Ceratocystis platani

Current scientific name: Ceratocystis platani (JM Walter) Engelbr. & TC Harr. 2005 Class: Sordariomycetes Order: Microascales Family: Ceratocystidaceae Genus: Ceratocystis Species: Ceratocystis platani Synonym(s): Ceratocystis fimbriata f. sp. platani C May & JG Palmer 1959 and Endoconidiophora fimbriata f. sp. platani JM Walter EPPO Code:  CERAFP  Common name: canker stain of plane, blue stain canker, canker of sycamore

Ceratocystis platani species was first described as Endoconidiophora fimbriata f. sp. platani (Walter, 1946), then moved to genus Ceratocystis (Bakshi, 1950; Hunt, 1956) and included in the Ceratocystis fimbriata species complex as C. fimbriata f. sp. platani (May and Palmer, 1959). The fungus was later reclassified as Ceratocystis platani based on phylogeny (Engelbrecht et al., 2004; Engelbrechtand Harrington, 2005).

Ceratocytis platani is a Union quarantine pest listed in Annex II Part B of  Commission Implementing Regulation (EU) 2019/2072 . Annex VII sets out special requirements for the introduction into EU territory of plants for planting other than seeds (point 39) and for wood except wood packaging material (point 95) of Platanus L. from Albania, Armenia, Switzerland, Turkey and the United States. Annex VIII sets out special requirements for the movement within the EU of machinery and vehicles operated for agricultural and forestry purposes (point 1), plants for planting (point 17) and wood (point 24) of Platanus L. originating in the Union territory. Annex XI part A and Annex XIII set out the requirements for phytosanitary certificates for the introduction into the Union territory of wood or plants for planting of Platanus L.

The general requirements for survey of quarantine pests in the EU territory are laid down in  Regulation (EU) 2016/2031  and  Commission Implementing Regulation (EU) 2020/1231 .

Ceratocystis platani is native to the eastern United States (Walter et al., 1952; Engelbrecht et al., 2004), and was originally described in 1935 in Pennsylvania on Platanus × hispanica, (initially reported as P. orientalis) (Jackson and Sleeth, 1935). Native range in the United States includes California, Nevada and Washington, in addition to several eastern states (EPPO, 2021).

The pathogen was accidentally introduced to southern Europe (Panconesi, 1999) during World War II, probably through infected wood packaging material (Santini and Capretti, 2000). The first reports of C. platani in Europe are from 1972 in Tuscany, Italy (Panconesi, 1972), and 1974 in Marseille, southern France (Ferrari and Pichenot, 1974). In the EU C. platani is currently present in France, Greece and Italy (Granata and Guastella, 1986; Chapin and Arcangioli, 2007; EPPO, 2014, 2021). The pathogen is also present in Albania, Switzerland, Turkey and Armenia (EPPO, 2021). Large outbreaks were described in Greece in natural stands of P. orientalis (Ocasio-Morales et al. 2007) and in European Istanbul, Turkey on both P. orientalis and P. x hispanica (Lehtijärvi et al., 2018). Local outbreaks in Corsica and Spain were eradicated (Chapin and Arcangioli, 2007; EPPO, 2016).

Note: the information included in this section is aligned with the EPPO map updated on 24-02-2021

