Corresponding author: Carmen Morales-Rodríguez (
Academic editor: Richard Shaw
The number of invasive alien pest and pathogen species affecting ecosystem functioning, human health and economies has increased dramatically over the last decades. Discoveries of invasive pests and pathogens previously unknown to science or with unknown host associations yet damaging on novel hosts highlights the necessity of developing novel tools to predict their appearance in hitherto naïve environments. The use of sentinel plant systems is a promising tool to improve the detection of pests and pathogens before introduction and to provide valuable information for the development of preventative measures to minimize economic or environmental impacts. Though sentinel plantings have been established and studied during the last decade, there still remains a great need for guidance on which tools and protocols to put into practice in order to make assessments accurate and reliable. The sampling and diagnostic protocols chosen should enable as much information as possible about potential damaging agents and species identification. Consistency and comparison of results are based on the adoption of common procedures for sampling design and sample processing. In this paper, we suggest harmonized procedures that should be used in sentinel planting surveys for effective sampling and identification of potential pests and pathogens. We also review the benefits and limitations of various diagnostic methods for early detection in sentinel systems, and the feasibility of the results obtained supporting National Plant Protection Organizations in pest and commodity risk analysis.
Morales-Rodríguez C, Anslan S, Auger-Rozenberg M-A, Augustin S, Baranchikov Y, Bellahirech A, Burokienė D, Čepukoit D, Çota E, Davydenko K, Doğmuş Lehtijärvi HT, Drenkhan R, Drenkhan T, Eschen R, Franić I, Glavendekić M, de Groot M, Kacprzyk M, Kenis M, Kirichenko N, Matsiakh I, Musolin DL, Nowakowska JA, O’Hanlon R, Prospero S, Roques A, Santini A, Talgø V, Tedersoo L, Uimari A, Vannini A, Witzell J, Woodward S, Zambounis A, Cleary M (2019) Forewarned is forearmed: harmonized approaches for early detection of potentially invasive pests and pathogens in sentinel plantings. NeoBiota 47: 95–123.
Invasive alien species (
Global trade of plants for planting is recognised as the principal pathway for accidental introductions of alien invasive forest and agricultural pests and pathogens worldwide (
Most National Plant Protection Organizations (
In principle, an early warning system is a major element of disaster risk reduction (
Two main strategies apply to the sentinel planting concept: sentinel plantations and sentinel nurseries (Figs
The aim of this paper is to provide an overview of the protocols and techniques useful in sentinel plantings with a focus on: 1) the capacity for sentinel systems to provide useful information to
Schematic representation of the sentinel plantation concept. Tree species native to the importing country are planted in the exporting country. Being exposed to the resident pest and pathogens, they should develop visible symptoms.
Schematic representation of the sentinel nursery concept. Tree species native to the exporting country and traded with the importing country are regularly inspected for resident pest and pathogens. Because of host-parasite coevolution, visible symptoms may not necessarily develop.
Schematic representation of the sentinel arboretum (botanical garden) concept. The exotic and native tree species cultivated in the same area/environment are cross-exposed to inoculum harbored by each of the species. The identification of causal agents of different symptomatologies provides a list of new pests or pathogens potentially harmful to those plants in their native environments.
The Food and Agriculture Organization of the United Nations (
Pest risk analysis (
Despite the great amount of data that can be derived from sentinel plantings, there are several issues that the scientific community and plant health regulators need to address in order to best optimize the use of these data:
1. There is currently a mismatch between the systems of identification and classification of pests and pathogens used by scientists (e.g. pathogen lineages, molecular OTUs, taxon) and those used by regulators (usually formal species). How data on higher or lower taxonomic levels could be used in plant health regulations or specifically
2. The number of unidentified taxa and new pest/pathogen-host relationships in recent sentinel planting studies remains high (
3. Reliance on DNA methods for detecting a pathogen does not reveal any indication of the viability of that particular organism. Hence, a limitation of high throughput sequencing (
4. If numerous potential pests and/or pathogens are detected, the limited resources available for carrying out the labour-intensive
5. Currently, the sharing of occurrence and disease data from existing sentinel plantings is rare, but a centralized database, as suggested by
In sentinel plantations, non-native plants are grown in a country out of their natural distribution range (e.g. native European trees planted in China) and monitored for potentially damaging agents which may provide useful data for
It is necessary to carry out
In a sentinel nursery, native plants are grown in their natural distribution range to identify potential pests or pathogens which could be spread with the international trade of these plants (Fig.
