Outbreak of Pitch Canker Caused by Fusarium circinatum on Pinus spp. in Northern Spain

Spanish ecological battleground: population structure of two invasive fungi, Cryphonectria parasitica and Fusarium circinatum

Introduction

Invasive fungi distributed worldwide through globalization have caused devastating diseases in different forests, causing economic and ecologic disturbances. Two such invasive species are Cryphonectria parasitica and Fusarium circinatum, which were introduced to Europe from North America, separated temporally: C. parasitica was introduced about nine decades ago, whereas F. circinatum was introduced around two decades ago. As C. parasitica had a longer time to undergo genetic changes, we hypothesized that it has higher genetic diversity than the recently introduced F. circinatum in Spain. In addition, we studied the genetic characterization of both fungi present in similar ecological conditions in Northern Spain with the aim of providing data for biocontrol measures.

Methods

Molecular genetic markers were used to test these hypotheses, including mating type and DNA sequencing of internal transcribed spacer (ITS) regions. In addition, we used vegetative compatibility (VC) type markers in C. parasitica as the information about VC type is essential to apply biocontrol against the fungus.

Results and discussion

All the isolates of C. parasitica from the studied area belonged to only one VC type (EU-1) and one mating type (MAT-2). However, three distinct haplotypes of C. parasitica were identified through ITS sequencing, showing that multiple introductions might have happened to Cantabria. Among F. circinatum, no diversity was observed in ITS and MAT loci in the studied area but isolates from other Spanish regions showed the presence of both mating types. Overall, C. parasitica had higher genetic diversity than F. circinatum, despite both organisms appearing to reproduce clonally. This study helped understand the invasion patterns of C. parasitica and F. circinatum in northern Spain and will be useful in applying biocontrol measures against both pathogens.

Crop diversity promotes the recovery of fungal communities in saline-alkali areas of the Western Songnen Plain

Introduction

Phytoremediation is an effective strategy for saline land restoration. In the Western Songnen Plain, northeast China, soil fungal community recovery for saline phytoremediation has not been well documented among different cropping patterns. In this study, we tested how rotation, mixture, and monoculture cropping patterns impact fungal communities in saline-alkali soils to assess the variability between cropping patterns.

Methods

The fungal communities of the soils of the different cropping types were determined using Illumina Miseq sequencing.

Results

Mixture and rotation promoted an increase in operational taxonomic unit (OTU) richness, and OTU richness in the mixture system decreased with increasing soil depth. A principal coordinate analysis (PCoA) showed that cropping patterns and soil depths influenced the structure of fungal communities, which may be due to the impact of soil chemistry. This was reflected by soil total nitrogen (TN) and electrical conductivity (EC) being the key factors driving OTU richness, while soil available potassium (AK) and total phosphorus (TP) were significantly correlated with the relative abundance of fungal dominant genus. The relative abundance of Leptosphaerulina, Alternaria, Myrothecium, Gibberella, and Tetracladium varied significantly between cropping patterns, and Leptosphaerulina was significantly associated with soil chemistry. Soil depth caused significant differences in the relative abundance of Fusarium in rotation and mixture soils, with Fusarium more commonly active at 0-15 cm deep soil. Null-model analysis revealed that the fungal community assembly of the mixture soils in 0-15 cm deep soil was dominated by deterministic processes, unlike the other two cropping patterns. Furthermore, fungal symbiotic networks were more complex in rotation and mixture than in monoculture soils, reflected in more nodes, more module hubs, and connectors. The fungal networks in rotation and mixture soils were more stable than in monoculture soils, and mixture networks were obviously more connected than rotations. FUNGuild showed that the relative proportion of saprotroph in rotation and mixture was significantly higher than that in monocultures. The highest proportion of pathotroph and symbiotroph was exhibited in rotation and mixture soils, respectively.

Discussion

Overall, mixture is superior to crop rotation and monocultures in restoring fungal communities of the saline-alkali soils of the Western Songnen Plain, northeast China.

Temporal and Spatial Variation in the Population Structure of Spanish Fusarium circinatum Infecting Pine Stands

Fusarium circinatum is an introduced fungal pathogen extended to the northern regions of Spain that causes Pine Pitch Canker (PPC) disease. In this work, we analyzed the pathogen's genetic diversity to study changes over time and space since the first outbreak occurred in Spain. Using six polymorphic SSR markers, 15 MLGs were identified in 66 isolates, and only three haplotypes were found with frequencies higher than one. In general, genotypic diversity was low and decreased shortly over time in the northwestern regions while maintained at País Vasco, where only one haplotype (MLG32) was detected 10 years. This population also included isolates of a single mating type (MAT-2) and VCGs identified in only two groups, while isolates from NW regions were of both mating types and VCGs represented in 11 groups. The existence of haplotype MLG32 maintained on time and widely distributed suggests its good adaptation to the environment and the host. Results showed that the pathogen in País Vasco remains clearly differentiated from other northwestern populations. This fact was supported with no evidence of migration among regions. Results are explained by the asexual reproduction, but also selfing at least to a lesser extent that leads to identification of two new haplotypes.