Ceratocystis platani infects trees through wounds caused by accidental injuries or animal activity on the branches, trunk or roots of trees (Vigouroux and Stojadinovic, 1990; Tsopelas et al., 2017; Panconesi et al., 2003). Conidia (asexual spores) of C. platani germinate when they come into contact with a wound, then the developing mycelium colonises xylem, proliferating both longitudinally and tangentially into the sapwood. Within 2–8 days following the infection, endoconidia are produced, typically visible as a grey powdery layer on pruning cut surfaces (Panconesi, 1999; Engelbrecht et al., 2004). Following the initial infection, aleurioconidia (or chlamydospores), a type of asexual resting spore, can be produced within xylem vessels (EPPO, 2014). The sexual stage starts 6–8 days after the infection and involves the production of perithecia containing ascospores on the surface of infected trees (pruning cut surfaces) (Panconesi, 1999). Mycelial proliferation within the xylem causes the wilting of leaves and the formation of stem cankers on branches and the trunk. A single infection can cause 2–2.5 m long canker stain within one year, and it can kill a tree of 30–40 cm in diameter within two years (Panconesi, 1999). Secondary infections by other fungi often occur on infected tissue. Ceratocystis platani can survive as aleurioconidia in the wood (including timber, packaging and debris) for several months or years (Grosclaude et al., 1996; Engelbrecht et al., 2004). The optimal temperature for the development of C. platani is 25°C. The pathogen does not grow above 45°C or below 10°C (CABI, 2019), but it tolerates low winter temperatures, while temperatures of 35–40°C in the summer impact survival in the soil (Mutto Accordi, 1998). The most suitable period for infectionand disease development in Europe is from May to September; canopy wilt usually appears in spring and summer when the tree has a high water demand, but when infections occur later in the season, the infected branch or the entire tree may not flush the following spring, with new leaves withering (Panconesi, 1999).

This section provides the information needed to characterise the population of host plants to target in a survey, as described in the  ‘General guidelines for statistically sound and risk-based surveys of plant pests’  (EFSA et al., 2020). This includes the pest’s host range and main hosts in the EU ( Section 2.1 ), the suitability of EU environments to the pest’s establishment ( Section 2.2 ), the ability of the pest to spread ( Section 2.3 ), and the identification of risk factors associated with an increased probability of presence ( Section 2.4 ).

Once the above parameters have been defined, the target population can be structured in multiple levels. At level 1 is the survey area, which corresponds to the entirety or part of the Member State. At levels 2 and 3 are the epidemiological units that can be distinguished within the survey area. Epidemiological units can be chosen as administrative regions (e.g. EU NUTS areas or Member State-level regions) if they are homogeneous, or further subdivided into the environments where host plants are present using a land-use categorisation (e.g. urban, agricultural and natural areas, nurseries). At level 4, if risk factors are identified, the risk areas are defined around the risk locations. At level 5 are the inspection units, the elementary subdivisions of the target population that are inspected for the detection of the pest (e.g. host plants), depending on the pest detection method ( Section 3 ). For the definitions of the target population, epidemiological units and inspection units, see also the  glossary  of terms available at the end of this document.

The hierarchical structure of the target population should be tailored to the situation in each Member State. A possible structure of the target population for surveys of C. platani within the EU is proposed in  Section 2.5 .

The only known hosts of C. platani are the Oriental plane tree P. orientalis L., the sycamore or American plane tree P. occidentalis L., and the London plane tree P. x hispanica (syn. P. acerifolia) which is the hybrid of the previous two species (Tsopelas et al., 2017). Platanus orientalis and P. x hispanica are more susceptible to the disease than P. occidentalis (Panconesi, 1999), and there is no record of C. platani infection for the remaining species of the genus Platanus, which includes 10 species and varieties with Paleartic and Neartic distribution (Panconesi, 1999; Tsopelas et al., 2017). Platanus orientalis grows naturally in riparian settings in southern Eurasia from the eastern Mediterranean to Iran (Grueva and Zhelev, 2011; Caudullo et al., 2017), but is also used extensively as urban trees in those regions. The London plane tree P. x hispanica has been widely planted in Europe since the 1800s as a major ornamental tree in city streets, parks, along channels and extra-urban roads (Santamour and McArdle, 1986; Santini, 2001), and is found in all the EU Member States (Santini, pers. comm., 2021). Overall, C. platani hosts in the EU are present in urban areas (parks and streets), extra-urban areas mostly along roads and canals, and in natural habitats along rivers.