Previous fungal studies in sentinel nurseries have not provided conclusive evidence of identified risks but rather provided information that must be analyzed to arrive at a selection of taxa for further study of whether these organisms pose a threat if introduced in a naïve habitat (
During arthropod studies in sentinel nurseries (
A sentinel arboretum (Fig.
Protocols used in sentinel arboreta should aim to characterize damage morphotypes, followed by isolation or collection, and species level identification of the organisms causing these symptoms. The non-native trees might harbour endophytic microflora since the time of their introduction into arboreta as propagation material (e.g. seeds, seedlings, cuttings).
For the three cases of sentinel plantings presented above, confirmation of pathogenicity on the host plant is an essential step for determining the causal agent of disease (Koch’s postulates). Thus, collection and isolation of the organism from symptomatic plants is crucial for establishing the causative relationship between a microbe and the disease or symptoms it produces. This procedure, however, is limited to mainly non-biotrophic organisms which can be cultured onto nutrient media. Once the causal agent is known, additional inoculation trials can be designed and carried out to evaluate its potential host range. Colonizing insects observed on sentinel plants must not be incidental, but clearly capable of completing the entire life cycle on the given host, especially when non-native plants are used in sentinel plantings. This process is difficult to ascertain because rearing possibilities on non-native plants could be limited when such plants are only growing within a sentinel plot. One way to distinguish between incidental species and potential pest could be to consider the number of successive colonization events attained over a number of years by an insect species on the same non-native tree.
A first step towards the identification of causal agents of damage is usually the observation and recording of symptoms and signs of infections in the field. In the framework of the COST Action FP1401 Global Warning (a global network of nurseries as early warning system against alien tree pests;
The successful detection of potentially harmful pests and pathogens in sentinel plantings relies on several conceptual, methodological and organizational factors. Among these, experimental design (i.e. how sentinel plantings are organized, e.g. how many replicates of each tree species), and sampling design (i.e. how, when and what should be sampled) are critical to making sampling as efficient and reliable as possible (Eschen et al. in prep). Similar-looking symptoms might have different causes, and for this reason, the diagnostic procedure can be challenging. Although sentinel plants might be colonized and/or damaged by a broad range of organisms, some general principles about sample collection and preservation apply to all organisms (
1. As different organisms can affect a single plant, the whole plant should be carefully checked for different damage morphotypes (hereinafter referred to as damage characteristic of a certain pest or pathogen) (Tables
2. Before collecting symptomatic plant material, high-resolution photographs of the whole plant, of the damaged organ(s), and, if present and visible, possible damaging agent(s) should be taken. Categorization of damage morphotypes (Tables
3. Cross-contamination from sampling instruments (e.g. secateurs, pruning saw, forceps) should be avoided; this is of particular importance when sampling for pathogens.
4. The best period for sampling varies according to the affected tissues and the suspected causal agents. If possible, at least three samplings per year (spring, summer and fall) should be conducted.
5. Samples should also be taken from apparently healthy tissue to know what healthy plant tissue looks like during normal growth, to potentially detect differences in microbial community composition between healthy and symptomatic tissues, and to study latent infection or endophytes.