Susceptibility of germinating seedlings of European and Eurasian populations of Pinus sylvestris to damping-off caused by Fusarium circinatum

The effect of inoculation with Fusarium circinatum on survival of seed and seedlings of 19 populations of Pinus sylvestris was examined under environmentally controlled conditions, with four treatments (0, 50, 10³, 10⁶ spores ml^-1). A single seed source of P. radiata was included as a positive control. Germination (emergence of the plumule above the compost) and health of seedlings was assessed daily, for 85 days. Spore density had a significant effect on germination: at 50 spores ml^-1, only germination of a Northeast Scotland population was reduced. Treatment with 1000 spores ml^-1, however, reduced germination of six populations of P. sylvestris and of P. radiata. Survival of emerged seedlings also varied with inoculum dose. Approximately 75% of seedlings survived 85 days after germination after inoculation with 50 spores ml^-1. Seedlings of all populations were killed within 12-16 days of germination by the 10³ and 10⁶ spores ml^-1 treatments. Emerged seedlings of the Austrian populations showed the highest susceptibility to F. circinatum following treatment with 50 spores ml^-1, although 15% of seedlings of one Austrian population (AU3) survived to the end of the experiment (85 days after germination). There was no clear pattern in survival rates of the P. sylvestris seedlings from other populations treated with 1000 or 1 million spores ml^-1 due to death of all emerged seedlings within a short period. Variations in susceptibility of different populations of P. sylvestris to F. circinatum may be used in future selection and breeding programmes to reduce the impact of the pathogen as it spreads over wider areas in Europe and Eurasia.

Tolerance of 2-Benzoxazolinone and Interactions with Grass and Pine Hosts in a Population of Fusarium circinatum

Fusarium circinatum, the causal agent of pitch canker in pines and a cryptic endophyte of grasses, was examined for heritable variation in tolerance of the grass defense compound 2-benzoxazolinone (BOA). A diverse population of F. circinatum progeny was assayed for growth rate on potato dextrose agar amended with BOA. Matings were conducted to allow for selection of progeny with lower and higher tolerance of BOA. The results confirmed heritable variation in BOA tolerance in F. circinatum. A subset of differentially tolerant progeny was used for inoculations of growth chamber-grown Zea mays and greenhouse-grown Pinus radiata. No differences were detected in the rate of infection or extent of colonization of Z. mays inoculated with F. circinatum progeny differing in tolerance of BOA. Pitch canker symptoms in inoculated P. radiata trees showed that high BOA-tolerating isolates induced significantly longer lesion lengths than those induced by low BOA-tolerating isolates. Results from this study were consistent with the proposition that F. circinatum evolved from grass-colonizing ancestors and that pathogenicity to pine is a relatively recent evolutionary innovation.

Influence of Temperature and Moisture Duration on Pathogenic Life History Traits of Predominant Haplotypes of Fusarium circinatum on Pinus spp. in Spain

Pathogen life history traits influence epidemic development and pathogen adaptive ability to interact with their hosts in different environments. Reduced traits variation may compromise pathogen evolutionary potential, which is particularly important for introduced pathogens. Fusarium circinatum (cause of pine pitch canker) is an invasive fungal pathogen in Europe, with current distribution restricted to forest stands of Pinus radiata and Pinus pinaster in northern Spain and Portugal. This study aimed to quantify pathogenic traits of Spanish isolates of F. circinatum, with two of the strains representing the two dominant haplotypes in the Spanish population. Disease severity was measured on P. radiata, analyzing the influence of temperature and moisture duration on infection as well as the influence of temperature on spore germination, sporulation, and mycelial growth. Results indicated that the isolate representing the most common haplotype caused more severe disease on P. radiata at 25 and 30°C compared with the second most common haplotype but caused less severe disease at 15°C. Spore germination was higher for the most common haplotype, which produced more spores at 20 and 25°C. The isolate showed hyphal melanization at 5°C, which has been associated with survival and may be important because no resting structures have been described for F. circinatum. Our study determined that longer moisture periods during infection result in more severe disease from 7 to 24 h, regardless of the isolate virulence. This is the first study on virulence of the most abundant haplotypes of F. circinatum in Spain as affected by temperatures and moisture.

Global Geographic Distribution and Host Range of Fusarium circinatum, the Causal Agent of Pine Pitch Canker