Ceratocystis platani is successfully established in some countries in the Mediterranean EU: Italy, France and Greece. These regions correspond to temperate and continental climates (EFSA Panel on Plant Health, 2014; EFSA Panel on Plant Health et al., 2016). Based on the biology of the fungus and the occurrence of these established populations, the potential establishment and further spread of the pathogen within the entire EU is not limited by major eco-climatic factors (La Porta et al., 2008; EFSA Panel on Plant Health, 2014). Suitable hosts are widely available mostly as urban and suburban planted trees in most of the EU territory but also as natural stands of P. orientalis in southern Member States. Considering this, C. platani is expected to become established and spread in all the areas where plane trees are available, but natural spread in the Member States of the Baltic region (Lithuania, northern Poland, Latvia, Estonia) might be reduced by the slower growth and limited abundance of hosts.

Ceratocystis platani could spread further within the EU due to a combination of natural and human-assisted dispersal, thus supporting local spread of outbreaks and the long-distance introduction in new areas. The fungus might be expected to spread up to very few hundred meter per year on land and up to several hundred through water along rivers, but these distances can be magnified by the movement of contaminated material and machinery that might occur from the infected area.

Natural spread

Ceratocystis platani can spread through the dispersion of spores via water flows, rain and wind, and animals (Panconesi et al., 2003; Engelbrecht et al., 2004; Ocasio-Morales et al., 2007, CABI 2019), including insects (Soulioti et al., 2015). Root anastomosis is a major means of spread of the pathogen on a local scale because plane trees tend to grow in small groups along rivers, and are planted contiguously in rows in the city (Mutto Accordi, 1988; Ocasio-Morales et al., 2007; Tsopelas et al., 2017). The root system of an infected tree can reach and infect another tree up to 20 m away (Alberto Santini and Panagiotis Tsopelas, personal communication). Contaminated sawdust and frass dispersed by wind, rain or running water contribute to local spread (Harrington, 2000). Conidia can be easily dispersed by the running water of canals, streams and rivers (Panconesi et al., 2003), and C. platani was detected up to 108 m downstream from infected logs in a controlled experiment (Grosclaude et al., 1991). The fungus was observed to spread several hundred meters downstream in rivers in Greece due to dead infected logs carried by the water (Panagiotis Tsopelas, personal communication). This is a major means of spread since P. orientalis is a riparian species and P. × hispanica is often planted along channels and rivers. Wind and rain are occasional dispersal mechanisms(CABI, 2019).

Human-assisted spread

Human activities are a major means for the introduction of C. platani into new areas and further local proliferation (McCrakenand Burkhardt, 1977; Panconesi et al., 2003; Tsopelas et al., 2017). The movement of infected seedlings, grafted plants, rootstocks and scions (Ocasio-Morales et al., 2007; Santini et al., 2013), and infected timber and wood products are a main pathway for the introduction to new areas (CABI, 2019; EFSA Panel on Plant Health et al., 2016). Ceratocystis platani can be transported long distances on wood used for packaging, as has been widely accepted as the mode of introduction of the fungus into the EU from the United States (Cristinzio et al. 1973; Panconesi 1999), pruning tools contaminated with infected sawdust/wood debris (Walter 1946; Walter et al., 1952; Panconesi, 1999; Tsopelas et al., 2017), and the movement of earth-moving equipment and machinery contaminated with infected soil cause local but also long-distance spread (Panconesi, 1999). The pathogen was introduced to northwestern Greece from southern Greece on roadwork machinery (Ocasio-Morales et al., 2007). Contaminated sawdust has been trapped up to 200 m fromthe nearest infected tree during tree-felling in Italy, even with phytosanitary precautions in place (Luchi et al., 2013).

Identification of risk factors and their relative risk estimation are essential for performing risk-based surveys. A risk factor is a biotic or abiotic factor that increases the probability of infestation by the pest in the area of interest. The risk factors that are relevant for surveillance need to be characterised by their relative risk (should have more than one level of risk for the target population) and the proportion of the overall target population on which they apply. The identification of risk factors needs to be tailored to the situation of each Member State. This section presents examples of risk factors for C. platani and is not necessarily exhaustive. 

