The effectiveness of phytosanitary controls in preventing the entry of Colletotrichum acutatum in the UK strawberry sector

Authors


E-mail: eman.calleja@um.edu.mt

Abstract

Phytosanitary legislation is widely thought to be the most effective means of limiting the entry of alien pathogens without hindering trade. However, there is increasing evidence of limitations and weaknesses in phytosanitary systems worldwide. One such case is the entry and establishment of Colletotrichum acutatum in the UK in the 1980s. To investigate this further, data sets of disease incidence gathered by the authorities responsible for plant health between 1984 and 2008 were analysed to determine how and why the pathogen entered the UK and became established on strawberries. Results suggest that the disease entered the UK in 1982 on plants originating from the USA, a year earlier than the first UK record of the disease was reported. At least 54 further cases of infested plant material breached phytosanitary procedures and entered the UK, 75% of which originated from EU countries. Moreover, internal trade transmission was found to be highly influential in the successful spread of the disease. The effectiveness of phytosanitary procedures is limited when the sector is importing large numbers of plants from nurseries that have a track record of selling infested plant material, even if they are accompanied by a plant passport. By taking greater responsibility in limiting the spread of quarantine diseases, nurseries throughout the EU have an important role to play in limiting the entry of alien pathogens.

Introduction

Colletotrichum acutatum is a common fungal pathogen of a wide range of crops and wild plant species, having been recorded on more than 40 hosts worldwide since it was first described by Simmonds in 1965 (Sreenivasaprasad & Talhinhas, 2005). It is now recognized as a cosmopolitan pathogen causing anthracnose diseases on a number of economically important crops (Peres et al., 2005). In strawberries, C. acutatum causes extensive losses in fruit production (Sreenivasaprasad & Talhinhas, 2005), and is considered the world’s second most important pathogen after Botrytis cinerea in terms of economic impact (EPPO, 2003). The pathogen is spread through rain-splash dispersal (Ntahimpera et al., 1999) and through secondary contamination via the introduction of inoculum on symptomless plants, soil or equipment (Sreenivasaprasad & Talhinhas, 2005). Once infected with C. acutatum, strawberry plants develop bud and crown rot, causing the collapse and death of the entire plant (Freeman et al., 2001). Symptoms are characterized by reddish-brown necrotic areas in the crown (Ureña-Padilla et al., 2002), and in some cases stunting and chlorosis that is associated with root necrosis (Freeman et al., 2001). The disease, also known as strawberry black spot in the UK, may also exist on symptomless leaves through the presence of secondary conidia and appressoria (Sreenivasaprasad & Talhinhas, 2005).

Colletotrichum acutatum may overwinter as mycelium and/or appressoria in or on different parts of the host (Wharton & Diéguez-Uribeondo, 2004). In a study by Ureña-Padilla et al. (2001), inoculum of Colletotrichum was found not to survive in buried plant debris between seasons. Moreover, due to its wide host range the pathogen is capable of colonizing weed species surrounding strawberry fields and could survive for 3 months on other crops such as tomato, pepper and aubergine leaves (Freeman et al., 2001). Thus overwintering of the pathogen could also be due to its spread from strawberry plants to other species adjacent to strawberry fields and then re-infesting new strawberry fields the following year (Peres et al., 2005). Colletotrichum acutatum was first identified as causing disease in strawberries in California in 1983 (Smith & Black, 1986). Since then, it has spread throughout the world through the trade of strawberry runners and propagating material (Sreenivasaprasad et al., 1996; Freeman et al., 2001; Denoyes-Rothan et al., 2003; Peres et al., 2005; Sreenivasaprasad & Talhinhas, 2005). The first record of C. acutatum in the UK was in 1978, on Anemone sp. grown on Jersey (Anonymous, 1978; Jones & Baker, 2007). Five years later, the first instance of disease in strawberries caused by C. acutatum was recorded in the UK in 1983. This outbreak was not linked to the previous record of C. acutatum in Jersey, and is instead attributed to the importation of infected strawberry runners of cultivar Brighton, from California that same year (Simpson et al., 1994). As indicated by the same authors, this first outbreak in strawberries was successfully controlled by eradicatory measures, and phytosanitary precautions had until the early 1990s been successful in controlling the disease. However, others within the horticultural industry (Lovelidge in Sreenivasaprasad et al., 1996) believed that the continued introduction of infected strawberry material from continental Europe and the USA was so widespread that the disease was destined to become endemic in the UK. This was exacerbated by the existence of a latent phase of infection of C. acutatum that exhibits no visible symptoms of disease infection (Peres et al., 2005; Sreenivasaprasad & Talhinhas, 2005), and the sensitivity of the early laboratory tests used to identify the pathogen responsible for the disease (Sreenivasaprasad et al., 1996). The latter consisted of an apple bait test, a paraquat test and an enzyme-linked immunosorbent assay (ELISA), the first two of which often only identified a percentage of the infected plants (Morzieres & Baudry, 1993; Cook et al., 1995). Molecular diagnostic tests developed since the 1990s became more reliable and were faster at identifying not just the pathogen but also pathogenic variability amongst isolates (Sreenivasaprasad et al., 1996; Garrido et al., 2009). These tools gave added insight into the potential source of the infection. One such study by Simpson et al. (2006) revealed evidence of considerable differences in pathogenicity among several UK isolates of C. acutatum. This suggested the presence of a heterogeneous population of the pathogen in the UK, most likely resulting from the multiple introductions of infected plant material through imports. Other studies involving molecular characterization of populations of C. acutatum have shown further evidence of the influence of trade on the geographical spread of the disease. These include Denoyes-Rothan et al. (2003), who found evidence of identical RAPD patterns and ITS sequence data in populations of the pathogen in strawberries originating from the USA and Europe.