Fusarium circinatum, the causal agent of pine pitch canker (PPC), is currently one of the most important threats of Pinus spp. globally. This pathogen is known in many pine-growing regions, including natural and planted forests, and can affect all life stages of trees, from emerging seedlings to mature trees. Despite the importance of PPC, the global distribution of F. circinatum is poorly documented, and this problem is also true of the hosts within countries that are affected. The aim of this study was to review the global distribution of F. circinatum, with a particular focus on Europe. We considered (1) the current and historical pathogen records, both positive and negative, based on confirmed reports from Europe and globally; (2) the genetic diversity and population structure of the pathogen; (3) the current distribution of PPC in Europe, comparing published models of predicted disease distribution; and (4) host susceptibility by reviewing literature and generating a comprehensive list of known hosts for the fungus. These data were collated from 41 countries and used to compile a specially constructed geo-database. A review of 6297 observation records showed that F. circinatum and the symptoms it causes on conifers occurred in 14 countries, including four in Europe, and is absent in 28 countries. Field observations and experimental data from 138 host species revealed 106 susceptible host species including 85 Pinus species, 6 non-pine tree species and 15 grass and herb species. Our data confirm that susceptibility to F. circinatum varies between different host species, tree ages and environmental characteristics. Knowledge on the geographic distribution, host range and the relative susceptibility of different hosts is essential for disease management, mitigation and containment strategies. The findings reported in this review will support countries that are currently free of F. circinatum in implementing effective procedures and restrictions and prevent further spread of the pathogen.

Compost Tea Reduces the Susceptibility of Pinus radiata to Fusarium circinatum in Nursery Production

Nutrition is one of the factors that most limits forestry plant growth; thus, current production in nurseries is based on conventional fertilization focused on enhancing vigor. However, an excessive intake of mineral nitrogen can cause morphological imbalances and the formation of more succulent tissues which, consequently, increase susceptibility to plant pathogens. Fusarium circinatum is the causal agent of pitch canker in plants of the Pinus genus, with Pinus radiata being the species most susceptible to this disease. This study compares the response of P. radiata seedlings to infection by F. circinatum as influenced by two fertilizers-conventional and aerated compost tea (ACT)-applied during the nursery phase. The potential of ACT against F. circinatum was first tested in vitro, where it was found to inhibit the pathogen's mycelial growth and conidial germination. In the greenhouse, infected plants fertilized with ACT exhibited less severe internal and external symptoms of pitch canker and lower levels of pathogen colonization of both stems and needles than with conventional fertilizer. An analysis of the hormone content and defense-related gene expression shows greater salicylic acid production and phenylalanine ammonium-lyase and chalcone synthase expression in ACT-fertilized pine. All of the parameters assessed are consistent in showing that biofertilization with ACT reduces the susceptibility of pine seedlings to the disease compared with conventional fertilization.

Potential Interactions between Invasive Fusarium circinatum and Other Pine Pathogens in Europe

Pines are major components of native forests and plantations in Europe, where they have both economic significance and an important ecological role. Diseases of pines are mainly caused by fungal and oomycete pathogens, and can significantly reduce the survival, vigor, and yield of both individual trees and entire stands or plantations. Pine pitch canker (PPC), caused by Fusarium circinatum (Nirenberg and O’Donnell), is among the most devastating pine diseases in the world, and is an example of an emergent invasive disease in Europe. The effects of microbial interactions on plant health, as well as the possible roles plant microbiomes may have in disease expression, have been the focus of several recent studies. Here, we describe the possible effects of co-infection with pathogenic fungi and oomycetes with F. circinatum on the health of pine seedlings and mature plants, in an attempt to expand our understanding of the role that biotic interactions may play in the future of PPC disease in European nurseries and forests. The available information on pine pathogens that are able to co-occur with F. circinatum in Europe is here reviewed and interpreted to theoretically predict the effects of such co-occurrences on pine survival, growth, and yield. Beside the awareness that F. circinatum may co-occurr on pines with other pathogens, an additional outcome from this review is an updating of the literature, including the so-called grey literature, to document the geographical distribution of the relevant pathogens and to facilitate differential diagnoses, particularly in nurseries, where some of them may cause symptoms similar to those induced by F. circinatum. An early and accurate diagnosis of F. circinatum, a pathogen that has been recently introduced and that is currently regulated in Europe, is essential to prevent its introduction and spread in plantings and forests.

Pine Pitch Canker (PPC): Pathways of Pathogen Spread and Preventive Measures

Fusarium circinatum (Nirenberg and O’ Donnell) is the causal agent of pine pitch canker (PPC) disease, one of the most devastating forest diseases worldwide. Long-distance spread occurs mainly through the movement of infected seeds whereas at regional level, the movement of seedlings, substrates, or containers may play an important role in fungal dispersal. Invasion of nurseries takes place via infected seeds and further spread can occur by planting contaminated seedlings, especially due to the possibility of infected plants remaining symptomless. Once established, F. circinatum spreads by rain, wind, and insects. The natural spread of the pathogen is limited due to the short dispersal distances of the spores and the fairly short flight distances of disseminating insects. In this review, we summarize the currently known dispersal pathways of the pathogen, discussing both natural and human-assisted processes. With the purpose of understanding how to best intervene in the disease’s development in nurseries and forests, we outline the epidemiology of the pathogen describing the key factors influencing its spread. Preventive measures to control the spread of F. circinatum locally and globally are described with special emphasis on the challenges in implementing them.