For the identification of risk areas, it is first necessary to identify the activities that could contribute to introduction or spread of C. platani. These activities should then be connected to specific locations. Around these locations, risk areas can be defined, knowing that their size depends on the spread capacity of the target pest and the availability of host plants around these locations.

The Member States can opt to utilise the information available on the EU Platforms of TRACES NT, EUROPHYT Interceptions and EUROPHYT Outbreaks. The information available, in particular, relating to the country of origin, type of commodity and hosts of intercepted or outbreak reports can be extracted from such platforms for specific harmful organisms. This information can allow Member States to identify potential pathways of introduction from previous historical findings. Thus, Member States might consider focusing their surveillance efforts around activities and locations related to previous interceptions and outbreaks.  

Such information should only be considered as indicative and given the possible dynamic changes, it should be reviewed and analysed periodically.

Example 1: Tree felling, pruning and roadworks. The movement of contaminated cutting and earth-moving equipment from areas and countries where the fungus is present is a major risk for the disease introduction and spread, as documented for Greece (Tsopelas et al., 2017). In addition, the dispersion of infected wood debris during the pruning and removal of infected trees is an important means of spread (Luchi et al., 2013). As a result, sites where plane trees are pruned or felled, and roadworks near plane tree sites (Figures above) should be considered risk locations, for both detection and delimiting surveys.

Example 2: Trade in infected timber and wood packaging material. This remains a risk activity for the introduction and spread of C. platani, since it has been widely accepted as the mode of introduction of the fungus into Europe originating from the United States (Cristinzio et al. 1973; Panconesi 1999). Considering the existing EU regulations (See Section 1.2), this risk is especially relevant from third countries suitable for the pathogen to establish and further spread (climate, availability of hosts, potential entry points). This risk factor might be considered especially for detection surveys. 

Example 3: Trade of infected plant material for planting. The movement of planting material such as Platanus nursery stock, from countries where the pathogen is present, is a pathway for its introduction (Ocasio-Morales et al., 2007). Infected young plants show symptoms within a few weeks since the infection and die rapidly, but during winter months the fungus grows slowly in plant tissues (Pilotti et al. 2016). As a result, infected nursery stock could escape the attention of inspectors. Ceratocystis platani can survive for long periods in the soil (Mutto Accordi 1988) and in this way it can be transferred with plants for planting to other areas. Similarly to example 2 this risk is especially relevant from third countries suitable for the pathogen to establish and further spread (climate, availability of hosts, potential entry points). This risk factor might be considered especially for detection surveys. 

Example 4: Presence of waterways. Considering the facility with which the inoculum is transmitted through water (Grosclaude et al., 1991; Panconesi et al., 2003; Panagiotis Tsopelas, personal communication), trees growing along rivers and channels have a higher probability of being infected once the pathogen is present in the area.

The figure on the right panel gives examples of the components of a target population for Ceratocystis platani and is not necessarily exhaustive.

A valid method for the detection of C. platani within surveillance programmes is visual examination of symptoms at tree canopy level and canker lesions and stains on or inside the wood of stems and roots, combined with molecular confirmation directly in the field using portable equipment or in the lab on sampled material. Once molecular identification has been made, detection in the infected area can be conducted through visual examination. In addition, novel techniques based on the identification in the field of volatile organic compounds emitted by symptomatic and asymptomatic infected trees can contribute to the early detection of the pathogen. Diagnostics for C. platani are described in the EPPO standard  PM 7/14 (2)  (2014).

3.1.1. Visual examination

Symptoms and signs

Ceratocystis platani can infect trees of all ages and sizes; small trees can be killed in few months while large trees take up to a few years (Tsopelas et al., 2017). The first noticeable symptoms are leaf chlorosis and wilting. The disease then progresses over the entire tree crown with yellowing, dwarfing and thinning of leaves, until extensive dieback and tree death (Panconesi, 1981; Ocasio-Moraleset al., 2007; EPPO, 2014; CABI, 2019). Symptoms initially affect the side of the canopy where the infection began but it can spread over the entire tree. Canker lesions can be noticed on the trunk and branches of infected trees; detecting lesions on the smooth bark of P. occidentalis and P. x hispanica is easier than on P. orientalis trees, due to their rough and thick bark. A very characteristic symptom is wood staining under cracked bark, which develops in large necrotic portions of the bark.