This influence of trade on the spread of plant pathogens is not only restricted to strawberry or C. acutatum. It is widely accepted that the existence of trade routes facilitates the movement of plants and the establishment of pests or pathogens that might accompany them (Ebbels, 2003; Jones & Baker, 2007; Brasier, 2008). The solution to limit this risk of alien pathogen entry, using measures that would not hinder trade, is nowadays widely accepted to be the use of phytosanitary legislation to control the movement of plant material. Control is usually maintained through visual checks and, where reasonable doubts exists regarding the health status of the plant, samples can be taken for laboratory analysis (EPPO, 2008b). In spite of phytosanitary control being internationally considered as a tool in reducing the risk of importing alien pathogens, it still has weaknesses. Many plant pathogens make it through border inspection posts undetected (Jones & Baker, 2007; Brasier, 2008). These often include diseases that do not show any visible signs of infection or have a latent phase (Brasier, 2008).

Notwithstanding the widespread evidence of entry of plant pathogens through national borders (Jones & Baker, 2007; Brasier, 2008; Matthews-Berry & Reed, 2009; Miller et al., 2009), little work has been conducted on following the entry and spread of an alien pathogen over a long time series. This is most probably due to the lack of field level data usually available over a long time period for further study and analysis. One exception is the case of the introduction of C. acutatum in the UK, for which data covering a period of three decades are available.

The aim of this paper is to investigate the reasons that led to the successful introduction and spread of C. acutatum in UK strawberries since the early 1980s, and the effectiveness of the existing UK phytosanitary system in halting or slowing the spread of the disease. It does so through three specific objectives which attempt to investigate: (i) the likely routes taken by the pathogen to enter the UK strawberry sector; (ii) the reasons that led to the successful establishment and spread of the disease and why the existing legislation was ineffective in counteracting this; and (iii) the spatial distribution of the disease by mapping the geographical spread over time, and studying the reasons for anomalous concentrations of incidence. The investigation is based on evidence collected over three decades by phytosanitary authorities within the Ministry for Agriculture, Food and Fisheries (MAFF), and later within the Department of Environment, Food and Rural Affairs (Defra), on the incidence of C. acutatum in the UK, then considered as being a harmful organism in terms of Commission Directive 92/103/EEC (Anonymous, 1992). The paper first gives a background on phytosanitary controls, and legislative tools present in the UK and Europe to control C. acutatum, followed by a description of the adopted methodology, results and a discussion.

Phytosanitary control of plant disease

Between 1970 and 2004, 234 pathogens were recorded for the first time in Great Britain (Jones & Baker, 2007). Although some of these pathogens may have spread through natural transmission, the authors of that study suspected that most were introduced with imported plant material, such as seedlings, plants, budwood, rootstocks, bulbs, corms and tubers. Phytosanitary control of plant pests has existed at an international and national (UK) level for a number of centuries (MacLeod et al., 2010). In Great Britain, the foundations of a phytosanitary system were set up in 1877 with the introduction of the Destructive Insects Act, later followed by the Destructive Insects and Pests Act in 1907 (Dehnen-Schmutz et al., 2010). These acts introduced inspection and notification systems; for the first time, they developed a list of quarantine species and gave the power to authorities to destroy infested crops (Ebbels, 2003). The Plant Health Act of 1967 later replaced the two earlier acts to favour a managed risk-based approach, whereby controls would ensure a sufficient level of protection without hindering trade (MacLeod et al., 2010). This meant that a subset of the plants or plant material entering the country would be inspected, based on a risk assessment depending on the species and the country of origin. The consolidation of EU plant health legislation in 1993 facilitated this by introducing the ‘Plant Passporting’ system, whereby propagators intending to export plant propagation material to other EU states would need to satisfy a number of plant health requirements, thus enabling their certified plant material free movement through the EU (MacLeod et al., 2010).

Despite these measures, many plant pathogens still make it through border inspection posts undetected (Jones & Baker, 2007; Brasier, 2008). Some suggest that, while the risk of introductions exist, for a phytosanitary system to be effective, it should be assessed by its ability to reduce the expected probability of successful invasion rather than the certainty of preventing it (Mumford, 2002). Others have gone as far as suggesting that a substantial reduction in the international trade of plants is the best means of reducing threats from alien pests (Brasier, 2008). Mwebaze et al. (2010), on the other hand, suggest focusing phytosanitary inspections on plants originating from countries that provide a higher risk and have an established track record of exporting infested plant material.

Colletotrichum acutatum was first listed in UK phytosanitary legislation through The Plant Health (Great Britain) Order 1987 which came into force on 1 November 1987. In this order, the import of diseased plants or fruit from third countries (non-EU countries) was forbidden (Anonymous, 1987). Moreover, plants from third countries were required to be accompanied by a certificate declaring that they were not grown in a field that was infected with C. acutatum or within 50 m of such a field. Prior to this order coming into force in November 1987, the Plant Health and Seeds Inspectorate (PHSI) alternatively had the power, through articles 2 and 3 of The Plant Health Act 1967, to order the destruction of plant material if they had serious concerns that they could pose a phytosanitary risk (Anonymous, 1967). In 1990, the introduction of The Plant Health (Great Britain) (Amendment) Order 1990 included C. acutatum in Schedule I, forbidding its entry into the UK on imported plant material, including from the EU (Anonymous, 1990). This order, which came into force on 14 September 1990, also made illegal the cultivation and sale of strawberry plants infected with C. acutatum within the UK, and obliged growers having their land infected by the disease to inform the relevant authorities responsible for plant health within England, Scotland or Wales.