6. Proper labelling of sampled material is an essential step without which biological specimens lose their scientific value (
7. The stringency of sample disinfection before processing represents an additional variable, especially for biological detection of culturable microorganisms. However, the adoption or not of surface sterilization of samples also represents a conceptual decision. Specifically, in the case of sentinel nurseries, superficial contamination of plants might represent an additional pathway of introduction of alien microorganisms that deserves further attention (
Apart from these general principles, which apply to all groups of damaging agents, there are approaches for sample collection that are specific to the affected plant tissues and causal agent groups (Table
Pathogens can affect all plant tissues and cause a broad range of symptoms, which could affect the whole plant (e.g. general dieback) or be more localized (e.g. wilting of individual branches). Based on the tissue affected and the type of damage induced (i.e. damage morphotype, Table
Similar to pathogens, sampling of invertebrates varies depending on the affected plant tissue (Table
Information on pests and pathogens are needed for pest- and commodity risk analysis including the organism’s identification to the species level and its associated hosts. A variety of traditional, inexpensive techniques and advanced molecular methods are available for identification purposes. The key problem, upon detection of a living pest or pathogen is its correct and rapid identification. Molecular tools can satisfy both of these criteria and have, to some extent, the advantage of being automated. These characteristics make molecular diagnostics as complementary methods to classical morphology-based identification (
Conventional detection of pathogens involves macroscopic and microscopic examination of symptomatic plant material and isolation of the causal agent. Often, specific isolation protocols, based on optimal requirements for types of pathogens are available, potentially increasing isolation success. However, when working with sentinel plants, there is a risk that causal agents are unknown to science. For this reason, sampled material should be analyzed using a variety of isolation methods, different culture media and temperatures.
Once isolated in pure culture, macroscopic traits, including colony shape, texture and color, and microscopic characteristics of vegetative and reproductive structures are useful criteria for characterization and identification of isolates (
One problem with the identification of pathogens is the impossibility to grow some organisms on artificial/synthetic media. Obligate parasites such as rust fungi, powdery mildews, viruses and mollicutes require a living host to grow and reproduce. For these organisms vegetative and/or reproductive structure characteristics must be observed on specimens directly from the living host using optical microscopy, or electron microscopy for viruses and mollicutes. Apart from the
Commercially designed kits, such as enzyme-linked immunosorbent assays (
Molecular-based techniques using polymerase chain reaction (
Rapidly evolving high-throughput sequencing (
Third-generation sequencing technologies such as PacBio (
The use of
The observation and evaluation of damage on plants is the first step towards a diagnosis of damaging arthropod and nematode pests. Damage morphotypes can be effectively utilized in sentinel planting surveys as an identifier to assign phytophagous pests to certain feeding guilds, prior to species identification using morphology-based taxonomy (
The rapid development of computer vision technologies has led to applications in highly promising automatized arthropod identification platforms based on multivariate biometric features of the taxon. This novel approach, based fully on classical taxonomy and computer algorithms, allows species identification procedures to be performed even by non-taxonomists, with a high degree of reliability (
DNA barcoding is a well-known molecular approach to species identification (
For arthropods, DNA barcoding uses a short genetic marker – a fragment of mitochondrial DNA (
As for pathogens, one of the limitations of DNA barcoding is the lack of appropriate reference databases, which would cover all formally described arthropods. To date, comprehensive databases have been accumulated mainly for certain insect taxa (e.g.
For nematodes, several genes are targeted for identification such as the mitochondrial cytochrome b locus (
Invasive pests and pathogens are major threats to the health of plants and forests. Key to controlling these invasions are preventative measures that will allow for early detection of potentially damaging organisms preferably before they are introduced to a new region. Sentinel plants can have a fundamental role in this early detection and help predict associated risks to plants in the importing country. The three sentinel plantings described offer different possibilities to provide information useful for
The protocols and diagnostic approaches to follow will therefore vary amongst these systems. For sentinel plantations, the main focus is on symptoms found on the plants and the identification of the causal agent(s) for which classical identification methods are the key. In contrast, the focus for sentinel nurseries and sentinel arboreta should be on identifying a large number of taxa associated with the host irrespective of whether they are causing damage.
The following recommendations can be given to promote the use of data collected through sentinel plantings: 1) better communication between scientists and
This work was supported by COST Action Global Warning (FP1401). DLM and YB contribution was also supported by the Russian Foundation for Basic Research (Grant No. 17-04-01486). MG was supported by Ministry of Education, Science and Technological Development of the Republic of Serbia, Grant III43002. MKA was supported by the Ministry of Science and Higher Education of the Republic of Poland. NK was supported by Le Studium foundation (France) and RFBR (Grant No. 19-04-01029). RE, IF and MK contribution was also supported by CABI with core financial support from its member countries (see
Diagnostic approach for the identification of plant pathogens.