Sampling and Detection Strategies for the Pine Pitch Canker (PPC) Disease Pathogen Fusarium circinatum in Europe

Fusarium circinatum Nirenberg & O’Donnel is listed among the species recommended for regulation as quarantine pests in Europe. Over 60 Pinus species are susceptible to the pathogen and it also causes disease on Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) and species in genera such as Picea and Larix. The European Food Safety Authority considers the probability of new introductions—via contaminated seeds, wood material, soil and growing substrates, natural means and human activities—into the EU very likely. Due to early detection, constant surveillance and control measures, F. circinatum outbreaks have officially been eradicated in Italy and France. However, the global spread of F. circinatum suggests that the pathogen will continue to be encountered in new environments in the future. Therefore, continuous surveillance of reproductive material, nurseries and plantations, prompt control measures and realistic contingency plans will be important in Europe and elsewhere to limit disease spread and the “bridgehead effect”, where new introductions of a tree pathogen become increasingly likely as new environments are invaded, must be considered. Therefore, survey programs already implemented to limit the spread in Europe and that could be helpful for other EU countries are summarized in this review. These surveys include not only countries where pitch canker is present, such as Portugal and Spain, but also several other EU countries where F. circinatum is not present. Sampling protocols for seeds, seedlings, twigs, branches, shoots, soil samples, spore traps and insects from different studies are collated and compiled in this review. Likewise, methodology for morphological and molecular identification is herein presented. These include conventional PCR with a target-specific region located in the intergenic spacer region, as well as several real-time PCR protocols, with different levels of specificity and sensitivity. Finally, the global situation and future perspectives are addressed.

Transferability of PCR-based diagnostic protocols: An international collaborative case study assessing protocols targeting the quarantine pine pathogen Fusarium circinatum

Fusarium circinatum is a harmful pathogenic fungus mostly attacking Pinus species and also Pseudotsuga menziesii, causing cankers in trees of all ages, damping-off in seedlings, and mortality in cuttings and mother plants for clonal production. This fungus is listed as a quarantine pest in several parts of the world and the trade of potentially contaminated pine material such as cuttings, seedlings or seeds is restricted in order to prevent its spread to disease-free areas. Inspection of plant material often relies on DNA testing and several conventional or real-time PCR based tests targeting F. circinatum are available in the literature. In this work, an international collaborative study joined 23 partners to assess the transferability and the performance of nine molecular protocols, using a wide panel of DNA from 71 representative strains of F. circinatum and related Fusarium species. Diagnostic sensitivity, specificity and accuracy of the nine protocols all reached values >80%, and the diagnostic specificity was the only parameter differing significantly between protocols. The rates of false positives and of false negatives were computed and only the false positive rates differed significantly, ranging from 3.0% to 17.3%. The difference between protocols for some of the performance values were mainly due to cross-reactions with DNA from non-target species, which were either not tested or documented in the original articles. Considering that participating laboratories were free to use their own reagents and equipment, this study demonstrated that the diagnostic protocols for F. circinatum were not easily transferable to end-users. More generally, our results suggest that the use of protocols using conventional or real-time PCR outside their initial development and validation conditions should require careful characterization of the performance data prior to use under modified conditions (i.e. reagents and equipment). Suggestions to improve the transfer are proposed.

Factors Affecting the Distribution of Pine Pitch Canker in Northern Spain

Fusarium circinatum is the causal agent of pine pitch canker disease (PPC), affecting Pinus species and other conifers (i.e., Pseudotsuga menziesii (Mirb.) Franco.), forming resinous cankers on the main stem and branches and causing dieback in the terminal guide. This pathogen is spreading worldwide, causing economic losses by converting plantations into standing timber without any potential for future production. The disease was recently detected in Northern Spain in plantations of Pinus radiata and forest nurseries. The aim of the work reported here was to study the role of climatic and topographic variables, soil properties, and stand characteristics on PPC. For this purpose, we surveyed 50 pine stands in Cantabria and quantified the percentage of trees showing three symptoms in each stand: canker, defoliation, and dieback. We investigated the predictive power of 30 variables using generalized linear models and hierarchical partitioning. Both approaches yielded similar results. We found that the three symptoms correlated with different explanatory variables. In addition, more trees exhibited cankers in the proximity of the coast and the Basque Country. Additionally, our results showed that low canopy cover is related to a high level of the dieback symptom. Overall, this study highlights the important variables affecting the distribution of PPC in Cantabria.

Fungal Communities Associated with Bark Beetles in Pinus radiata Plantations in Northern Spain Affected by Pine Pitch Canker, with Special Focus on Fusarium Species

Fusarium spp., as well as other endophytic or pathogenic fungi that form communities, have been reported to be phoretically associated with bark beetles (Coleoptera; Scolytinae) worldwide. This applies to Fusarium circinatum Nirenberg and O’Donnell, the causal agent of pine pitch canker (PPC), which threatens Pinus radiata D. Don plantations in northern Spain. The main objective of this study was to study the fungal communities associated with bark beetles and their galleries in stands affected by PPC, with special attention given to Fusarium species. Funnel traps and logs were placed in a P. radiata plot known to be affected by F. circinatum. The traps were baited with different attractants: four with (E)-pityol and six with ethanol and α-Pinene. In addition, fresh green shoots with Tomicus piniperda L. feeding galleries were collected from the ground in 25 P. radiata plots affected by PPC. Extracts of whole insects and gallery tissues were plated on agar medium to isolate and identify the associated fungi. A total of 24 different fungal species were isolated from the bark beetle galleries constructed in logs and shoots, while 18 were isolated from the insect exoskeletons. Ten different Fusarium species were isolated from tissue and insects. Fusarium circinatum was isolated from bark beetle exoskeletons (1.05% of the Pityophthorus pubescens Marsham specimens harboured F. circinatum) and from the galleries (3.5% of the T. piniperda feeding galleries harboured the pathogen). The findings provide information about the fungal communities associated with bark beetles in P. radiata stands in northern Spain.