Colletotrichum acutatum was first listed in European legislation in 1992, through Commission Directive 92/103/EEC on protective measures against the introduction into the Community of organisms harmful to plants or plant products and against their spread within the Community (Anonymous, 1992). It was listed in Annex I Section 2 as a harmful organism whose ‘introduction into, and spread within, all member states shall be banned’. This directive also introduced the Plant Passporting system, whereby plants that were crossing borders between EU member states were required to be accompanied by a plant passport as evidence of their pest or disease-free status. This Directive was enacted into British Legislation through The Plant Health (Great Britain) Order of 1993, which also declared the disease ‘notifiable’, meaning that land known to have grown infected plants now had to be notified to the PHSI (Anonymous, 1993). The order came into force on 1 June 1993. The Plant Health Orders of 2005 for England and Scotland and The Plant Health (Wales) Order of 2006 reconfirmed a ban on the import and sale of plants infected with C. acutatum, and declared the disease notifiable only in nurseries.

In 2008, following a Pest Risk Analysis (PRA) initiated by France and complemented by a survey among the Member States, C. acutatum was deregulated as a harmful organism for the European Union (De Hoop et al., 2008). The decision was taken by the member states following evidence that the disease had become widespread within the Community (Anonymous, 2008). The pathogen ceased being considered as a quarantine organism on 30 September 2008, through the introduction of Commission Directive 2008/64/EC (Anonymous, 2008). The Plant Health (England) (Amendment) Order 2008 consequently adopted this Directive and declassified C. acutatum in England. Wales and Scotland adopted similar measures in the same year.

Notwithstanding the declassification of C. acutatum as a quarantine organism, the European and Mediterranean Plant Protection Organization (EPPO) certification scheme requires that plants recommended for certification as Nuclear stock or Propagation Stock I should be free of the pathogen (EPPO, 2008a). They also recommend laboratory testing through a paraquat test to detect any latent infection and that ‘for Colletotrichum acutatum at least one plant should be sampled per 1000 plants with a minimum of 50 plants for small lots and a maximum of 300 plants taken from different parts of the lot’ (EPPO, 2008b).

Materials and methods

In this study, data sets for C. acutatum held by the authorities responsible for Plant Health during the last three decades were used to assess the effectiveness of UK phytosanitary legislation to halt the entry of quarantine disease into the UK. These data sets were obtained from a computer-based database called Pathdiary which was kept between 1984 and 2002. Data were collected by the Ministry of Agriculture, Fisheries and Food (MAFF) in the 1980s at the Harpenden Laboratory and later by the Central Science Laboratory (CSL) in York (now the Food and Environment Research Agency – Fera). Data were gathered through three sources: the PHSI, which collected most of the information through specimens taken during crop inspections and from border inspection posts and subsequently sent for identification to Harpenden and later CSL; ADAS (the Agricultural Development and Advisory Service), which sent specimens for identification to CSL; and the public including growers, who have occasionally sent specimens to CSL for identification. After 2002, records of C. acutatum were collected only for tests carried out by CSL (now Fera) on samples sent to them for testing by the public.

Samples were often taken when disease symptoms were encountered and especially when plants were suspected of having the disease. The disease was then confirmed by CSL following laboratory analysis; thus all records included in this study are positive records. Each record included the date the entry was made, farm location (through an address or name of farm or farmer), the disease and cultivar name of the strawberry plants affected. On some occasions, there was also reference to the cultivation method, infection level and action taken by the PHSI. In almost all cases, the farm was identified and a postcode obtained indicating the exact location of the outbreak. The records of infected plant material also noted cases where plants had been imported from abroad. These consisted of infected material stopped at the point of entry and at border inspection posts (BIPs). In all cases, the country from which the samples originated was also listed.

As these data were collected by the UK phytosanitary authorities as part of their control procedures to counter the entry and establishment of alien pathogens, they reflect instances of infected plant material that were either detected at BIPs or in the field following inspections by the PHSI or, alternatively, cases that were brought to the authorities’ attention and investigated. They do not reflect the precise or actual number of outbreaks of C. acutatum in UK strawberries between 1982 and 2008. The figures represented through this study, therefore, potentially underestimate the actual number of outbreaks of C. acutatum in UK strawberries. However, the investigations carried out do provide information on the source of the pathogen and the proportions of outbreaks attributed to trade, repeated outbreaks or those that were untraced to either of the previous two. The proportions are in fact more important than the actual figures, because they give an indication of the routes taken and the reasons that enabled the successful entry and establishment of this disease.

To facilitate the analysis of the introduction and spread of the disease, the records were grouped into five different phases covering the period between 1982 and 2008 (Fig. 1). This enabled disaggregation of more than 200 records of disease outbreaks into manageable blocks in order to better understand how the disease had spread throughout the country and to trace the sources of the infested plant material. For each phase, records of outbreaks on farms were traced back to a source by using information gathered from the PHSI investigations and subsequently recorded in Pathdiary. Records of infested plant material intercepted at the BIPs were also divided into five phases.

Figure 1.

 Cumulative number of outbreaks of Colletotrichum acutatum in the UK, 1982–2008.