Damage morphotype | Main symptoms and/or signs | Causal agent(s) | Diagnostic approach |
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Foliage (leaves and needles) | |||
Discolouration and necrosis | Necrotic spots or patches of different shapes and colours, ring- or net-shaped lines, bands, reduced leaf size; possible presence of reproductive structures on necrotic area | 1. Isolation from symptomatic tissue | |
2. Molecular barcoding from cultures | |||
3. Serological test from symptomatic tissue | |||
4. Morphological description of signs (OM1) | |||
Mould | Soot-like or powdery deposit on the surface; mycelial mats, reproductive structures |
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1. Isolation from symptomatic tissue |
2. Molecular barcoding from cultures | |||
Rust | Blisters and/or pustules on the surface (fruiting bodies) | 1. Morphological description of signs (OM1) | |
2. Molecular barcoding from symptomatic tissue/signs | |||
Mildew | White powdery mycelium and reproductive structures (including fruiting bodies) on the surface | 1. Morphological description of signs (OM1) | |
2. Molecular barcoding from symptomatic tissue/signs | |||
Reproductive structures (flower, catkins, cones, fruits, seeds) | |||
Discolouration and necrosis | Discolorations, necrotic spots; reproductive structures (fruiting bodies) | 1. Morphological description of signs (OM1) | |
2. Molecular barcoding from symptomatic tissue/signs | |||
Rust | Blisters and/or pustules on the surface (fruiting bodies) | 1. Morphological description of signs (OM1) | |
2. Molecular barcoding from symptomatic tissue/signs | |||
Mould | Soot-like or powdery deposit on the surface; mycelial mats, reproductive structures |
|
1. Isolation from the symptomatic tissue |
2. Molecular barcoding from cultures | |||
Mildew | White powdery mycelium and reproductive structures (including fruiting bodies) on the surface | 1. Morphological description of signs (OM1) | |
2. Molecular barcoding from symptomatic tissue/signs | |||
Fruit rot (mummification) | Entire or partial discolourations, chalky or sponge-like appearance, necrotic spots; fungal mycelium and reproductive structures |
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1. Isolation from symptomatic tissue or signs |
2. Molecular barcoding from cultures | |||
Stems, branches and twigs | |||
Butt and stem rot | Bark lesions, eventually with exudates; fruiting bodies | 1. Isolation from symptomatic tissue or signs | |
2. Molecular barcoding from cultures | |||
Bark necrosis (canker) | Localised necrotic lesions, swollen or sunken, eventually with exudates; reproductive structures (fruiting bodies) | 1. Isolation from symptomatic tissue or signs | |
2. Molecular barcoding from cultures | |||
Witches’ broom | Concentration of young shoots, which are thicker and shorter than normal ones; reproductive structures (fruiting bodies) | 1. Direct symptom observation | |
2. Isolation from symptomatic tissue | |||
3. Molecular barcoding from cultures or symptomatic tissues (e. g mollicutes) | |||
Epicormic shoots/fasciation | Sprouts growing from dormant buds, flattened, elongated shoots and flower heads | 1. Direct symptom observation | |
2. Isolation from symptomatic tissue | |||
3. Molecular barcoding from cultures or symptomatic tissues (e. g mollicutes) | |||
Shoot blight or dieback | Discolorations, wilting or crooking from the tip of the shoots, eventually exudates | 1. Direct symptom observation | |
2. Isolation from symptomatic tissue | |||
3. Molecular barcoding from cultures or symptomatic tissues (e. g mollicutes) | |||
Roots | |||
Root rot | Wood decay and eventually staining, root exudates; fruiting bodies | 1. Isolation from symptomatic tissue or signs | |
2. Molecular barcoding from cultures |
1 Optical Microscopy
Diagnostic approach for the identification of invertebrate plant pests.
