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SID 5 Research Project Final Report

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NoteIn line with the Freedom of Information Act 2000, Defra aims to place the results of its completed research projects in the public domain wherever possible. The SID 5 (Research Project Final Report) is designed to capture the information on the results and outputs of Defra-funded research in a format that is easily publishable through the Defra website. A SID 5 must be completed for all projects.

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ACCESS TO INFORMATIONThe information collected on this form will be stored electronically and may be sent to any part of Defra, or to individual researchers or organisations outside Defra for the purposes of reviewing the project. Defra may also disclose the information to any outside organisation acting as an agent authorised by Defra to process final research reports on its behalf. Defra intends to publish this form on its website, unless there are strong reasons not to, which fully comply with exemptions under the Environmental Information Regulations or the Freedom of Information Act 2000.Defra may be required to release information, including personal data and commercial information, on request under the Environmental Information Regulations or the Freedom of Information Act 2000. However, Defra will not permit any unwarranted breach of confidentiality or act in contravention of its obligations under the Data Protection Act 1998. Defra or its appointed agents may use the name, address or other details on your form to contact you in connection with occasional customer research aimed at improving the processes through which Defra works with its contractors.

Project identification

1. Defra Project code PH0303

2. Project title

Scoping study on methods for sanitisation of bio-waste for control of pathogens and pests of plant health importance

3. Contractororganisation(s)

Central Science LaboratorySand HuttonYorkYO41 1LZ          

54. Total Defra project costs £ 15,000

5. Project: start date................ 01 April 2004

end date................. 31 March 2005

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6. It is Defra’s intention to publish this form. Please confirm your agreement to do so...................................................................................YES NO (a) When preparing SID 5s contractors should bear in mind that Defra intends that they be made public. They

should be written in a clear and concise manner and represent a full account of the research project which someone not closely associated with the project can follow.Defra recognises that in a small minority of cases there may be information, such as intellectual property or commercially confidential data, used in or generated by the research project, which should not be disclosed. In these cases, such information should be detailed in a separate annex (not to be published) so that the SID 5 can be placed in the public domain. Where it is impossible to complete the Final Report without including references to any sensitive or confidential data, the information should be included and section (b) completed. NB: only in exceptional circumstances will Defra expect contractors to give a "No" answer.In all cases, reasons for withholding information must be fully in line with exemptions under the Environmental Information Regulations or the Freedom of Information Act 2000.

(b) If you have answered NO, please explain why the Final report should not be released into public domain

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Executive Summary7. The executive summary must not exceed 2 sides in total of A4 and should be understandable to the

intelligent non-scientist. It should cover the main objectives, methods and findings of the research, together with any other significant events and options for new work.

A scoping study was conducted to identify the range of practical treatment options for elimination from biowastes of high-risk pathogens and pests of plant health concern from biowastes. Findings of the study were presented to a stakeholder group to assess the suitability of biowaste processing and disposal options for high-risk biological waste, including municipal and industrial biowaste, crop residues and produce contaminated with quarantine pests and pathogens. Recommendations were made on practical measures for processing and disposal of biowastes containing high-risk organisms of plant health concern. Gaps in current knowledge were identified and future research needs recommended.

A portfolio of information on currently used biowaste processing methods was assembled. These included disposal to landfill and on-farm burial, incineration and on-farm burning, direct heating, industrial processing, feeding to animals, aerobic composting processes and anaerobic digestion. Each process was assessed based on (a) ease of containment of the high-risk organism during transport, processing, storage and recycling/disposal of processed material, (b) conformity to national and European legislation and best practice codes and quality standards, and (c) expected efficiency of the process in eliminating target organisms. Key organisms of plant health concern included in the assessment were (a) the quarantine bacteria Ralstonia solanacearum, Clavibacter michiganensis subsp. sepedonicus, (b) biotrophic fungi with hardy resting spores which are either quarantine organisms (including Synchytrium endobiotcium, Tilletia indica and Phytophthora ramorum) or vectors of quarantine viruses (including Polymyxa betae) (c) Pepino mosaic virus and Potato spindle tuber viroid (d) the regulated nematodes Globodera pallida, G. rostochiensis, Ditylenchus dipsaci and Meloidogyne chitwoodii, and (e) alien insect pests Bemisia tabaci and Liriomyza huidobrensis.

Deep burial at an approved landfill site or incineration at an approved facility are safe disposal methods for biowaste of high plant health risk. However, a number of alternative options for sanitisation of biowastes are appropriate for many, but not all, organisms of concern, which will allow either recycling of biodegradable materials or at least reduce the mass to be disposed. Feeding to animals and industrial processing have been considered as alternative methods of disposal/reduction, mainly from the aspect of the need to ensure containment of high-risk organisms so that there is no identifiable risk of them escaping into the environment and spreading to potential host plants.

Critical temperatures for direct heating treatments have been established to ensure elimination of the most heat tolerant organisms. Accurate determination of the full range of lethal temperatures and required exposure times will allow less stringent and more cost-effective specifications to be established individually for many key target organisms of concern. Knowledge of the exact requirements for elimination of high-risk plant pathogens and pests will also facilitate selection of relevant indicator organisms for use in process validation procedures.

EA licensed composting processes, which are managed according to current quality standards (BSI PAS100), should provide suitable conditions for eradication of many of the organisms of concern, including the bacterial, nematode and insect targets. However, for the obligate biotrophic fungi with hardy resting spores and heat tolerant viruses and viroids, the risk of survival of the composting processes justifies additional containment requirements and restrictions on the uses and destinations of the processed material. Factors other than temperature contribute to the sanitisation process during composting and can be particularly important in the assessment of the suitability of processes for biowaste containing organisms for which critical temperature-time conditions may be difficult to achieve in full. Such factors include the type and mixture of feedstock, particle dimensions, moisture content, turning frequencies, chemical analyses (especially ammonia and pH) and microbiological interactions. More accurate specification of the conditions required during commercial composting and digestion processes will allow more informed selection of suitable processes for high risk wastes containing key organisms and more accurate assessment of the risks involved. Development of a range of methods for detection of key target organisms in feedstocks and processed material will also support risk assessment and process validation.

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Project Report to Defra8. As a guide this report should be no longer than 20 sides of A4. This report is to provide Defra with

details of the outputs of the research project for internal purposes; to meet the terms of the contract; and to allow Defra to publish details of the outputs to meet Environmental Information Regulation or Freedom of Information obligations. This short report to Defra does not preclude contractors from also seeking to publish a full, formal scientific report/paper in an appropriate scientific or other journal/publication. Indeed, Defra actively encourages such publications as part of the contract terms. The report to Defra should include: the scientific objectives as set out in the contract; the extent to which the objectives set out in the contract have been met; details of methods used and the results obtained, including statistical analysis (if appropriate); a discussion of the results and their reliability; the main implications of the findings; possible future work; and any action resulting from the research (e.g. IP, Knowledge Transfer).

SCOPING STUDY ON SANITISATION OF BIOWASTE TO CONTROL PATHOGENS AND PESTS OF PLANT HEALTH CONCERN

The first draft of the scoping study review has been circulated to the stakeholder group for consultation (copy attached at Appendix 1).

1. Introduction

International trade and associated processing and handling of plant produce generate a risk of introduction or spread of plant pests and pathogens if associated waste is not managed properly. Safe management procedures are also needed for crops and associated wastes at high-risk of contamination with pests and pathogens of quarantine or other regulatory importance following known introductions or outbreaks. A code of practice for the management of agricultural and horticultural waste was published by Defra (formerly MAFF) Plant Health Division (Anon., 1998). A number of waste management procedures are recommended for solid and liquid wastes which are suspected to contain plant pathogens. Recommended procedures for disposal of high-risk wastes are currently:

Incineration authorised under the Environmental Protection Act 1990. Disposal at a landfill site under licence from the relevant environment agency. Eliminated by heat treatment either by boiling for at least 30 minutes, steaming to a minimum of 80 ºC for at

least 1 hour, or dry heating to a minimum of 120ºC for at least 1 hour;

The code also gives other disposal options, e.g. composting, anaerobic digestion or aerobic digestion, though primarily for low risk pathogens. Such methods are expensive and often impractical to apply, particularly where disposal of large tonnages of infected crops and associated waste is required. The code also includes treatment options for disposal of liquid wastes.