The overall records of infested plant material that were related to trade sources were analysed by country of origin to determine provenance. This included listing the importers involved in the introduction of the plant material. Farms affected were also located spatially by using postcodes obtained for all incidences, indicating the approximate location of each outbreak. These were then used to plot the cumulative number of outbreaks of disease by time, using ArcGIS 9 (ArcMap v. 9.2). This was done by plotting the total number of outbreaks per county that had been recorded from the first record in 1982, up to the end of each phase. The output consisted of five maps of the UK showing where outbreaks had occurred up to the end of each phase, with the county being the main unit of analysis.

The cumulative incidence of C. acutatum in UK strawberries was tested statistically using two tools in ArcMap v. 10: (i) Spatial autocorrelation Moran’s I index which, given a set of features and an associated attribute, evaluates whether the pattern expressed is clustered, dispersed or random. The Global Moran’s I tool calculates a z-score and P-value to indicate whether or not one can reject the null hypothesis, which in this case states that the outbreaks of strawberry black spot are randomly distributed across the study area. For this study and data analysis, a z-score value higher than +1·96 indicates that the null hypothesis is rejected and the outbreaks are indeed clustered. (ii) High/Low Clustering Getis-Ord General G index, which measures the degree of clustering for either high values or low values using the Getis-Ord General G statistic. The General G tool calculates a z-score which is a measure of statistical significance showing whether or not to reject the null hypothesis, which in this case is that the outbreaks of strawberry black spot were randomly distributed. The higher the z-score, the stronger the intensity of the clustering.

The cumulative disease incidence per county was then divided by the mean strawberry crop area per county during those 27 years. This gave an indication of the disease pressure or intensity per county through the number of outbreaks per square kilometre of cultivated strawberries. The number of nurseries per county known to have imported diseased plant material was also mapped.

Results

First entry

The first incidence of C. acutatum in UK strawberries encountered through this study occurred in 1982 on plants of the cultivar Hekker. The record was entered in the Pathdiary database on 26 April 1984. The plants were found to have been imported from the USA by a plant nursery in the autumn of 1982, to be used in trials. They were planted on land belonging to the importer in Kent but, following scientific advice from the Harpenden Laboratory (MAFF), the plants were destroyed. This record places C. acutatum in UK strawberries a year before the first published record reported by Simpson et al. (1994).

First phase (1982–1987)

Between 1982 and 1987, a total of 10 outbreaks of C. acutatum had been recorded in the UK in two counties (Kent and Cornwall), two of which were from direct imports. In addition, there were another seven intercepted imports of runners or fruits that were found to be infested with C. acutatum (Table 1). The runners were stopped at the border inspection posts, by having the material destroyed. However, the fruit was allowed to enter, because it was destined for consumption. All of the records of identification during this phase were made by visual means. Where doubts existed, a sample of the crop was sent to either the MAFF Harpenden Laboratory or the Central Science Laboratory (CSL) for morphological identification by a mycologist.

Table 1. Incidences of Colletotrichum acutatum on farms in the five phases from 1982 to 2008, and the number of cases of intercepted plant material found at border inspection posts during the same period
PhaseYearDiseased material that was intercepted at the border inspection postsDiseased material that was not interceptedRecords of C. acutatum on farmsa
  1. aThe incidences recorded on farms belonged to one or a combination of: diseased plants found to have been imported (also listed in adjacent column); diseased plants originating from UK nurseries; and diseased plant whose source of infestation could not be traced back to a nursery or import.

First1982–1987 7 2 10
Second1988–19923533 51
Third1993–199710 9 39
Fourth1998–2002 3 9 56
Fifth2003–2008 2 2  7
Total 5755163

From records in Pathdiary, it was observed that during these first 5 years in only two instances was there a recommendation for any action to be taken to stop the further spread of the disease. In both instances, this involved importation of diseased material. Although the Cornish farm that had eight incidences was monitored on an annual basis, there was no record of any further action taken to curtail the disease. The action taken by authorities on outbreaks reported during this phase was carried out under the authority given to them through the Plant Health Act 1967, because The Plant Health Great Britain Order 1987 came into action in the last 2 months of 1987.

Second phase (1988–1992)

The second 5-year phase saw the spread of the disease throughout the south and south east of England. Fifty-one records of C. acutatum were reported, of which 33 were from imported plant material that failed to be intercepted at the BIPs (Table 1). A total of 29 new farms were involved and the disease had now spread to 14 different counties, 12 of which were first records in that county. Another 35 cases of infected plant material were recorded as being intercepted at the BIPs, resulting in recommendations that the material be destroyed or re-exported. Morphological identification of disease during this phase was augmented and confirmed through two other tests. The apple bait test was used in 1988 and 1989. Paraquat tests were applied from 1990 onwards. In the latter case, samples of up to 300 petioles were tested from each batch.

Whereas material intercepted during importation was ordered for destruction or re-exportation, only three eradication notices were served for the cases involving incidences on farms. In two of these cases, it was recommended that the grower did not plant strawberries in the infected field, whereas in the third the PHSI recommended the destruction of the plants.