EU Legislation on Forest Plant Health: An Overview with a Focus on Fusarium circinatum

The increase in arrivals of new forest pests highlights the need for effective phytosanitary legislation and measures. This paper introduces legislation targeted at prevention and management of potential introductions of forest pests and pathogens. An overview is given on plant health regulations on global and regional level with detailed information on the situation in the European Union (EU). The current and new European legislation is discussed, and a particular focus is given on eradication and contingency plans for Fusarium circinatum. We identified key aspects relevant for the improvement of the efficacy of measures aimed to prevent alien pests.

A Comparative Study of the Pathogenicity of Fusarium circinatum and other Fusarium Species in Polish Provenances of P. sylvestris L

The fungal pathogen Fusarium circinatum is the causal agent of Pine Pitch Canker (PPC), a disease which seriously affects different species of pine in forests and nurseries worldwide. In Europe, the fungus affects pines in northern Spain and Portugal, and it has also been detected in France and Italy. Here, we report the findings of the first trial investigating the susceptibility of Polish provenances of Scots pine, Pinus sylvestris L., to infection by F. circinatum. In a greenhouse experiment, 16 Polish provenances of Scots pine were artificially inoculated with F. circinatum and with six other Fusarium species known to infect pine seedlings in nurseries. All pines proved highly susceptible to PPC and displayed different levels of susceptibility to the other Fusarium spp. tested. The findings obtained indicate the potentially strong threat of establishment of an invasive pathogen such as F. circinatum following unintentional introduction into Poland.

Links between Soil Fungal Diversity and Plant and Soil Properties on the Loess Plateau

Previous studies have revealed inconsistent correlations between fungal diversity and plant/soil properties from local to global scales. Here, we investigated the internal relationships between soil fungal diversity and plant/soil properties on the Loess Plateau following vegetation restoration, using Illumina sequencing of the internal transcribed spacer 2 (ITS2) region for fungal identification. We found significant effects of land use types (Af, Artificial forest; Ns, Natural shrub; Ag, Artificial grassland; Ng, Natural grassland; Sc, slope cropland) on soil fungal communities composition, and the dominant phyla were Ascomycota, Basidiomycota, and Zygomycota, which transitioned from Basidiomycota-dominant to Ascomycota-dominant community due to vegetation restoration. The Chao1 richness, Shannon's diversity and ACE indices were significantly influenced by land use types with the order of Ns > Af > Ng > Ag > Sc, and the total number of OTUs varied widely. In contrast, Good's coverage and Simpson's diversity indicated no significant difference among land use types (p > 0.05). Correlation analysis showed that plant and soil properties were closely related to fungal diversity regardless of land use types. In addition, soil organic carbon (SOC) and Hplant (plant richness, Shannon-Wiener index) were strong driving factors that explained fungal diversity. As revealed by the structural equation model (SEM) and generalized additive models (GAMs), fungal diversity was directly and indirectly affected by soil and plant properties, respectively, providing evidence for strong links between soil fungal diversity and plant and soil properties on the Loess Plateau.

Incorporating exposure to pitch canker disease to support management decisions of Pinus pinaster Ait. in the face of climate change

Climate change is gravely affecting forest ecosystems, resulting in large distribution shifts as well as in increasing infection diseases and biological invasions. Accordingly, forest management requires an evaluation of exposure to climate change that should integrate both its abiotic and biotic components. Here we address the implications of climate change in an emerging disease by analysing both the host species (Pinus pinaster, Maritime pine) and the pathogen's (Fusarium circinatum, pitch canker) environmental suitability i.e. estimating the host's risk of habitat loss and the disease`s future environmental range. We constrained our study area to the Spanish Iberian Peninsula, where accurate climate and pitch canker occurrence databases were available. While P. pinaster is widely distributed across the study area, the disease has only been detected in its north-central and north-western edges. We fitted species distribution models for the current distribution of the conifer and the disease. Then, these models were projected into nine Global Climate Models and two different climatic scenarios which totalled to 18 different future climate predictions representative of 2050. Based on the level of agreement among them, we created future suitability maps for the pine and for the disease independently, which were then used to assess exposure of current populations of P. pinaster to abiotic and biotic effects of climate change. Almost the entire distribution of P. pinaster in the Spanish Iberian Peninsula will be subjected to abiotic exposure likely to be driven by the predicted increase in drought events in the future. Furthermore, we detected a reduction in exposure to pitch canker that will be concentrated along the north-western edge of the study area. Setting up breeding programs is recommended in highly exposed and productive populations, while silvicultural methods and monitoring should be applied in those less productive, but still exposed, populations.