Implementation of the Landfill (England and Wales) Regulations 2002, which reflects the requirements of the European Union (EU Landfill Directive 1999/31/EC) for waste disposal, is causing a move away from landfill to more environmentally sensitive waste disposal methods. All waste containing biological material (“biowaste”), will progressively be diverted towards composting or other modes of waste processing. The disposal of biowaste through composting or other digestion processes is a cost-effective and environmentally friendly option. Soil and vegetable by-products represent a potential source of income if recycled as growing media, soil improvers, mulches, composts, green manures or animal feeds. Careful selection and validation of specifications for the range of commercially-used procedures may allow certain procedures to be used for safe and efficient disposal of biowaste which is known or suspected to contain quarantine pests or pathogens.

This report summarises the findings of a scoping study, conducted on behalf of Defra Plant Health Division, to identify the range of available management options and procedures for elimination of plant pathogens and pests from biowastes, and to assess their suitability for application in relevant waste streams where quarantine organisms are known or suspected to occur. Specific objectives were:

1. To produce a portfolio of information on currently-used biowaste treatments including various composting, digestion and direct heating methods.

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2. To summarise current literature on the behaviour of key plant pathogens and pests during biowaste treatments and identify biowaste streams at risk of spreading plant quarantine organisms.

3. To assemble a working group of relevant stakeholders representing science, policy and industry sectors to assess the range of potential methods for risk-free treatment of biowastes.

4. To identify knowledge gaps and recommend future research. 2. Biowastes requiring sanitation

This study examines the range of processes for management of biowastes, which are known, or are suspected to be, contaminated with quarantine pests or pathogens. Such biowastes may include:

Municipal solid waste (MSW) Biodegradable waste from private gardens, parks and other public amenities Kitchen and catering wastes Retail and supermarket wastes Infected or contaminated crops or consignments of produce (identified during surveys, outbreaks or

interceptions). Associated crop residues (e.g. plant remains, soil and other debris). Other infected host plants (e.g. weed hosts identified during surveys). Waste from grading or on-farm packing of contaminated crops (rejected crop material, soil and other debris). Forest or wood processing waste (e.g. bark, wood, sawdust) Liquid and solid waste from vegetable processing such as potatoes and sugar beet (washings, peelings,

rejected crop material, soil, sludge and other debris).

3. General considerations for management of biowaste.

In selecting an appropriate process for management of biowaste, known or suspected to contain quarantine pathogens or pests, it is useful to consider 3 general areas:

3.1. Containment.Successful disposal of biowaste will require careful implementation to ensure pathogen containment throughout storage, transport and treatment of the material in question. If the material has to be transported, this should be under official control and under containment conditions in such a way that there can be no loss of material in transit. Containers and packaging should be incinerated or disinfected and not used further for seed potatoes. Vehicles and other surfaces coming into contact with the contaminated material should be thoroughly washed and disinfected and the washings should be disposed of in such a way that there is no identifiable risk of the pathogen escaping into the environment. Further containment of treated waste may be required in cases where the treatment is not guaranteed to eliminate the pathogen.

3.2. Compliance with legislation, quality standards and best practice advice.Choice of suitable biowaste disposal methods will be governed by national and EU legislation covering issues such as plant health, environmental protection and animal welfare as well as relevant quality standards and codes of practice. These potentially include:

Plant Health (Great Britain) Order 1993 (as amended) and related EC plant health legislation including: o Council Directive 2000/29/EC on protective measures against the introduction into the Community of

organisms harmful to plants or plant products and against their spread within the Community, o Council Directive 69/464/EEC on the control of potato wart disease,o Council Directive 69/465/EEC on the control of potato cyst nematode,o Council Directive 93/85/EEC on the control of potato ring rot, o Council Directive 98/57/EC on the control of Ralstonia solanacearum (Smith)Yabuuchi et al.

Environmental Protection Act 1990 and associated Regulations. EU Landfill Directive 1999/31/EC, Waste Framework Directive 75/442/EEC as amended by 91/156/EEC, Incineration of Waste Directive 2000/76/EC, Possible EU directive on the biological treatment of biowaste, Council Directive 96/25/EC and amendments on the circulation and use of animal feed materials, Defra code of practice for the management of agricultural and horticultural waste, BSI PAS 100 specification for composted materials, Proposed EPPO phytosanitary procedure on the management of plant health risks associated with the use of

biowaste of plant origin.

3.3. Effectiveness of pathogen elimination. Initial validation followed by regular monitoring and auditing of disposal procedures will ensure efficient and effective pathogen elimination prior to release from containment of treated biowaste. A range of disposal

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methods and their suitability for biowaste is presented in Section 4. Specific risks associated with key pathogens and pests of plant health importance are reviewed in Section 5.

4. Recommended methods for safe treatment and disposal of biowaste containing plant pests or pathogens of plant health concern.

4.1. Disposal to landfillLegislation resulting from environmental concerns, and associated increased costs, are creating a need to reduce the disposal of biodegradable waste sent to landfill by 75%, 50% and 35% of that produced in 1995 by 2010, 2013 and 2020 respectively (http://www.environment-agency.gov.uk). Furthermore it is no longer acceptable to dispose of liquid wastes in landfill sites. Alternative methods (listed below) for disposal or decomposition of biowastes containing high-risk plant pathogens or pests, will either eliminate or reduce the volume of biowaste going to landfill, depending on the reliability of elimination of the pest or pathogen in question.

Where disposal of biowaste containing high-risk organisms at landfill sites is necessary, the material should be transported under official control to an approved landfill site with an appropriate licence from the relevant environment agency. There should be no identifiable risk of escape of the organism into the environment. Burial should be at a minimal depth of 2m. New landfills usually use liners to contain leachate. The leachate is collected and treated before being discharged or returned to the landfill site.

4.1.1. On-farm, disposalIn cases where transport to landfill is impractical, or containment during transport can not be guaranteed, it may be preferable to dispose of contaminated crops or residues at the place of production under official supervision from the plant health authority. Extension of the implementation of Waste Framework Directive 75/442/EEC (as amended by 91/156/EEC) to cover agricultural wastes now means that such activities would need to be performed under licence from the relevant environment agency. Defra Plant Health Division is currently exploring the possibility of exemptions in the case of plant health emergencies.

If a crop is found to be infected during the growing season, destruction of the growing plants (e.g. by glyphosate herbicide application) before harvest may be preferred. For disposal in the field of origin, or other suitable site (not destined for arable production), there would need to be no identifiable risk of escape of the pathogen into the environment. Deep burial, at least 2 metres below the surface would provide the highest level of containment. Incorporation of an infected crop or distribution of infected produce or crop residues in the field of origin may be a more practical option. In such cases, covering with soil to a depth of at least 10 cm would help to prevent dispersal (e.g. by wind or wild animals and birds). It would also be necessary to strictly control growth of any volunteer plants under official inspection (e.g. by regular application of glyphosate herbicide). Subsequent growth of susceptible crops in the same field would probably be restricted or prohibited depending on the expected survival of the organism in question.

4.2. IncinerationIncineration involves controlled mass burning to ash, at an approved municipal site authorised under the Environmental Protection Act 1990, during which temperatures must be raised to 850 to 1200 °C (the optimum being 1100 °C) for 40-70 minutes (Ares and Bolton, 2002). Biowaste would require transport under official control to an approved incinerator. After incineration, ash is usually removed to approved landfill sites because of its toxicity. The ash presents no plant health risks.

