting mix at different temperature/duration combinations that reflect soil conditions present in the mid‐Atlantic United States. Impact of P. cinnamomi on fine root lengths of red and white oak saplings varied considerably over time. Significant periods of fine root loss occurred primarily during spring, when bud break and leaf flush began for both oak species. Red oaks had 17% fine root loss on average, while white oaks appeared more resistant to P. cinnamomi infection with a 2% decrease in fine roots over the course of the experiment. Phytophthora cinnamomi CFU declined significantly with exposure to all incubation temperatures except 8°C. This was in contrast to in vitro experiments, where the optimum temperature for mycelial growth was determined to be 21°C and above. Significant fine root loss caused by P. cinnamomi depended on plant phenology and the oak species tested. Extreme soil temperatures have a significant adverse impact on temporal changes of P. cinnamomi population.     

日本落叶松杂种及亲本 4CL基因的克隆和SNP多态性分析

以日本落叶松(Larix kaempferi)不同杂交组合为材料,依据转录组测序信息,成功获得了11种日本落叶松材料的4 CL基因的编码区序列,该序列长1 537 bp,包含完整ORF片段1 368 bp,编码455个氨基酸。qRT-PCR结果分析表明,除日本落叶松Larix16×Larix61组合外,Larix82×Larix13、Larix40×Larix10和Larix82×Larix61子代的表达均高于双亲,推测4 CL基因差异表达与落叶松杂种的木质素生物合成有关。进一步采用SNP标记方法和DNAMAN软件对不同日本落叶松杂种和亲本组合的单核苷酸多态性进行分析。共检测到66个SNP多态性位点,表明日本落叶松4CL 具有丰富的遗传多样性。66个SNP变异中,属于三突变1个;转换类型45个,占总变异的68.18%;20个属于颠换类型,占总变异的30.30%,转换:颠换=9:4。在转换类型中,A/G和C/T转换分别占28.79%和39.39%;颠换类型中A/C、A/T、G/C、G/T分别占4.55%、9.09%、9.09%和7.58%。采用NJ 聚类法对克隆片段的氨基酸序列进行了遗传多样性分析。