Damage morphotype | Main symptoms and/or signs | Causal agent(s) | Diagnostic approach1 |
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Foliage (leaves and needles) | |||
Discolouration | Spots, galleries of different shapes, size and colours, mosaic-like discoloration | Insects (leaf-mining, sucking), mites | 1. Collecting damaged leaves for presence of damaging agent |
2. Sampling insects from mines, or on leaf surface; herbarizing leaves with typical damage | |||
3. Rearing larvae to adults | |||
4. Morphological identification and/or DNA barcoding (MI & DNA2) | |||
Chlorosis, yellowing or browning. External symptoms reflect infestation of wood or roots | Nematodes | See the sections “Stems, branches and twigs” and “Roots” | |
Lack of surface/ tissue parts | Skeletisation, perforation, holes, cut-outs, rough eating | Insects, snails and slugs | 1. Identifying damage type |
2. Sampling feeding larvae and adults directly from leaves or by beating branches. | |||
3. MI & DNA | |||
Other coating/covering | Foth, wax, spittle, webbing | Insects, mites | 1. Sampling damaging agent by removing the coating or opening the construction (nests) |
Construction | Nests | 2. MI & DNA | |
Deformation | Rolling, curling, twisting, reduced size | Insects, mites | 1. Collecting damaged leaves for damage type identification |
2. Sampling arthropods by opening the rolls and deformed tissues; herbarizing leaves with typical damage | |||
Outgrowth of plant tissue | Galls | 3. MI & DNA | |
Reproductive structures (flower, catkins, cones, fruits, and seeds) | |||
Discolouration | Entire or partial (spotted) discolouration, necrotic spots | Insects, mites | 1. Sampling mites or insect larvae by opening the affected organ |
2. MI & DNA | |||
Other coating/covering | Presence of resin flow, white dusting, shield or felt-like covering, etc. | Insects (sap-feeders) or mites | 1. Sampling mites, sucking aphids, etc. from the affected organ |
2. MI & DNA | |||
Internal damage: tunnels, holes | Damage invisible at the beginning; later detected as tissue deformation, presence of openings and insect frass on the surface | Insects | 1. Sampling larvae/adults from damaged organs/tissue |
2. At early-stage, X-ray seeds for the presence of the damaging agent inside | |||
3. Rearing larvae in damaged organs to adults | |||
4. MI & DNA | |||
External injuries | Gnawing, rough eating (lack of tissues parts) | Insects | 1. Sampling feeding larvae (nymphs) or adults directly from damaged organs |
2. MI & DNA (any development stage) | |||
Deformation | Distorted or shrivelled organs/tissues (especially flowers, conelets) | Insects, mites | 1. Sampling by opening damaged organs/tissues |
Outgrowth of plant tissue or abnormal growth | Swollen organs, gall formations | 2. MI & DNA (any development stage) | |
Apparently sound seeds | Apparently sound | Insects | X-raying to reveal presence of larvae |
Stems, branches, and twigs | |||
Coating/covering | Presence of white dust shield or felt-like covering, etc. | Insects (sap-feeders) | 1. Sampling insect from damaged surface |
2. MI & DNA | |||
Internal damage: galleries | Damage invisible at the beginning; later detected through the presence of holes on the bark, insect frass on the surface | Insects | 1. Sampling by opening bark with holes or insect frass on the surface |
2. Collecting fragments of bark or wood with typical galleries for damage morphotype identification | |||
3. MI & DNA | |||
Internal damage: embolism of xylem tissue | Disruption of water transport in the tissues (timber) accompanied by external symptoms: plant stunting, wilting and foliage discoloration | Nematodes | 1. Remove bark and inspect sapwood |
2. Collect nematodes | |||
3. MI & DNA | |||
External injuries | Scars on bark, debarking/bark stripped (girdling or pruning) | Insects | 1. Sampling the damaging agent feeding on the bark or by opening swollen plant tissue |
Outgrowth of plant tissue | Swollen tissues, gall formations | 2. MI & DNA | |
Roots | |||
Deformations, root knot or galls, necrosis, atrophy | Thickenings in a variety of shapes, stunting, appearance of necrotic spots, dying-off roots. Accompanied by plant stunting, wilting and foliage discoloration. | Insects, nematodes | 1. Sample externally feeding larvae |
2. Collect affected fragments of roots, examine externally and dissect knots and galls to find insect larvae or nematodes (using magnification) | |||
3. MI & DNA | |||
Injuries (internal and/or external) | Debarking/bark stripped, tunnels, holes and/or frass at root collar | Insects | 1. Sampling damaging agent |
Coating/covering | Wax, dust | 2. MI & DNA (any development stage) |
1As a rule, morphological identification of damaging agent is applicable to adult stage solely, whereas for DNA-barcoding any development stage can be used; 2MI & DNA: Morphological identification and/or DNA barcoding.