Of the 7,000 incinerators in England and Wales, 12 large-scale municipal incinerators burn around 2 million t of municipal solid waste (MSW) each year (8% of the total). There are also currently around 3,000 small on-farm incinerators. The DETR has calculated that between 28 and 165 new average-size incinerators may be needed over the next 20 years in England and Wales to meet targets for diverting wastes from landfill set out in the EU Landfill Directive.

4.2.1. On-farm burningBurning on the farm of origin may also be a practical means for safe disposal of small quantities of produce or plant residues contaminated with quarantine plant pests or pathogens. Burning should be done under official control and in such a way that there is no identifiable risk of escape of the pathogen into the environment. Any residual material not burned to ash should either be re-burned or disposed of by another acceptable method.

4.3. Heating A draft EPPO Phytosanitary Procedure (EPPO, unpublished) recommends that biowaste containing quarantine organisms should be heat-treated to 74 °C for 4 hours preferably using wet heat. However, for many individual pathogens, less stringent heat treatment may be effective. For elimination of pathogens, the Defra code of practice for management of agricultural and horticultural waste (Anon., 1998) currently recommends:

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Boiling in water for 10, 20 or 30 minutes for suspended solids or particles of up to 2 cm or 2-10 cm in diameter respectively,

Steaming in a covered pit so that the whole bulk reaches 80 °C for at least 1 hour, or Dry heating so that the whole bulk reaches at least 120 °C for at least 1 hour.

Lethal temperature for pests and pathogens is a function of both time and temperature and is based on the fact that most plant pathogens and pests are mesophylic (with temperature ranges of 20-45 °C) . For mesophylic organisms, a temperature threshold of about 37 °C is critical; the accumulation of heat at this or higher temperatures over time is lethal. With increasing temperature, less time is required to reach a lethal combination of time and temperature. For example, at 37 °C, a killing temperature (LD90) for many mesophylic fungi), may require from 2-4 weeks exposure, whereas at 47 °C, only 1-6 hours exposure is required (DeVay, 1990). Hence, Pullman et al., (1981) showed that four mesophilic plant pathogenic fungi were controlled at constant temperatures ranging from 37 to 50 ºC. The time required for pathogen mortality decreased as temperature increased. Rhizoctonia solani, for example, was killed in 10 min at 50 ºC but required 14 days at 39 ºC. Similarly

Some plant pathogens however are known to be tolerant of higher temperatures (Bollen, 1985). These include thermo-tolerant viruses and viroids such as Tobacco necrosis virus (TNV), Tobacco mosaic virus (TMV) and Potato tuber spindle viroid (PSTVd). Several fungal plant pathogens which produce hardy resting spores (such as Plasmodiophora brassicae, Olpidium spp. and Fusarium oxysporum) have also been reported to be heat tolerant. Specific risks associated with heat tolerance of key pathogens and pests of plant health concern are addressed in Section 5. Whereas heat tolerant pathogens would not survive the three heat treatments as recommended in the Defra code of practice, there could be a risk of survival if pockets of lower temperature were allowed to develop within or at the edges of the bulk of material during treatment. Effective heat transfer throughout the bulk can be obtained by ensuring that:

Particle size is as small and uniform as possible, The material is thoroughly mixed during the treatment, and Temperature and/or exposure time are increased above critical levels.

4.4. Industrial processingIndustrial processing can be a useful means of disposing of infected produce whilst providing some income for the producer. Processes of choice usually involve high temperature treatment and critical requirements are usually of the high temperature/short duration type. Official approval of the site and process by the plant health authority should ensure that there is no identifiable risk of escape of the organism into the environment during transport, handling and storage prior to processing. Liquid and solid wastes generated during the processing of the contaminated material, which do not undergo the approved process, will require officially approved disposal measures. Untreated soil recovered from the process should not be returned to agricultural land or otherwise present an identifiable risk of the pathogen coming into contact with potential host plants. Possible alternatives to contained removal of contaminated solid processing waste to landfill include:

4.4.1. Anaerobic/aerobic sewage digestionAerobic or anaerobic digestion of liquid and solid waste as a slurry with a retention time in the reactor of at least 20 days. With shorter retention times additional aerobic composting of the digested solids may be required. Disposal of the treated solids on non-agricultural land is recommended.

4.4.2. Heating Heating of solid and liquid wastes as slurries may be particularly suitable for processing facilities which generate excess steam. Temperatures of at least 60 ºC should be maintained throughout the volume of waste for at least 30 minutes. Shorter times may be suitable for higher temperatures. Wet heat is more effective than dry heat.

4.4.3. Starch production Starch production from process wastes may be considered a safe process. Protein recycling from “fruit water” during starch production requires heating of the liquid to 110 ºC at pH 5 and potato starch fibres are dried for animal feed at 80-90 ºC. Any leftover waste is usually fermented in citric acid.

4.4.4. Feeding to animalsUse of suitable outgrades or untreated waste to animals should be done according to the conditions described in 4.5.

4.4.5. Approved compostingBiodegradable waste could be safely disposed of through an officially approved composting process (as in 4.6).

4.4.6. Long-term storage of soil on-site Untreated soil recovered from the process could be stored at the processing site provided no identifiable risk of escape of the pathogen is identified. Volunteer plants would require regular control under official inspection.

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4.4.7 Storage lagoonsStorage lagoons may be used to hold liquid waste if aerobic/anaerobic digestion is not available at the processing site. Storage for at least two months is usually suggested to allow sedimentation. A secondary treatment of the clarified effluent to remove possible residual pathogen populations may be required (see 4.4.9). Solid sludge should be dredged from the lagoons regularly and transported to an approved disposal site or treated as above.

4.4.8. Disinfection of liquid waste Disinfection of liquid waste effluents (including wash water) usually first requires sedimentation or chemical coagulation and flocculation to remove suspended solids. Disinfection methods for clarified liquid waste include:

Sand filtration Heating UV irradiation Use of non-persistent disinfectants (such as ozone, chlorine dioxide or peracetic acid formulations) with

approval from the relevant environment agency.

4.5. Feeding to animals

It may be convenient to feed suitable crops or produce, known to contain pests or pathogens of plant health concern, directly to livestock on the farm of origin under officially controlled conditions on a contained hard-standing area where there is no identifiable risk of escape of the organism to the environment. Risks would be minimised by collecting and composting the manure in a contained area for a period of at least 2 months and not returning it to arable land. Feeding of crops to animals will need to satisfy the conditions of Commission Decision 2004/217/EC which prohibits use for animal nutrition if they have undergone specific treatment with certain plant protection products.

For additional security, the crop or product can be pre-steamed on the farm of origin using mobile equipment, thus avoiding risks associated with transport of contaminated material. Official approval by the plant health authority would ensure that treatment was conducted in an officially approved area or on the field of origin, with no identifiable risks of escape of the organism into the environment. Risks would also be reduced by containment of the material during treatment (e.g. in a steam-retaining covered pit or tank), circulation of pressurised steam so that all parts of the bulk reach a minimum temperature of 60 ºC for at least 30 minutes and washing and disinfection of mobile steaming units before leaving the site.

Fermentation of contaminated potatoes during silage production may also be a convenient pre-treatment prior to feeding to animals. In this case the silage should be produced on the farm of origin contained in such a way that there is no identifiable risk of escape of the pathogen to the environment during silage preparation and fermentation. Feeding of the silage should be done on the farm of origin on a contained hard-standing area. Manure should be collected and composted in a contained area for a period of at least 2 months and should not subsequently be returned to arable land.

Processing of infected crops into animal feeds has also been successfully used to reduce risks of dispersal of pathogens such as Tilletia indica in wheat (see 5.2.4.).