Genome-Based Selection and Characterization of Fusarium circinatum-Specific Sequences

Fusarium circinatum is an important pathogen of pine trees and its management in the commercial forestry environment relies largely on early detection, particularly in seedling nurseries. The fact that the entire genome of this pathogen is available opens new avenues for the development of diagnostic tools for this fungus. In this study we identified open reading frames (ORFs) unique to F. circinatum and determined that they were specific to the pathogen. The ORF identification process involved bioinformatics-based screening of all the putative F. circinatum ORFs against public databases. This was followed by functional characterization of ORFs found to be unique to F. circinatum. We used PCR- and hybridization-based approaches to confirm the presence of selected unique genes in different strains of F. circinatum and their absence from other Fusarium species for which genome sequence data are not yet available. These included species that are closely related to F. circinatum as well as those that are commonly encountered in the forestry environment. Thirty-six ORFs were identified as potentially unique to F. circinatum. Nineteen of these encode proteins with known domains while the other 17 encode proteins of unknown function. The results of our PCR analyses and hybridization assays showed that three of the selected genes were present in all of the strains of F. circinatum tested and absent from the other Fusarium species screened. These data thus indicate that the selected genes are common and unique to F. circinatum. These genes thus could be good candidates for use in rapid, in-the-field diagnostic assays specific to F. circinatum. Our study further demonstrates how genome sequence information can be mined for the identification of new diagnostic markers for the detection of plant pathogens.

Reducing the risk of invasive forest pests and pathogens: Combining legislation, targeted management and public awareness

Intensifying global trade will result in increased numbers of plant pest and pathogen species inadvertently being transported along with cargo. This paper examines current mechanisms for prevention and management of potential introductions of forest insect pests and pathogens in the European Union (EU). Current European legislation has not been found sufficient in preventing invasion, establishment and spread of pest and pathogen species within the EU. Costs associated with future invasions are difficult to estimate but past invasions have led to negative economic impacts in the invaded country. The challenge is combining free trade and free movement of products (within the EU) with protection against invasive pests and pathogens. Public awareness may mobilise the public for prevention and detection of potential invasions and, simultaneously, increase support for eradication and control measures. We recommend focus on commodities in addition to pathways, an approach within the EU using a centralised response unit and, critically, to engage the general public in the battle against establishment and spread of these harmful pests and pathogens.

Sentinel trees as a tool to forecast invasions of alien plant pathogens

Recent disease outbreaks caused by alien invasive pathogens into European forests posed a serious threat to forest sustainability with relevant environmental and economic effects. Many of the alien tree pathogens recently introduced into Europe were not previously included on any quarantine lists, thus they were not subject to phytosanitary inspections. The identification and description of alien fungi potentially pathogenic to native European flora before their introduction in Europe, is a paramount need in order to limit the risk of invasion and the impact to forest ecosystems. To determine the potential invasive fungi, a sentinel trees plot was established in Fuyang, China, using healthy seedlings of European tree species including Quercus petreae, Q. suber, and Q. ilex. The fungal assemblage associated with symptomatic specimens was studied using the tag-encoded 454 pyrosequencing of the nuclear ribosomal internal transcribed spacer-1 (ITS 1). Taxa with probable Asiatic origin were identified and included plant pathogenic genera. These results indicate that sentinel plants may be a strategic tool to improve the prevention of bioinvasions.

Adaptive potential of maritime pine (Pinus pinaster) populations to the emerging pitch canker pathogen, Fusarium circinatum

There is a concern on how emerging pests and diseases will affect the distribution range and adaptability of their host species, especially due to different conditions derived from climate change and growing globalization. Fusarium circinatum, which causes pitch canker disease in Pinus species, is an exotic pathogen of recent introduction in Spain that threatens its maritime pine (P. pinaster) stands. To predict the impact this disease will have on the species, we examine host resistance traits and their genetic architecture. Resistance phenotyping was done in a clonal provenance/progeny trial, using three-year-old cuttings artificially inoculated with the pathogen and maintained under controlled environmental conditions. A total number of 670 ramets were assessed, distributed in 10 populations, with a total of 47 families, 2 to 5 half-sibs per family, and 3-7 ramets per clone. High genetic variation was found at the three hierarchical levels studied: population, family and clone, being both additive and non-additive effects important. Narrow-sense and broad-sense heritability estimates were relatively high, with respective values of 0.43-0.58 and 0.51-0.8, depending on the resistance traits measured (lesion length, lesion length rate, time to wilting, and survival). These values suggest the species' high capacity of evolutionary response to the F. circinatum pathogen. A population originated in Northern Spain was the most resistant, while another from Morocco was the most susceptible. The total number of plants that did not show lesion development or presented a small lesion (length<30 mm) was 224 out of 670, indicating a high proportion of resistant trees in the offspring within the analyzed populations. We found large differences among populations and considerable genetic variation within populations, which should allow, through natural or artificial selection, the successful adaptation of maritime pine to pitch canker disease.