Sampling methods used in sentinel plantings.
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Foliage | • Whole leaves/needles should be collected, not only symptomatic parts • If symptoms occur on foliage at different stages all developmental stages should be collected • If symptoms concern whole shoots (e.g. wilting), it is likely that the causal agent has infected the twig/branch and not the foliage, which should also be checked | • Leaves/needles should be collected dry and rapidly processed, avoiding long storage • Leaves with diagnostic damage type should be stored in herbarium collection |
Reproductive structures1 | • Whole reproductive structures should be collected • If symptoms occur on foliage at different stages all developmental stages should be collected | • Apart from cones, seeds and some fruits are better kept dry |
Shoots, twigs, branches, stems | • Samples should include the region where healthy tissue borders infected tissue. If symptoms occur on a small branch or sprout, the entire symptomatic section of the branch or shoot should be collected • For vascular diseases and to a lesser extent butt and stem rots, symptoms are often only seen when the bark is removed, and the wood exposed | • Wood tissues should be kept in humid conditions and stored cold (5–8 °C) |
Roots | • Carefully remove the soil to expose the main superficial roots. Samples should include the region where healthy tissue borders infected tissue • Since roots are generally infected by soil-borne organisms, soil samples should be collected from the rhizosphere of trees with symptomatic roots | • Roots tissues should be kept in humid conditions and stored cold (5–8 °C) |
Visible signs of pathogen damage2 | • Fruiting bodies and mycelial fans (below the bark) are reliable indicators of pathogen presence and should be sampled either alone or with the substrate on which they grow | • Samples should be stored cold (5–8 °C) and processed rapidly to avoid long storage |
Sampling invertebrates | ||
Tissue | Collection | Preservation |
Foliage | • Leaves with typical damage caused by endophagous arthropods (mines and galls), which are often host plant specific, should be preserved as herbarium specimens as they might provide essential information for taxon identification at a later stage | • Preserve arthropods in ethanol, either at 70% for morphological identification or 96% for molecular identification • Slugs and snails can be stored in water in sealed containers • Mites should be preserved in a mixture of ethanol and lactic acid • Plant tissues can be preserved until their processing as described above. |
Reproductive structures | • Organs with visible damage symptoms should be collected, with immature individuals present inside • The fruits, cones or seeds can be collected from the ground under a tree or by beating branches over sheets or netting • Seeds can be extracted from fruits or cones and a subset of seeds with no visible signs of damage must be X-rayed to assess the possible presence of larvae inside. Collected seed can also be kept in the laboratory until adult emergence | |
Shoots, twigs, branches and stems | • Pests feeding on plant tissues can be sampled directly from the surface or by debarking • Immature insect stages hidden in plant tissues can be sampled together with a healthy plant fragment and reared in the laboratory • For assessing the presence of wood nematodes, wood discs, chips or sawdust should be collected from the sapwood of symptomatic trees, if possible at different stem heights for further diagnostics • Stem sections with dark staining in the sapwood often indicating the presence of blue stain fungi, or signs (holes, galleries) of xylophagous insects should also be sampled | |
Roots | • The base of the trunk and the roots should be first inspected for the presence of holes and sawdust (frass) and dissected to find pests • Fine feeder roots showing disease symptoms should also be sampled • Litter and soil around the damaged roots should be inspected • For diagnostics of root-knot nematodes fine roots and soil must first be collected |
1 i.e. flowers, fruits, catkins, cones and seeds; 2 The term ‘visible’ means everything observable in the field to the naked eye, or with simple, portable magnifying instruments