4.6. Aerobic Composting

The literature relating to risks of pathogen and pest survival during composting of agricultural and horticultural waste was reviewed for Defra Plant Health Division in 1998 (Sansford and MacLeod, 1998) in support of their code of practice for the management of agricultural and horticultural waste (Anon., 1998). More recent studies (Noble and Roberts, 2004) commissioned by the Waste & Resources Action Programme (WRAP) have reviewed the effects of temperature–time combinations and other sanitizing factors during composting on 64 plant pathogenic fungi, plasmodiophoromycetes, oomycetes, bacteria, viruses and nematodes. Both studies conclude that most plant pathogens and nematodes should be eliminated during a well managed compost process in which uniform temperatures of 60-65 ºC are maintained for several days. However, a number of plant pathogens were also identified (mostly heat tolerant viruses and obligate biotrophic fungi with hardy resting spores) which were capable of surviving the composting process. WRAP-commissioned research has also studied the behavior of a number of plant pathogens during laboratory and large-scale composting processes and has monitored typical conditions within commercial scale processes (Noble et al., 2004).

The review of Noble and Roberts (2004) reports that for 33 out of 38 fungal and oomycete pathogens, all seven bacterial pathogens and nine nematodes, and three out of nine plant viruses, a peak temperature of 64–70°C and duration of 21 days, were sufficient to reduce numbers to below the detection limits of the tests used. Plasmodiophora brassicae (clubroot of Brassica spp.), Fusarium oxysporum f.sp. lycopersici (tomato wilt) and Macrophomina phaseolina (dry root rot) were more temperature-tolerant, as they survived a peak compost

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temperature of at least 62°C (maximum 74°C) and a composting duration of 21 days. Tobacco mosaic virus (TMV) was found to be the most tolerant pathogen where peak compost temperatures in excess of 68 °C and composting for longer than 20 days were needed to reduce numbers below detection limits.

Heat generated during the thermophilic phase of the composting process is thought to be the most important factor for the elimination of plant pathogens (Ryckeboer 2001). A sufficient and uniform moisture content is also important to ensure efficient heat transfer throughout the batch. Factors other than temperature and moisture which also influence pathogen survival include:

The production of toxic compounds such as organic acids and ammonia, Lytic activity of enzymes produced in the compost, Microbial antagonism, competition and predation/parasitism, The initial population and stage of the life cycle of the pathogen, and Properties of the feedstock (including C:N ratio, particle size and pH).

Given the number of interacting variables, it is not surprising that published data on pathogen survival varies from study to study and persistence in composts is often shorter than that observed in laboratory cultures at the same temperature. For example, eradication of Plasmodiophora brassicae was demonstrated in some studies at 54-55 °C over 21 days (Lopez-Real & Foster, 1985; Bollen et al., 1989) but required 60–70 °C for 7-21 days in others (Ylimaki et al. 1983; Bruns et al., 1993; Christensen et al., 2001).

Nobel et al. (2004) systematically studied the behaviour of indigenous plant pathogens of current commercial concern during composting under both controlled laboratory and commercial conditions. Propagules of Fusarium oxysporum f.spp. lycopersici and radicis-lycopersici, Pythium ultimum, Thielaviopsis basicola, Rhizoctonia solani, Verticillium dahliae and Xanthomonas campestris pv. campestris were shown to be eradicated from affected plant material in laboratory tests by a compost temperature of 52 °C held for 7 days. Eradication of Fusarium oxysporum f.sp. lycopersici and Verticillium dahliae were shown to be eradicated from infected plant material in a large-scale tunnel when compost temperatures exceeded 50 °C (with a maximum of 70 °C) for 4 days. Eradication of Plasmodiophora brassicae from infected Chinese cabbage plants was achieved with composting batch temperatures of 60 °C for 1 day or 65°C for 1 day when initial moisture content was at least 59% or 51% (w/w) respectively. Propagules of Phytophthora nicotianae required a temperature of up to 58 °C for 7 days for eradication, whereas, propagules of Microdochium nivale required a higher temperature of up 64°C for 7 days for eradication from composting material. It was concluded that a minimum composting temperature of 65 °C maintained for 7 days with a minimum moisture content of 51% w/w at the start is required to eradicate all the pathogens examined, with the exception of TMV. A composting temperature of at least 80 °C for 7 days was required for eradication of TMV from affected leaf material. Monitoring of plant-based waste during two large-scale commercial composting processes indicated that temperatures exceeded 65 °C for at least 1 day at 50 cm depth and exceeded 64 °C for at least 2 days at 10 cm depth. Temperatures in excess of 60 °C can be readily achieved in different composting systems, with a wide range of organic feedstocks (Noble and Roberts, 2004). Providing the type, mixture and condition of feedstock is suitable, it should be possible to select suitable composting processes able to achieve the conditions required for sanitisation of most types of biowaste containing many, but probably not all, plant pathogens of concern. Various composting processes, currently used in the UK, are described by the Composting Association (http://www.compost.org.uk):

4.6.1. Turned-WindrowsMost large-scale composting facilities in the UK tend to be run on a low-tech basis using the open-air turned-windrow technology to compost green wastes from domestic or public gardens. This is where the composting materials are laid out in long triangular prisms (called ‘windrows’). They are broken up and mechanically ‘turned’ periodically to mix and oxygenate the material. Windrows can be insulated by covering with straw after each turning to avoid low temperatures at the surface.

Nobel and Roberts (2004) indicate that non-insulated static systems are particularly prone to significant cool zones. These can also occur in dry zones when moisture content is not well managed (Bollen & Volker, 1996). Gale (2002) recommends turning of windrows at least three times during a thermophilic phase of at least 14 days to allow uniform distribution of heat and moisture throughout the batch. This should allow the temperature to reach at least 60 °C for at least 2 days. Christensen et al. (2002) recommend five turns within a 14 day period for windrow systems.

4.6.2. Aerated Static PilesThese are un-turned piles through which air is forced during composting via pipes or ducts laid beneath the composting mass. The air may either be blown (positive aeration) or sucked (negative aeration). The forced air can prevent pockets of anaerobic activity and allow better temperature control throughout the batch. However, Nobel et al. (2004) found that composting batch aeration had no significant effect on the temperature and time required to eradicate P. brassicae.

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4.6.3. In-Vessel SystemsIn-vessel systems apply the same process of aerobic composting through thermophilic microbial activity as for windrow composting. The enclosed conditions allow better insulation and easier optimisation of humidity, aeration and temperature through forced aeration and recycling of warm, moist air. Furthermore, the waste is fully contained within the vessel during the process. For this reason, in-vessel processing may be more suited for waste potentially containing high-risk plant or animal pathogens. The enclosed in-vessel system also prevents access by vermin and birds and better containment of leachates, bioaerosols and odours. A wider range of feedstocks, including animal by-products and catering waste, can therefore be safely processed in-vessel within the Animal By-Products Regulations following site approval by the State Veterinary Service. Since residence times are usually short in in-vessel systems, further aerobic composting is usually required to stabilise the compost following its sanitation during the in-vessel process. In-vessel systems are also often used to reduce the mass and biological activity of MSW prior to landfill.

There are over 30 different in-vessel systems available within the UK ranging from those suitable for annual processing of 2 tonnes to those processing tens of thousands of tonnes of municipal waste. There are continuous flow and batch processes. In the former, new feedstock is regularly added at the start of the process whereas the latter system treats all waste in a single batch. For high-risk material, single batch processes are recommended since all material is guaranteed to undergo the full process. Mobile systems also allow for on-site composting and delivery of composted material to final destination prior to opening of the vessel. A Guide to In-Vessel Composting – Plus a Directory of System Suppliers has been published with the support of Defra through the Environmental Action Fund and is available from The Composting Association through www.compost.org.uk.