Evidence for multiple introductions and clonality in Spanish populations of Fusarium circinatum

Fusarium circinatum is thought to have been moved around the world with pine planting stock consisting, most probably, of infected seed. In this effort, we investigate the genetic structure of F. circinatum in Spain and globally. In total, 223 isolates were studied from five regions in northern Spain and eight countries. Eight microsatellite markers revealed 66 multilocus genotypes (MLGs). Minimum spanning network analysis of MLGs by region within Spain as well as globally, discriminant analysis of principal components, and analysis of molecular variance revealed that Spanish populations are significantly differentiated and structured into two distinct groups, each one including one of the dominant genotypes observed. This result suggests that two independent introductions occurred into Spain that subsequently underwent clonal divergence and admixture. This result is further supported by the linkage disequilibrium and clonality observed for F. circinatum populations in northern Spain. The maintenance of differentiation between the clusters could result from the lack of or rare sexual reproduction in Spain. Possible introduction pathways from other countries and subsequent routes of dispersion of F. circinatum in Spain are discussed.

Environmental maternal effects mediate the resistance of maritime pine to biotic stress

The resistance to abiotic stress is increasingly recognised as being impacted by maternal effects, given that environmental conditions experienced by parent (mother) trees affect stress tolerance in offspring. We hypothesised that abiotic environmental maternal effects may also mediate the resistance of trees to biotic stress. The influence of maternal environment and maternal genotype and the interaction of these two factors on early resistance of Pinus pinaster half-sibs to the Fusarium circinatum pathogen was studied using 10 mother genotypes clonally replicated in two contrasting environments. Necrosis length of infected seedlings was 16% shorter in seedlings grown from favourable maternal environment seeds than in seedlings grown from unfavourable maternal environment seeds. Damage caused by F. circinatum was mediated by maternal environment and maternal genotype, but not by seed mass. Mechanisms unrelated to seed provisioning, perhaps of epigenetic nature, were probably involved in the transgenerational plasticity of P. pinaster, mediating its resistance to biotic stress. Our findings suggest that the transgenerational resistance of pines due to an abiotic stress may interact with the defensive response of pines to a biotic stress.

Sanitation of a South African Forestry Nursery Contaminated with Fusarium circinatum Using Hydrogen Peroxide at Specific Oxidation Reduction Potentials

Pitch canker, caused by Fusarium circinatum, was first reported in a forestry nursery in the Mpumalanga Province of South Africa in 1990, and it has since spread to almost all forestry nurseries in the country, where it causes significant economic losses. The aim of the current study was to (i) identify sources of F. circinatum contamination in the Karatara forestry nursery in the Western Cape Province and (ii) manage the disease by implementing an oxidation reduction potential (ORP)-based sanitation method using hydrogen peroxide. The irrigation water, planting tray inserts and seeds were screened for fungal contamination. Fusarium circinatum colonies were identified morphologically and confirmed by polymerase chain reaction using species-specific primers. Both the irrigation water and planting tray inserts served as sources of inoculum that introduced the pathogen into the nursery. The irrigation water was amended with hydrogen peroxide at an ORP level of 400 mV for an exposure time of 6 h because it was observed that such a treatment effectively killed all F. circinatum spores and was not phytotoxic to pine seedlings under laboratory conditions. In addition, the contaminated planting tray inserts were cleaned in water amended with hydrogen peroxide at an ORP value of 360 mV for 6 h, which was shown to efficiently eliminate all inoculum from planting tray inserts. Since the introduction of the ORP-based sanitation method at Karatara nursery, losses of pine seedlings were reduced to insignificant levels, and field losses were minimized.

A genetically homogenous population of Fusarium circinatum causes pitch canker of Pinus radiata in the Basque Country, Spain

Pitch canker, caused by Fusarium circinatum, is a destructive disease of Pinus species and has recently been shown to represent a substantial threat to natural and commercial forests in northern Spain. The genetic diversity of F. circinatum in the Basque Country of Spain was assessed by characterising 96 isolates based on vegetative compatibility groups (VCGs), mating type assays, polymorphic DNA-markers and amplified fragment length polymorphism (AFLP) analyses. For this purpose, F. circinatum isolates were collected from diseased Pinus radiata as well as from insects associated with this host. Overall, a low level of diversity was detected in the population. The isolates represented only two VCGs and they were all of the same mating type. AFLP analyses revealed three genotypes and polymorphic DNA-markers specific for F. circinatum showed nine genotypes. The most common genotypes represented 97% of all isolates for AFLP analysis and 68% of isolates for the polymorphic DNA-marker sets. Over all, this indicates that pitch canker in the Basque Country of Spain is caused by a clonally propagating population of F. circinatum, typical of a recently introduced pathogen.