3.1.2. Other methods of detection in the field

LAMP

A loop-mediated isothermal amplification (LAMP) assay for the detection of C. platani can be conducted with portable instruments in the field to confirm visual identification; this method is highly specific and sensitive (limit of detection of LAMP assays is as low as 0.02 pg μL ⁻¹ ) and allows pathogen detection in 30 min (Aglietti et al., 2019) Platanus plant tissue.

VOCs

Brilli et al. (2020, available  here ) identified the blend of volatile organic compounds (VOCs) emitted by C. platani in vitro and have successfully validated in vivo that VOCs uniquely emitted by the fungus were released from the infected bark of plane trees, both symptomatic and asymptomatic. It is therefore possible to exploit the VOCs emitted specifically by C. platani as biomarkers to recognizeinfected plane tree plants using a portable gas-chromatographer. This is an emerging methodology that can help the early detection of the pathogen.

3.1.3. Sample collection

Plant tissues (bark, wood cores, wood fragment and sawdust) sampled from Platanus trees, soil collected from the vicinity of the tree root system, and water samples from river waterways along which Platanus trees are growing are brought to the laboratory for subsequent analyses (fungal isolation and/or molecular identification). Detailed descriptions of sample collection procedures are included in the EPPO standard  PM 7/14 (2)  (EPPO, 2014).

3.1.4. Timing of detection and identification

The most appropriate time to observe symptoms of C. platani infections on trees is from May to September, when all symptoms are visible. During the winter months canker stains on stems and roots can still be observed. Collection of water samples and detection through VOCs are best in spring and summer, while LAMP and sampling of plant tissue and soil can be conducted all year round.

Both morphological identification of isolated cultures and molecular identification from samples can be used to confirmdetection of C. platani but the molecular method is quicker, easier and usually preferred.

3.2.1. Morphological identification

Ceratocystis platani can be isolated from wood, soil and water with a variety of techniques, cultured and identifiedmorphologically based on the characteristics of mycelium and conidia. Refer to EPPO  PM 7/14 (2)  (2014) for isolation and culturing techniques, and for identification keys.

3.2.2. Laboratory testing and other methods of identification

Molecular identification is used to confirm results of morphological identification of C. platani cultures, or to directly detect the pathogen in wood, soil and water samples without the need for isolation using real-time PCR, PCR-RFLP tests or genome sequencing. Refer to EPPO standards  PM 7/14 (2)  (EPPO, 2014) and PM 7/129(1) (EPPO, 2016) for these methodologies. Different quantitativePCR techniques can be used to detect C. platani in wood samples (Pilotti et al., 2012) and in small wood particles such as sawdust (Luchi et al., 2013), with detection limits of 10 and 2 fg/µl, (femtograms/microliter) respectively and an overall diagnostic sensitivity of 100% when within detection limits.

Information on what, where, when and how to conduct survey activities for C. platani is summarised in the Table on the right panel. The identification of the target population needs to be tailored to the situation in each Member State (example shown below).

The figure below shows the next steps after the survey preparation for designing statistically sound and risk-based detection and delimiting surveys of C. platani. Guidance on selecting the type of survey, related survey preparation and design, is provided in the the  EFSA general guidelines for pest surveys  on the right panel (EFSA et al., 2020).

Scroll down the right panel to access the definitions included in the glossary.

 General guidelines for statistically sound and risk-based surveys of plant pests 

 Index of EFSA Plant Pest Survey Toolkit 

 Plant Pest Survey Cards Gallery 

 Pest survey cards: what, when, where and how to survey? 

 The statistical tool: RiBESS+ 

 The RiBESS+ manual 

 The RiBESS+ video tutorial