Third phase (1993–1997)

During the third 5-year phase, the pathogen spread to another six counties within England. The trend for incidence in the south and south east continued, but this period also experienced spread to two counties in the Midlands and one in the north of England. A total of 39 cases of C. acutatum were reported in this phase in 15 different counties. The majority of cases were found on new farms (26 new farms) encountered during the strawberry black spot (SBS) survey (carried out in 1995 by the PHSI). Of the 39 cases recorded, nine were traced back to imported material. Another 10 cases of infected plant material were intercepted at the BIPs where once again plant material destruction or re-export was recommended. Morphological identification of the disease during this phase was augmented or confirmed using paraquat tests and ELISA. Paraquat tests were performed up until 1995, after which ELISA tests were carried out. All of the diagnostic tests carried out during the SBS survey of 1995 made use of the ELISA technique.

During this phase, more direct action was taken on behalf of the authorities to curtail the spread of the disease and in most cases the destruction of the beds was recommended. However, from 1996 onwards, no further requests for destruction were made due to an apparent new ‘relaxed’ policy that now focused on monitoring rather than eradication. A list of infected farms was still maintained and those nurseries found to have incidences of C. acutatum were excluded from the Plant Health Propagation Scheme (PHPS). However, these nurseries were allowed to sell non-PHPS material to farms already infected by the disease.

Fourth phase (1998–2002)

The fourth 5-year phase saw the disease spread to another seven new counties within the UK, two of which were in Scotland and one on Jersey. A total of 56 cases of C. acutatum were reported in 16 different counties. Half of the cases were found on new farms (28), most of which were encountered during a countrywide survey carried out in 1999 by the PHSI. This was conducted following an outbreak found on PHPS material of cv. Everest in a major strawberry nursery. The PHSI then inspected all downstream farms within the supply chains of the nursery involved. Of the 30 cases recorded that year, 21 were on plants of cv. Everest, even though the most common variety cultivated at that time was cv. Elsanta. Of the 56 cases, only nine were traced back to imported material (Table 1), evidence that the disease had become well established in the UK by this stage. Only three cases of infected plant material were intercepted at the BIPs. As in previous phases, plant material destruction or re-export was recommended.

Fifth phase (2003–2008)

The fifth 5-year phase saw the least number of recorded incidences of C. acutatum since it became established (Table 1). By the end of this phase, C. acutatum was known to have spread throughout the EU and, following a Pest Risk Assessment initiated by France, the pathogen was deregulated as a quarantine organism (De Hoop et al., 2008). Subsequently, records of C. acutatum were kept only for tests resulting in positive identification of the pathogen carried out by CSL (now Fera) on samples of strawberry sent to them for testing by the public.

Spatial assessment of disease spread

The cumulative incidence of C. acutatum in UK strawberries, over the 27 years covered by this study, shows a clear prevalence of outbreaks in the south and south east of the country (Fig. 2). Spatial assessment using hotspot analysis through the Getis-Ord Gi statistic shows two statistically significant hotspots of disease incidence: one in the eastern counties from East Anglia to Kent, and another in the south west, in Cornwall. On testing whether this spatial pattern is due to chance, the Spatial autocorrelation Moran’s I index, and the High/Low Clustering Getis-Ord General G index, gave a z-score of 16·9 and 11·5, respectively, suggesting that there is a less than 1% likelihood that this high-clustered pattern could be the result of random events.

Figure 2.

 Statistically significant hotspots of incidence of Colletotrichum acutatum.

The disease pressure, taken as the number of outbreaks per km2 of strawberries (Fig. 3) for the whole UK over the 27-year period, gave a value of 3·3 outbreaks per km2 of strawberries. A number of counties obtained a much higher disease count per km2 than the national average, such as in Cornwall and in the Isle of Wight, both of which had a relatively small strawberry crop area at that time. One reason for this could be multiple outbreaks involving the same farm(s) in these two counties. Somerset, Kent, Suffolk and Norfolk also obtained a higher disease count than the national average. On the other hand, apart from a small pocket in Fife and Dundee in Scotland, the disease seems to be absent north of East Riding, Yorkshire, even though strawberries are grown in these counties. Moreover, the disease had not been encountered in Wales, even though strawberries were cultivated there at the time.

Figure 3.

 Disease pressure per county, measured by dividing the cumulative number of outbreaks per county by the mean strawberry crop area per county. The circles show the number of nurseries per county, which are known (through this study) to have imported diseased plant material.

No relation was identified between the location of the nurseries and the spatial distribution of outbreaks.

Entry into the UK and establishment of the disease

When studying the data obtained from these databases, it was found that over two-thirds (68%) of the outbreaks recorded on farms were trade-related (112 out of 164 cases). Of these, 50% could be traced back to an import (55 cases), while the rest (57) were domestic trade transmissions occurring as a consequence of acquisitions made from a nursery that was already found to have infected plant material. Of the non-trade related cases, 43 were in farms having a second or repeated outbreak. Only nine of the recorded outbreaks, or 5·5%, could not be traced to a trade-related case or explained by a previous outbreak on the same land. Of the 55 cases of imported diseased plant material, 42 originated from other EU countries; 25 of these were from the Netherlands (Fig. 4) making it the biggest source of infected plant material. Of these latter cases, 17 occurred between 1 November 1987 and 13 September 1990, when The Plant Health (Great Britain) Order 1987 was in place. This legislative tool had imposed control restrictions only on plants originating from third countries. In fact, almost half (25 out of 55 cases all being from EU countries) of the cases where infected plant material passed undetected at the BIPs and found their way to the field occurred between 1988 and 1990 (Fig. 5). In the meantime, a total of 57 cases of plant material were stopped at the BIPs between 1982 and 2007. On dividing these into four phases, depending on the prevalent UK legislative tools at the time (Table 2), the highest rate of interceptions was obtained with the coming into force of The Plant Health (Great Britain) (Amendment) Order 1990, with around 6·75 interceptions for every 12 months. This legislative tool imposed control procedures not only on plants entering the UK from third countries, but also on plants originating from the EU or EEC countries. The lowest rate of interceptions was obtained between June 1993 and October 2008, when the EU’s plant passporting system, as established by Commission Directive 92/103/EEC, was in place.