4.6.4. VermicompostingVermicomposting uses earthworms to facilitate the composting of organic wastes by microorganisms. The process takes about the same time as traditional composting (6-12 weeks) but the final compost quality is usually higher. The ideal conditions for vermicomposting are 15-25 ºC, pH 5-9 and 70-90% moisture with a C:N ratio of 15:1 - 35:1. The process is suited to wetter feedstocks such as potatoes. As a low-temperature composting process, vermiculture is not suitable as a direct process for treating high-risk waste, however vermiculture has been accepted by Defra as a viable process for catering waste after pre-sanitisation in compliance with the Animal by-Products Regulations.

4.6.5. On-farm compostingOn-farm composting can range from a simple open heap to a fully licensed and certified sophisticated facility which may be permanent or a mobile facility brought onto the farm when needed. Composting on-farm currently requires a waste management licence from the relevant environment agency, although it is possible to obtain an exemption if composting under 1000 m3 of material for use on the farm itself.

The conditions required for sanitisation of waste suspected to contain pests or pathogens of plant health importance will be difficult to obtain in a simple open compost heap. However, mobile in-vessel facilities which are certified to achieve quality standards (see 4.8) may provide opportunities for small-scale processing of suitable waste materials.

4.7. Anaerobic digestion

Anaerobic digestion (also known as biogas generation) involves a process by which methanogenic bacteria break down organic matter under oxygen-deficient conditions to biogas (mainly methane and carbon dioxide), fibrous solid “digestate” and nutrient rich liquor. Anaerobic digestion plants can be built for individual farm use or provide a centralized facility to collect waste from several sources including MSW, food processors and supermarkets.

Although usually suited to high water/low solid wastes, some systems can process medium solid organic wastes (16-35%). All digestions are carried out in a sealed tank with automatic mixing and external heating (generated from the biogas produced). Both batch and continuous-flow systems are available. In the batch system, all of the material is processed for a set time before emptying whereas digestate and liquor are regularly removed and fresh waste is added during the continual process. The batch process is most suitable for higher solid waste and also ensures that all material is processed for the full time period. Two types of methanogenic bacteria can be employed; mesophilic bacteria are active at 30-40 ºC, and thermophilic bacteria at 55–77 ºC. Mesophilic anaerobic digestion typically takes 15-30 days at 35 ºC, whereas 12-14 days at 55 ºC are usually required for thermophilic anaerobic digestion. The Animal By-Products Regulations 2003 requires catering wastes to be anaerobically digested at either:

57 °C for 5 hours with a particle size of 5 cm. or; 70 °C for 1 hour with a particle size of 6 cm.

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If a lower temperature is used, the digestate must be sanitised separately (before or after digestion) to ensure adequate pathogen kill. The way in which an anaerobic digestion plant is run depends on the primary objective of the plant, e.g., biogas production or digestate (fertilizer) production. Between 30% and 60% of the input materials may be converted into biogas.

The digestate is generally in the form of a high water content anaerobic slurry. The digestate may be used directly for improving soil fertility. However more typically it will be dewatered before use. Liquid from dewatering is re-used in the process or used as a liquid fertilizer. Although de-watered solids may be used directly, they are still anaerobic and odorous, so more frequently they are composted aerobically before final use. The gas produced is a combination of methane (60%), carbon dioxide (39%) and some small amounts of ammonia and other gases. The liquor can be re-used in the digestion process or used as liquid fertiliser. The digestate can be used to improve soil fertility usually after dewatering and an aerobic composting phase.

The majority of systems currently available to digest wastes that contain solid residues are designed for low or medium solid contents and can be used to treat MSW, agricultural and many industrial residues. Designs include two-stage digestion (in which digestion and methane generation are carried out in two separate stages) and batch digestion (the design predominantly used for MSW). In batch digestion, the reactor containing the organic material is inoculated with previously digested feedstock from another reactor, sealed and allowed to digest naturally. The liquor at the bottom of the reactor is heated and recirculated to promote digestion. ‘High solids’ systems have been developed recently, principally for the organic fraction of municipal solid waste (MSW). The digestion occurs at solids contents between 16% and 35%. When the solids concentration is between 25% and 35%, the free water content is low and these systems may be referred to as ‘dry phase digestion’ or ‘anaerobic composting’. ‘Low solids (<10%)’ systems or slurry digestion, the simplest, most widely used system is a continuously stirred tank reactor (CSTR). Mixing is provided either mechanically or by gas recirculation to help prevent scum forming at the surface. For a medium solids content (of up to 16%), similar systems are available. The higher solids content means that scum is not formed, so mixing is only provided to aid gas release and material flow through the digester. Residence times are much shorter, being measured in days.

4.8. Quality Standards for composting and anaerobic digestion processes.

In the United Kingdom, the British Standards Institution’s Publicly Available Specification for Composted Materials (BSI PAS100) sets criteria for minimum quality of compost. The Composting Association’s Compost Certification Scheme provides independent assessment and verification of conformity with BSI PAS 100. The standard considers acceptable levels of human pathogens (Salmonella sp. and E. coli), toxic and phytotoxic elements, physical contaminants and weed propagules and modifications, currently under consultation, aim to ensure that the standards also address risk of survival of plant pathogens.

PAS100 currently states that: ‘Particle size, temperature, moisture, pH, ammonia concentration, mixing of composting materials and oxygen supply affect sanitization performance. Temperature should be maintained within the optimal range of 55 °C to 70 °C for a duration of at least 7 days (in-vessel) or 14 days (windrows). Moisture should be maintained within the optimal range of 40% to 60% mass/mass’. Noble et al. (2004) concluded that temperature and moisture content were the factors most affecting sanitisation and that variation in pH, ammonia concentration and oxygen supply found within compost processes played little or no significant role.

A proposed EPPO phytosanitary procedure addresses the management of plant health risks associated with the use of biowaste of plant origin. The current draft recommends that: ‘Water content should be at least 40%; the pH value should be approximately 7. In the course of the composting process, the entire quantity of materials being treated must be exposed to a temperature of at least 55° C during an uninterrupted period of two weeks, or, alternatively, to a temperature of 65° C (or, in the case of enclosed composting facilities, 60° C) over a continuous period of one week. In anaerobic digestion facilities, the waste matrix must be treated in such a way that a minimum temperature of 55° C is maintained over a period of 24 hours without interruption and that the hydraulic dwell time in the reactor is at least 20 days. For both types of treatments, if lower operating temperatures or shorter periods of exposure occur, either thermal pre-treatment of input materials (74° C for 4 hour, preferably wet heat) or adequate secondary treatment of the products (heating up to 74 °C for 4 hour, preferably wet heat), or, in the case of anaerobic digestion facilities, an aerobic secondary decomposition of the separated anaerobic digestion residues (composting) is required’

The EPPO procedure has further requirements that biowaste of plant origin known or suspected to contain any quarantine pest should be heat-treated (preferably wet heat) to 74 °C for 4 hours before entering into biowaste treatment processes. It is also proposed that exceptions could be made when:

the organism is well known to be susceptible to the biowaste treatment process, and the treatment can be performed in facilities separated from other biowaste.

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In every case, it is suggested that the process should be supervised by the national plant protection organisation, and the resulting product must be tested and found free from the quarantine pest. It is proposed that testing be conducted by means of 1) direct process validation, 2) indirect process supervision, and 3) product analyses. For direct process validation, indicator organisms are used to determine the efficiency of a process from a sanitation point of view for all process stages. Two independent validation tests are recommended, at least one to be conducted during winter for outdoor processes. A standard procedure involves the use of TMV and Plasmodiophora brassicae as indicators. The procedure has been tested under controlled conditions at CSL, where both organisms survived in infected plant material placed into commercial feedstock and maintained at 55-60 °C and 40% water content for 21 days (Elphinstone et al., 2004). Indirect process supervision involves regular temperature monitoring (preferably continual) in at least three representative zones in the process stages. Product analysis involves representative sampling of the final product and testing for viable seeds or vegetative plant parts which would indicate incomplete sanitisation.

Risks associated with key pathogens and pests of plant health significance in the light of the above mentioned quality standards are discussed in Section 5.