Sensitive detection of Fusarium circinatum in pine seed by combining an enrichment procedure with a real-time polymerase chain reaction using dual-labeled probe chemistry

Fusarium circinatum is the causal agent of pitch canker disease on numerous Pinus spp. This aggressive fungus may infect pine seed cryptically and, therefore, can easily be spread long distances by the seed trade. F. circinatum has recently been listed as a quarantine organism in numerous countries throughout the world, which prompted the development of a specific and sensitive tool for the detection of this pathogen in conifer seed. A new detection protocol for F. circinatum based on a biological enrichment step followed by a real-time polymerase chain reaction (PCR) assay was developed. Several enrichment protocols were compared and a 72-h incubation of the seed with potato dextrose broth was the most efficient technique to increase F. circinatum biomass before DNA extraction. The relative accuracy, specificity, and sensitivity of the real-time PCR assay was evaluated in comparison with a previously published conventional PCR test on 420 seed DNA extracts. The real-time PCR described here proved to be highly specific and significantly more sensitive than the conventional PCR, and enabled the detection of F. circinatum in samples artificially contaminated with less than 1/1,000 infected seed, as well as in naturally infected samples. Last, in order to routinely check the quality of the seed DNA extracts, a primer-probe combination that targets a highly conserved region within the 18S ribosomal DNA in plants or fungi was successfully developed. This assay allows for quick and reliable detection of F. circinatum in seed, which can help to prevent long-distance spread of the pathogen via contaminated seed lots.

Quantitative association of bark beetles with pitch canker fungus and effects of verbenone on their semiochemical communication in Monterey pine forests in Northern Spain

The association between 11 species of bark beetles (Coleoptera: Scolytinae) and one weevil (Coleoptera: Entiminae) with the pitch canker fungus, Fusarium circinatum Nirenberg and O'Donnell, was determined by crushing beetles on selective medium and histone H3 gene sequencing. Pityophthorus pubescens (Marsham) (25.00%), Hylurgops palliatus (Gyllenhal) (11.96%), Ips sexdentatus (Börner) (8.57%), Hypothenemus eruditus Westwood (7.89%), Hylastes attenuatus Erichson (7.40%), and Orthotomicus erosus (Wollaston) (2.73%) were found to carry the inoculum. In addition, the root weevil Brachyderes incanus L. (14.28%) had the second highest frequency of occurrence of the fungus. The responses of the insects to a range of verbenone doses were tested in field bioassays using funnel traps. Catches of P. pubescens, a species colonizing branch tips of live trees, were significantly reduced in a log-linear dose-dependent relationship. Catches of I. sexdentatus, an opportunistic species normally attacking fresh dead host material, were also gradually reduced with increasing verbenone dose. Catches of Tomicus piniperda L., O. erosus, Dryocoetes autographus (Ratzeburg), H. eruditus, Xyleborus dryographus (Ratzeburg), Hylastes ater (Paykull), Hylurgus ligniperda (F.), H. attenuatus, and B. incanus were not significantly affected by verbenone. The effects of verbenone were consistent with differences in host-age preference. Semiochemical disruption by verbenone in P. pubescens and I. sexdentatus could represent an integrated pest management strategy for the prevention of the spread of pitch canker disease between different stands. However, several species associated with F. circinatum were unaffected by verbenone, not supporting this compound for prevention of the establishment of potential vectors in Northern Spain.

Characterization of Fusarium circinatum from Pinus spp. in northern Spain

Pitch canker caused by Fusarium circinatum was recently reported on Pinus spp. in Spain. In this study, a collection of 157 isolates of F. circinatum obtained from different geographical origins and hosts in northern Spain were identified and characterized by cultural and morphological features, PCR-RFLPs of the histone H3 gene, IGS region, and the translation elongation factor 1-alpha gene (TEF). Mating types were determined by multiplex PCR and sexual compatibility was performed under laboratory conditions. Both mating types were present in Spain and were able to form the teleomorph Gibberella circinata. Morphological differences between mating types, not previously reported, were observed: MAT-1 isolates showed clear, coiled, sterile hyphae characteristic of F. circinatum, whereas MAT-2 isolates presented sterile hyphae but not coiled. Virulence of representative isolates was tested on seven to eight-month-old P. nigra, P. pinaster and P. sylvestris seedlings. All isolates tested were pathogenic to these pine species, MAT-1 isolates being more virulent than MAT-2 isolates.

Potential diversity in vegetative compatibility groupings in the California population of Gibberella circinata

Pitch canker, caused by Gibberella circinata, is a disease affecting pines throughout the world. Although the pathogen is capable of sexual reproduction, natural populations are often comprised of very few vegetative compatibility groups (VCGs), which implies a predominance of asexual reproduction. However, even where outcrossing occurs, a population could have limited VCG diversity due to a low level of polymorphism at the loci governing vegetative compatibility (=vic loci). To determine whether this was the case for the California population of G. circinata, inter-fertile strains were crossed under laboratory conditions. Progeny of this cross included a minimum of 29 VCGs, which was consistent with segregation of vic alleles at five loci. Only two of these VCGs were known to occur naturally in California. Three VCGs accounted for 32.4% of the progeny but 0% of the California population. Overall, the results support the conclusion that outcrossing has been rare or absent in the California population of G. circinata.