Figure 4.

 Imports of diseased plant material recorded in the Pathdiary database. These included both interceptions of diseased material at the BIPs which were subsequently destroyed, and also non-intercepted imported infected material.

Figure 5.

 Annual records of interceptions at the BIPs and records of cases when infected plants were not intercepted and ended up being planted in the field.

Table 2. Incidences of Colletotrichum acutatum from 1982 to 2008 on farms and cases of intercepted plant material found at border inspection posts during the same period. These were divided according to the UK legislative tool in force during that period
Time period during which legislation was in forceLegislative tool in force at that timeNo. of interceptions at the BIPsMean no. of interceptions/per yearaNo. of non-intercepted cases of imports of infected plantsMean no. of non-intercepted cases of imports per yearaReference to C. acutatum in legislation
  1. aFor each legislative tool, the mean number of interceptions or non-interceptions per year were obtained by dividing the number of cases by the total number of months during which the legislation was valid, and then converting the figure into a yearly mean.

1982–31 October 1987Plant Health Act 1967 71·4 20·4 C. acutatum not specifically mentioned in legislation. Legislation gave powers to authorities to destroy plants if authorities had serious concerns that disease/pest could pose a phytosanitary risk
1 November 1987–13 September 1990Plant Health (Great Britain) Order 1987144·8258·57 C. acutatum mentioned in legislation. The import of diseased plants or fruit from third countries (non-EU countries) was forbidden
14 September 1990–31 May 1993Plant Health (Great Britain) (Amendment) Order 1990186·75 83 C. acutatum mentioned in legislation. The cultivation within the UK, and import of diseased plants or fruit from any country was forbidden
1 June 1993–30 September 2008Plant Health (Great Britain) Order of 1993181·12201·30 C. acutatum mentioned in UK and EU legislation. Plant passporting system in force

In Figure 6, the three major peaks in annual disease incidence show that the largest records of disease were accompanied by a spread of C. acutatum onto new farms (steeper increase in growers involved to date). The three peaks in strawberry black spot incidence correspond to the following events: (i) in the first, between 1988 and 1990, 25 of the 43 reported cases involved imports of infected material, all of which originated from EU countries. (ii) The peak in records in 1995 is attributed to the strawberry black spot survey carried out by the PHSI that same year. Most of these records (20 out of 25) were due to domestic trade of plant material that was found to be infected with C. acutatum. (iii) The peak in 1999 was due to another investigation carried out by the PHSI that year. In this case, 14 of the 30 reported cases were on farms having previous outbreaks of the disease.

Figure 6.

 The annual records of disease incidence in UK farms plotted against the accumulated number of growers having Colletotrichum acutatum in their land to date.

When investigating the number of importers associated with entry of diseased plant material into the UK, it was found that 60% of the cases were attributed to two importers.

Discussion

Through this study, the first record of C. acutatum on strawberries in the UK was reported to occur in the autumn of 1982, when diseased plants of the cv. Hekker were imported from the USA and planted by an importer based in Kent. This is a year earlier than the first UK record in 1983 reported by Simpson et al. (1994). It also precedes the first record of the disease in strawberries in the USA by a year (Smith & Black, 1986), even though the diseased plants recorded in Kent in 1982 originated from the USA itself. However, it is not the first record of C. acutatum in the UK. Colletotrichum acutatum was detected in 1978 on Anemone sp. grown in Jersey (Jones & Baker, 2007). This presents the prospect of the pathogen having existed undetected in the UK on other plants or weeds that might have been the source of the infection on strawberries in 1982 in Kent. Although this could be the case, it is highly unlikely. This is mainly because there are no other records of C. acutatum in mainland UK between the record in Jersey in 1978 and that encountered through this study in 1982. Secondly, there were no nurseries dealing in strawberries in Jersey during the time (Calleja, 2011), which could have otherwise acted as a medium for the pathogen to spread to strawberries on mainland Great Britain through trade. Thirdly, the geographical isolation of Jersey as an island would have made the natural transmission of the pathogen to strawberries on mainland Great Britain highly unlikely. Thus the most likely source of the pathogen in 1982 was through the importation of infected plants of cv. Hekker from the USA, referred to earlier. In fact, the disease on strawberries was known to exist in at least five separate states in the USA by 1983 (Smith & Black, 1986), potentially placing the pathogen in the US strawberry industry when the infected plants were bought in 1982.

The first entry of the disease in 1982 did not necessarily lead to the successful establishment of the disease in the UK. This leads to the first objective set in this paper – identifying the likely routes taken by the pathogen to enter the UK. The disease probably occurred due to successive imports of infested plant material in the 1980s and 1990s. In fact, this study has identified trade as being the main route of entry and establishment of C. acutatum in UK strawberries. At least 55 cases of imports of infested plant material that were not intercepted by BIPs, and were subsequently planted in the field, were identified. These occurred over a period of two decades, involving various nurseries that were importing plants into the UK. This outcome may provide supporting evidence for Simpson et al.’s proposition (2006) that the heterogeneous nature of the pathogen in the UK, which they had observed through molecular analysis, was likely to have resulted from multiple introductions of infected plant material.