5. Risks associated with key pathogens and pests of plant health concern.

5.1. Bacteria

Plant pathogenic bacteria are not generally expected to survive the temperature-time standards set by PAS100 or the proposed EPPO phytosanitory standards (Sansford and MacLeod, 1988; Noble and Roberts, 2004).

5.1.1. Ralstonia solanacearumIn Korea, R. solanacearum was eliminated from recycling nutrient solution by heating to 70 °C for only 3 minutes (Lee et al. 1998). In the laboratory the organism was eliminated in liquid culture or infected potato pieces at temperatures as low as 55 °C applied for at least 10 minutes (Elphinstone, unpublished). All biovars of R. solanacearum were eliminated from liquid cultures by exposure to 43 °C (Date et al., 1993). However, the bacterium survived in diseased plant residues and in soil at 43 °C for 2 days and at 40 °C for 5 days. Hot air treatment at 75% relative humidity of infected ginger rhizomes eliminated R. solanacearum when their internal temperatures were allowed to reach either 49 °C for 45 minutes or 50 °C for 30 minutes (Tsang and Shintaku, 1998).

Ryckeboer et al. (2002) demonstrated that R. solanacearum could be destroyed to below detectable limits within one day during anaerobic digestion at 52 °C of source separated household wastes. Similarly, Termorshuizen et al. (2003) showed that R. solanacearum could be reduced below detection levels following 6 weeks mesophilic (maximum temperature 40 °C) anaerobic digestion of vegetable, fruit and garden waste. Elphinstone (unpublished) also showed reduction of R. solanacearum below detectable levels within 48 hours during anaerobic digestion of sewage sludge at 37 °C, further indicating that factors other than temperature alone are important in elimination of this pathogen.

5.1.2. Clavibacter michiganensis subsp. sepedonicus and related pathogensA minimum temperature of 82 ºC for at least 5 min has been shown to completely inactivate the ring rot bacterium (Secor et al., 1988). No information is available on the behaviour of this organism during composting or anaerobic digestion but Turner et al. (1983) concluded that the related pathogen Clavibacter michiganensis subsp. michiganensis (Cmm) was effectively reduced during anaerobic digestion at 35 ºC. Cmm, the causal agent of bacterial canker of tomato, was also shown to be eradicated from naturally infected tomato seeds that were soaked in water at 52 ºC for 20 min or at 56 ºC for 30 min (Shoemaker and Echandi, 1976; Fatmi et al., 1991). Treatment at 51 deg C for 1 h was also shown to control the related pathogen Clavibacter xyli subsp. xyli in infected sugarcane seed (Ramallo and de Ramallo, 2001).

5.1.3. Other bacteria of plant health significanceOther bacteria of plant health significance which can be eliminated from infected plant material during composting for at least 7 days at temperatures of at least 50 ºC include Erwinia amylovora (Bruns et al., 1993), Erwinia chrysanthemi (Hoitink et al., 1976) and Pseudomonas savastanoi pv. phaseolicola (Lopez-Real & Foster, 1985).

5.2. Fungi

Hyphae of most fungal pathogens will be killed under the conditions experienced in composting and anaerobic digestion processes managed according to PAS100 and EPPO quality standards. However, some fungi produce hardy resting spores which can resist elevated temperatures, can persist in the environment for many years and are unaffected by microbial competition and antagonism. Heat tolerant fungi may also remain protected from external influences inside infected plant tissues. A review of the literature was conducted to assess the potential for survival during composting of the following fungal pathogens of plant health significance:

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5.2.1. Polymyxa betae and P. graminis Dickens et al. (1991, unpublished) investigated the effect of heat treatment on effluent from sugar beet processing on the survival of Polymyxa betae, the vector of beet necrotic yellow vein virus (BNYVV) which causes sugar beet rhizomania. Viable resting spores (cystori) containing infective virus could be recovered from soil/water suspensions following 30 minutes exposure to temperatures up to 75 ºC. Populations were reduced but not eliminated at 55 ºC but not at 50 ºC.

Nishinome et al. (1996), however, found that lethal temperatures were achieved during composting of dewatered waste soil (80% soil) from sugar beet processing, using an aerated pile process. The lethal temperature for rhizomania infectivity was determined as 40 ºC for 14 days or 60 ºC for 1 day, and temperatures within the aerated pile reached 60 ºC over the one month duration. 5.2.2. Phytophthora ramorum and P. kernoviaePhytophthora spp. have the potential to produce two types of hardy resting spores, oospores and chlamydospores. Nobel et al. (2004) have shown in laboratory studies that the number of surviving propagules of Phytophthora nicotianae cultures containing oospores and mycelium declined as temperature increased from 18 – 52 ºC with complete eradication occurring in onion waste and pure cultures at 58 ºC over 7 days and in green waste at 52 ºC over 7 days. The results obtained for Phytophthora nicotianae were similar to those obtained previously for other Phytophthora species. Bollen et al. (1989) showed that peak compost temperatures of 55 and 60°C and composting for 21 days eradicated P. infestans and P. cryptogea respectively. Hoitink et al. (1976) eradicated P. cinnamomi in compost at only 40°C over 7 days.

New research on the eradication of Phytophthora ramorum (the cause of sudden oak death) during composting of infected plant material at 55°C over 14 days (Garbelotto, 2003) is providing hope that composting will provide a reliable means of sanitising host material (bay laurel leaves and woody chips and stems) infected with this quarantine pathogen. One week of heat treatment ‘sanitised’ the woody substrates (chips and stems) but not the bay laurel leaves, probably because chlamydospores rarely form in wood but frequently form in leaves. Because the chlamydospores are embedded in the leaf parenchyma they are more resistant to high temperatures than when tested on agar in the absence of leaf tissue. Significant variation detected among different plant substrates, post treatment environmental conditions, and composting parameters (specifically temperature) mean that further investigation will be required to assess the risks. It is possible that time of year may play an important role in the final outcome of the sanitation process.

The effect of temperature alone under laboratory conditions on chlamydospores and sporangia of P. ramorum in the absence of plant material was investigated by CSL in the Defra-funded Project PH0194. Viability of chlamydospores decreased with time at 40 ºC and no spores could be germinated after 24hrs. Sporangia were less robust and none were able to germinate after 1 hour at 40 ºC. It is however, questionable whether chlamydospores of P. ramorum are dormant or dead when they appear to be non-viable. It is not possible to get 100% of chlamydospores to germinate in the laboratory and it is therefore difficult to determine whether chlamydospores have been fully killed during composting or whether some of the remaining population may be dormant rather than dead.

The newly identified species Phytophthora kernoviae produces sporangia and the more hardy oospores in nature but has not yet been found to produce chlamydospores and may therefore be more easily eradicated than P. ramorum during a well managed composting process.

5.2.3. Synchytrium endobioticum Resting bodies (sori) of the potato wart disease fungus are able to survive for over 30 years in soil and are resistant to temperature extremes and microbial antagonism and competition. Survival of S. endobioticum in water at 60°C for 2 hours has been reported (Nobel and Roberts 2004), but there is little information on its behaviour during composting. One report from Russia showed that the pathogen was not recovered from waste from processing of infected potatoes which had been composted for 2 to 3 months together with animal manure and saturated with ammonia (Efremko and Yakoleva, 1981). Zoosporangia were able to maintain viability for 2-3 months in untreated processing waste (sludge) at temperatures below 21 ºC but were effectively killed by heat treatment.

The only reports of effective control of this organism involve fumigation of infested soils, e.g. using 98% methyl bromide at 50-200 g per m2 for 72 h (Noehr Rasmussen and Mygind, 1977). Potocek (1991) reported successful fumigation of soil using granulated urea at 500 g per m2, followed by calcium cyanamide (500 g per m2) and dazomet (as Basamid, 60 g per m2). Other effective treatments included AITK (as Allyspol 75 EC, 50 ml per m2), DNOC (as Nitrosan 25, 100 g per m2), metam Na (as Nematin, 100 ml per m2) and MITK + dichlorpropene (as Di-Trapex, 50 ml per m2).