The source of the infected imports and when these occurred demonstrates the impact of different legislative tools in controlling the spread of pathogens across borders. Over three-quarters (42 of the 55) of cases of infected plant material were from other EU countries. The majority of these cases were identified as originating from the Netherlands. This finding is similar to that of Jones & Baker (2007) where 47% of the introductions of alien pathogens into the UK, whose source was known, were from the Netherlands. This could be due to two reasons. First, the EU, and especially the Netherlands, was the largest source of runners to the UK strawberry industry during the time (Calleja, 2011). Secondly, almost half of these imports (25 out of 55) occurred when existing UK legislation had control procedures for C. acutatum only on plants originating from third countries. In fact, the coming into force of The Plant Health (Great Britain) (Amendment) Order 1990, which regulated imports from all countries including EU member states, brought about the highest rate of interceptions of infected plants (Table 2). Positive identification of these intercepted plants was then achieved through the use of a paraquat test of strawberry petioles, which took around a week to complete. This caused delays in dispatching the inspected consignments, which meant that fresh plants lost their vigour. Sreenivasaprasad reported that by 1994, the import trade for fresh runners had been destroyed due to this tighter regulation (Sreenivasaprasad et al., 1996).

On the other hand, the bringing into force of the EU plant passport system in 1993 was accompanied by a drop in the rate of interceptions of diseased plant material (Table 2). The latter most likely was not caused by the reduction in imports of fresh runners reported by Sreenivasaprasad et al. (1996). The importation of cold-stored plants was not affected by delays caused in the testing procedure, and continued to increase as the demand for strawberry runners increased in the late 1990s (Calleja, 2011). However, it is suggested that the plant passporting system was not effective as a barrier to the spread of pathogens across national borders within the EU. The infected plants that were either intercepted or made it through the BIPs since 1993 were all accompanied by a plant passport. This is not an uncommon occurrence, as was reported by Jones & Baker (2007) for a number of other pathogens. Although the plant passporting system is meant to put the burden of responsibility on the plant nurseries growing the plants, to ensure that the plants are free of pathogens, doubts have repeatedly been raised on its efficacy in preventing the movement of disease between EU member states (Jones & Baker, 2007; Brasier, 2008). In fact, as long as diseased plants continue to be sold at nurseries, the effectiveness of the phytosanitary system is hindered or limited in its capacity to halt the introduction and spread of alien pathogens throughout Europe. Not only was the UK phytosanitary system unable to prevent entry and spread of this disease, but so too was the wider EU phytosanitary system, whereby just 16 years after C. acutatum was first listed as a quarantine pest (Anonymous, 1992) it was declassified due to its widespread occurrence throughout the European Union (Anonymous, 2008). In contrast, there are cases where the system has been efficient enough to prevent the establishment of a quarantine pest even after entry, such as the case for Xanthomonas fragariae in the UK. This pathogen had entered the UK in May 2004 on plants from the Netherlands, which were then sold to four different farms (Matthews-Berry & Reed, 2009). Following notification to Fera by an affected grower, an investigation was carried out and growers were given the option to destroy the plants or long-term containment, i.e. the plants would be kept for fruit production and measures taken to curtail the spread of the disease. The second option was chosen by the growers and, following the implementation of the PHSI and the Plant Health Service recommendations, by 2009 the disease was deemed to have been successfully eradicated (Matthews-Berry & Reed, 2009).

In view of the evidence obtained through the data sets used in this study, the pathogen is thought most likely to have become established in the UK through a combination of three factors (second objective of this study). First, C. acutatum is not easily detected through visual inspections. The existence of a quiescent phase means that the disease is not always evident and does not show any discernible symptoms during this phase (Peres et al., 2005; Sreenivasaprasad & Talhinhas, 2005). This is a recurring problem for phytosanitary controls, as there are many pathogens that have quiescent phases and can easily pass undetected through preliminary visual inspections (Brasier, 2008). In fact, there is circumstantial evidence that the disease might have spread undetected further than was known to authorities. In a study carried out through grower interviews on the Kentish strawberry sector in 2009, 90% of the growers (20 interviewed) declared that they had encountered the disease on their land during their lifetime (Calleja, 2011). In addition, this study identified that two of the 3 years with the highest recorded incidences of strawberry black spot coincided with an intensification of farm inspections, and these were confirmed only after laboratory analysis. This suggests that the disease was more widespread than was thought, but was not recognized because of the lack of visible signs throughout most of the year.

The second factor that aided the establishment of the disease on UK strawberries was the repeated imports of undetected infected material. This created many opportunities for the disease to establish itself and spread. In fact, were it not for the persistent entry of diseased plant material, C. acutatum might not have become established in the UK at all. Adding to this was the influence of internal trade. Two-thirds of the recorded incidences identified in this study were traced back to internal UK trade, suggesting that internal trade transmission was also highly influential in the successful spread of the disease.

The third factor that contributed to the establishment of C. acutatum in UK strawberries was the effectiveness of the detection methods used to confirm presence or absence of the disease. The tests used by the UK phytosanitary authorities to detect C. acutatum on strawberry did not include molecular analysis. Molecular diagnostic tools had been available since the early 1990s (Mills et al., 1992; Sreenivasaprasad et al., 1996); however, the authorities continued to use classical mycological methods (Garrido et al., 2009) up until the mid-1990s, combined with ELISA after 1995. The earlier tests, in particular the apple bait and paraquat tests, were relatively unreliable and time-consuming. These tests would positively identify only a proportion of the infected batch of plants (Morzieres & Baudry, 1993; Cook et al., 1995), and the pathogen was often undetectable at low infection rates during the quiescent phase. ELISA tests were, on the other hand, much more reliable and accurate (Cook et al., 1995). However, the testing procedure was relatively slow as petioles were first sent to the Harpenden Laboratory or, later on, to the Central Science Laboratory in York for diagnosis. This meant that plants would often have been planted in the ground by the time the test result was received. The availability and use of faster diagnostic tools such as real-time PCR, particularly at the points of entry and BIPs, would have allowed identification of the pathogen (Garrido et al., 2009) within a few hours. This would have potentially allowed the PHSI to take quick decisions in limiting the instances of entry of infected strawberry plants.

In view of weaknesses in the methods of diagnosis, particularly until 1995 when ELISA tests first started being used on UK strawberries, it is likely that the disease may have been more widespread, and disease incidence in the earlier years may have been under-reported. Evidence of this lies in the higher rates of disease incidence during years when an intensification of disease surveys or farm inspections was apparent. In spite of this uncertainty in the actual extent of disease incidence and distribution across the UK, the main factor affecting the spread of the disease is still thought to have been the trade in runners. If more of the outbreaks occurred as a result of dispersion by natural means, or the pathogen moving from a viable wild host to a clean strawberry plantation, then there would have been more cases of untraced outbreaks. Only 5·5% of the outbreaks found in the UK through this study could not be traced to a previous outbreak or a trade-related entry. This implies that the spread by natural means was relatively uninfluential in the establishment of C. acutatum in UK strawberries.

The spatial distribution of strawberry black spot highlighted in this study shows a tendency for higher incidence in the south west and south east of the UK. The geographical distribution was statistically not due to chance, and was also seen to be independent of where strawberries were grown. In fact, if the disease were to occur randomly, depending on where infested plant material was planted, its spread through trade would have ensured that the diseased plants followed the route to wherever strawberries were grown. Despite this, some counties had a higher proportion of outbreaks than the national average, or the disease was not recorded in certain regions where strawberries were grown. The most likely explanation for this is climatic variation. Apart from rain and a high relative humidity during the germination stage, the key environmental variable for strawberry black spot is temperature, with the optimum temperature for conidial germination being between 23 and 27·7°C (Leandro et al., 2003) and that for infection of mature or immature fruit being 25–30°C (Wilson et al., 1990; Maas, 1998; Leandro et al., 2003). In the open field in the UK, these relatively high temperatures are most often achieved in the south and south east of the country, where higher mean temperatures are achieved. This would explain why the disease was not recorded from Wales and the northern counties apart from two incidents in Scotland. Although infected plants might have found their way there, relatively lower temperatures in these regions meant that the disease might have remained in the quiescent phase on symptomless plants, or was expressed at too low an infection level to have been observed. However, once it is established in a field its spread is aided by rain-splash (Wilson et al., 1990; Maas, 1998). This makes open field cultivation more vulnerable to strawberry black spot. In view of this, the increased use of protected cultivation and controlled irrigation methods in the strawberry sector (Lieten, 2006) are helping to eliminate rain-splash on the plants and consequently the likelihood of the disease occurring under protection. Evidence of this has already been encountered in a study of the sector where growers using polytunnels for strawberries described strawberry black spot as not being a problem any more, even though it was in the past when polytunnels were not available. This provides a solution for the sector in controlling this disease with minimum use of plant protection products.

Success and failure of a phytosanitary system does not always depend on detecting the pathogen at the border. The EPPO phytosanitary procedures for strawberry plants accept an infection rate of up to 1% with a confidence level of at least 99% (EPPO, 2008b). It has been proposed that a successful quarantine system could be measured by the number of interceptions and the action taken to eradicate pest outbreaks (Mumford, 2002). This would make the UK strawberry black spot episode a relative success, because not only was the disease intercepted at the BIPs in 57 different instances, but action and recommendations were taken by the Plant Health Service every time it was informed of an outbreak.

The reasons why eventually the systems in place did not succeed in eradicating the disease are thought to be twofold. First, although recommendations were made by the Plant Health Service in the 1980s and 1990s on each occasion an outbreak was detected, there is little evidence that the recommendations for grubbing and destruction of the infected plant material had been taken up by the growers. Secondly, and more important, unlike the case with Xanthomonas fragariae, the sheer volume of diseased plants that were being imported into the UK made it very difficult for the Phytosanitary Authorities to stop the disease effectively at the BIPs. Lessons learnt from the entry and spread of this disease in the UK, and the weaknesses that enabled it, could help the horticultural industry safeguard itself from future alien disease epidemics. As increasingly more plants and propagation material are being imported (Calleja, 2011), it is of utmost importance that high phytosanitary standards are kept at the point of source nurseries. Voluntary certification schemes, such as the Plant Health Propagation Scheme (PHPS) that encourages greater controls, should be embraced more by the sector to ensure that source nurseries are compliant to the minimum certification requirements. Through greater control from within industry and an improvement in plant health at source nurseries, coupled with the use of more efficient disease detection methods, intra-European movement of plants could pose a much reduced risk to phytosanitary controls.

Acknowledgements

The authors would like to thank Defra and Fera for funding this research and providing the data sets. They are especially thankful to Peter Reed and Sharon Elcock who provided the data sets Pathdiary and DOMERO. Thanks are also due to two anonymous referees for their comments on an earlier version of the manuscript.

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