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5.2.4. Tilletia indicaT. indica causes Karnal bunt of wheat and survives in the form of teliospores which are protected within hardy resting structures (sori). Current EC-commissioned research (Sansford et al., 2004) has fully investigated potential disposal methods for infected grain and crop residues. No data exist on the survival of teliospores during composting. Smilanick et al. (1998) report that in water or moist soil, heating to 50-55 ºC killed teliospores within 30 minutes to 5 hours. Heating infected grain reduced teliospore germination to less than 2% (compared to 46% for the untreated control) after 1, 12 and 88 hours exposure to dry heat at 125, 110 and 100°C respectively (Smilanick et al., 1988). However, no dry heat treatment methods tested have been successfully applied to bulk quantities of grain using commercial heating systems In small scale trials, teliospores of T. indica in deliberately contaminated millfeed were killed when the product reached temperatures of 84, 101 or 110ºC for 12, 5 or 2 minutes respectively. Using the same process for infected wheat grain, Peterson (unpublished results) showed that teliospores were killed when the grain reached temperatures of 115, 122, or 119ºC for 12, 5 and 2 minutes respectively. When the grain was tempered with water five minutes before treating, teliospores were killed at 110, 101, or 84ºC for 2, 5 and 12 minutes, respectively (Peterson, unpublished data).

No practical treatment method was found for killing teliospores in grain while preserving milling quality. Although e-beam and Cobalt-60 Gamma irradiation of grain (10 KG) killed teliospores of T. controversa and T. tritici while preserving milling quality, it also killed the seed, thus causing it to break down quickly (Schultz and Maguire, 1991; Sitton et al., 1995). The dosage of methyl bromide required to kill teliospores of these species also reduced the storage life of the wheat and presented residue problems.

An extensive study using grain infested with teliospores of T. controversa, (Bechtel et al., 1999) supported the view that flour obtained from milling wheat grain in an urban area is not considered a high dissemination risk. However, the milling by-products will most likely contain viable teliospores. Processing grain into animal feed using the steam flake milling process (109 °C for 30 minutes) was found 100% effective in eliminating the pathogen while preserving some economic value for the crop. This also eliminated the necessity to regulate livestock manure provided livestock have not been fed unprocessed grain. This method was tested on a commercial scale and has been used extensively by the United States Department of Agriculture (USDA).

Pelletisation or extrusion of millfeed (the by-product of flour production) also killed teliospores of T. tritici and T. controversa. At mills that routinely pelletise the by-products, milling Karnal bunt affected grain and pelleting the by-products would maintain some of the economic value of the commodity and reduce the majority (but not all) of the risk of spread to the conveyance pathway from the mill. The telliospores are likely to survive ingestion by livestock and chickens (Smilanick et al., 1986; APHIS, 2003) so manure produced up to 5 days after feeding infected grain will require safe disposal by deep burial or composting followed by restricted use where there is no identifiable risk of spread to crops.

In field studies, methyl bromide (ca. 565 kg/ha), methyl iodide (ca. 453 kg/ha) (see Table 13) and soil solarisation (30 to 45 days) were found to kill over 95% of the viable teliospores down to a maximum depth of 20 cm in the soil depending on the study concerned. In the case of an isolated or limited introduction, it may be practical and economically feasible to fumigate the infested field.

5.3. Viruses and viroids

Some viruses are know to tolerate long periods of exposure to high temperatures. Tobacco mosaic virus (TMV) requires temperatures in excess of 68 ºC for longer than 20 days for complete eradication from infected plant material (Noble and Roberts, 2004). For this reason it has been selected as a useful indicator of successful sanitisation. Tobacco necrosis virus (TNV) was inactivated by exposure to 55 ºC for 72-96 hours or 70 ºC for 24-48 hours. Other viruses known to be temperature tolerant include Cucumber Green Mottle Mosaic Virus, Pepper Mild Mottle Virus, Tobacco Mosaic Virus (TMV) and Tobacco Rattle Virus (TRV). TRV survived exposure over 6 days to 68 ºC during composting of infected plant material (Bollen, 1985) although this process effectively eliminated the nematode vector of the virus. Tomato Mosaic Virus (ToMV) was very temperature tolerant when tested in an incubator (Avgelis and Manios,1989), but eradicated from a compost heap at 47°C for 10 days. Cucumber Mosaic Virus, Melon Necrotic Spot Virus and Tobacco Necrosis Virus could be eradicated by a composting temperature of 55°C held for 14 days whereas Tomato Spotted Wilt Virus required a temperature of 60°C for 3 days, (Noble and Roberts, 2004).

5.3.1. Pepino mosaic virus

Studies at CSL (Mumford, unpublished) demonstrated Pepino mosaic virus (PepMV) was successfully eradicated from shredded infected tomato plants during windrow composting in which all parts of the heap attained a minimum of 60 ºC for at least 5 days. As a further precaution, the finished compost was not used for horticultural purposes.

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5.3.2. Heat tolerant viroids e.g. Potato spindle tuber viroidViroids are particularly persistent and heat tolerant. The Potato spindle tuber viroid (PSTVd) can remain infective in hydrated plant material for several months and in dry material for over a year is expected to survive composting processes. The only suitable disposal methods for high-risk biowaste infected with PSTVd are incineration or containment in an approved landfill site.

5.4. Nematodes

Nobel and Roberts (2004) found that all seven nematode species affecting plants studied during composting were eliminated within 1 day at temperatures of at least 52 ºC. These were:

Globodera pallida Globodera rostochiensis Heterodera schachtii Meloidogyne hapla Meloidogyne incognita Meloidogyne javanica Pratylenchus penetrans

However, some cyst-forming spp. such as the beet cyst nematode (Heterodera schachtii) has the ability to survive in compost at lower temperatures (31°C) for long periods (Ryckeboer et al. 2002b). All nematode pests are expected to be eliminated during well managed composting processes under quality standards such as PAS100.

5.4.1. Globodera pallida and Globodera rostochiensisThe lethal temperature for potato cyst nematodes (PCN) in sugar beet compost has been determined as 40 ºC for 10 days or 50 ºC for 5 days (Nishinome et al., 1996). PCN was also killed in potato processing sludge below the lethal temperature at only 34 ºC, probably due to toxicity of the sludge (Bollen, 1985). However viable PCN cysts have been recovered from sewage sludge after anaerobic digestion although populations were reduced (Turner et al., 1983). Similar studies by Catroux et al. (1983) showed that almost 100% of the cysts of G. rostochiensis and G. pallida were killed during anaerobic sewage digestion; composting with temperatures rising to more than or equal to 40 ºC also killed most of the cysts whereas aerobic digestion destroyed only a small proportion of the cysts. Live cysts could survive up to 3 months of storage in the outer layer of sludge. In limed sewage sludge at pH 10 or more, the cysts were killed in 14 days.

5.4.2. Ditylenchus dipsaciDipping of garlic bulbs for at least one hour in water at 49 ºC or 50 ºC but not at lower temperatures eradicated stem nematodes (Ditylenchus dipsaci) (Jaehn, 1995). Full control was also obtained by dipping garlic bulbs in hot water at 45 ºC for 20 min (del Toro and Mavrich, 1977). Satisfactory control of D. dipsaci within daffodil bulbs was also achieved by dipping in water at 44 ºC for 150 minutes with 0.37% formaldehyde or at 44 ºC for 240 minutes without chemicals (Qui et al., 1994). An annual treatment of 2 h at 43.5 ºC also controlled stem nematodes in narcissis bulbs (Vreeburg, 1981). The nematode was also eradicated by exposure of rice and maize seed to moist heat at 40 deg C for 30 min, followed by 60 deg C for 8 min (Tenente et al., 1999). Summer treatment at 48 ºC for 17.5 min has also been used to eradicate the nematodes from unrooted strawberry crowns (Akinin et al., 1983).

5.4.3. Root inhabiting Meloidogyne spp.The review of Nobel and Roberts (2004) showed that various Meloidogyne spp. (M. hapla, M. incognita and M. javanica) would be eradicated within 1 day at temperatures of 46-57 ºC. No data are available for the quarantine nematode M. chitwoodii. Rykeboer et al. (2002) showed that M. incognita was effectively destroyed to below detectable limits within one day of anaerobic digestion at 52 ºC. Complete eradication of M. incognita from mulberry saplings, grapevine rootstocks and Mentha spicata roots has been achieved with hot water treatment at 48-52 degrees C for 10-30 minutes (Gokte and Mathur, 1990 & 1995; Sharma and Govindiah, 1996).

5.5. Insects

Control options for eradication of quarantine insects have been reviewed by Cheek (1999). An integrated approach is usually taken to the selection of quarantine disinfestation treatments and is usually site specific. Treatments may include, washing to remove surface invertebrates, heating or cooling, insecticide or fumigant applications, and controlled atmospheres. Increasing restrictions on the use of chemicals, particularly on edible crops, have led to a greater emphasis on biological and other non-chemical options.

There is virtually no published information on the behavior of insect pathogens during composting, digestion or other heating processes, although it is widely acknowledged that insect eggs, lavae or pupae will not survive the conditions prevailing during such treatments (Sansford and MacLeod, 1998).

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For fumigation, the most effective product remains methyl bromide. This product is being phased out because of environmental concerns, although there are exemptions for critical uses. Investigation into alternatives is ongoing but have had limited success (MacDonald and Mills, 1994).

Some biological control treatments can be used as integrated control components but must be compatible with any chemicals being used.

5.5.1. Bemisia tabaciInvestigations into fumigation with methyl bromide and potential alternative fumigants did not result in a commercially acceptable method to eliminate B. tabaci from poinsettia (MacDonald and Cheek, 1994; MacDonald and Mills, 1994).

Larval stages occur on the underside of leaves, making insecticides difficult to target. Buprofezin has been effectively used, partly because its volatility means it is active in the vapour phase which reaches scales on the lower leaf surface. This is a novel chitin synthesis inhibitor and is the primary insecticide recommended against the larval stage in the UK. Imidaclorprid is also effective against both Bemisia and Liriomyza.

The entomopathogenic fungus Verticillium lecanii has been effective against B. tabaci on poinsettia cuttings and tropical glasshouses under high humidity conditions >95%). The parasitoid Encarsia formosa was also effective but expensive at the rate of introduction of one parasitoid per plant.

5.5.2. Liriomyza leaf minersA method for removal of Liriomyza trifoli from infested chrysanthemum consignments which combines cold storage with methyl bromide fumigation was developed by Mortimer and Powell (1984) and has become an EPPO standard procedure (OEPP/EPPO, 1984). The method has been successfully used in the UK following outbreaks of L. huidobrensis.

The larval stages are protected within the leaf tissue and so require systemic and translaminar insecticides, able to penetrate the tissue. Conventional products such as aldicarb, triazophos and trichlorfon have been used successfully in addition to more recent products such as abamectin and imidacloprid (Buxton and MacDonald, 1994). Abamectin is an important component of eradication programmes. The pupae are soil dwelling, so soil sterilisation treatments such as steaming, methyl bromide or dazomet are most effective. Resistance to some insecticides such as pyrethroids and organophosphates has been observed, particularly within L. huidobrensis populations (MacDonald, 1991).

L. huidobrensis has been successfully eradicated from low infestations in heated tomato crops by high rates of introduction of the parasitoids Dacnusa sibirica and D. isaea (van der Linden, 1991 and 1992). Williams and MacDonald (1995) have also shown potential for the use of foliar applications of entomopathogenic nematodes, such as Steinernema feltiae against L. huidobrensis.

6. Summary of recommendations for elimination of plant pathogens and pests of plant health concern from biowastes.

6.1. DisposalAlthough waste disposal to landfill is decreasing for economic and environmental reasons, it is still a reliable means for disposal of biowaste containing high-risk organisms. Landfill sites should be licensed by the appropriate environment agency. Processes which can reduce the mass of biowaste going to landfill are recommended, even if full sanitation is not achieved. Incineration provides an alternative to landfill for disposal of high-risk waste. Incinerators should be authorised under the Environmental Protection Act 1990.

6.2. ContainmentFor biowastes containing most high-risk plant pathogens and pests there are alternatives to landfill and incineration provided that full containment of the organisms is observed under official control of the relevant plant health authority at all stages of transport, treatment and final disposal of the material. Where waste treatment processes are not fully guaranteed to eliminate infective pathogens, their use in reducing or recycling biowaste may still be justified provided there is no identifiable risk of the final processed material coming into contact with potential host plants.

6.3. Heating Current specifications for biowaste sanitisation by boiling, steaming or dry heating will eliminate even the most heat resistant organisms. Where lower temperature-time combinations are known to be lethal for particular high-risk target organisms, less stringent treatments may be fully effective and less costly. Efficiency of heat transfer should be constantly monitored at all levels through the bulk to ensure that critical temperatures and exposure times are exceeded.

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6.4. Industrial processingIndustrial processing can be a useful means of disposing of produce containing high-risk organisms whilst providing some income for the producer. Official approval of the site and process by the plant health authority should ensure that there is no identifiable risk of escape of the organism into the environment during transport, handling and storage prior to processing. All waste generated during processing of the contaminated material should be treated or disposed of in such a way that there is no risk of the organism coming into contact with potential host plants.

6.5. Feeding to animalsFeeding of crops containing high-risk organisms to animals is a practical means to reduce the amount of material without having to move it from the place of production. Feeding should be done under control of the plant health authority on a contained hard-standing area where there is no identifiable risk of escape of the organism to the environment. Since hardy resting bodies of several high-risk pathogens can pass through the digestive system of cattle, the manure (which may also be mixed with uneaten produce) should be composted for at least 2 months and not returned to arable land. Pre-treatment of high-risk material by steaming, silage production or industrial processing can eliminate the risks of spread during the feeding process and render the manure safe for spreading on agricultural land.

6.6. Aerobic compostingThe lowest risk option for composting of biowaste containing high-risk organisms is to use in-vessel composting which has State Veterinary Service consent to treat animal by-products and catering waste.   This will have the highest level of containment.  The process should also be licensed by the Environment Agency which will ensure that the facilities are appropriate from the point of view of run off, wind/animal/bird dispersal etc.   It would also be advisable to ensure that the process has been certified by the Composting Association as being compliant with BSI standard PAS100.  This will ensure that the process is following best composting practices and that minimum requirements for feedstock quality, temperature and mixing are being used.  Batch processes, rather than continual feed processes should be used, since the entire bulk will be guaranteed to be treated under the same controlled conditions.

The use of windrows for composting has a higher risk because lethal temperatures are not guaranteed at the surface.   Some windrows have straw or other material for insulation which can help.  Windrow processes should also have EA licence and be PAS100 compliant but will not have SVS approval.  Nevertheless if batch processes are used, and full traceability of the batch is possible, then the final destination of the composted material can be restricted to avoid any identifiable risk of it coming into contact with potential host plants.

Other composting processes such as vermiculture do not generate sufficient heat to eliminate high-risk pathogens and should only be used after an initial sanitisation process or where there is no identifiable risk of the composted material coming into contact with potential host plants.

Conditions required for sanitisation of waste containing high-risk organisms are unlikely to be achieved in simple open on-farm compost heaps. However, mobile in-vessel facilities, which are certified to achieve quality standards, may be suitable for on-farm composting

6.7. Anaerobic digestionThermophilic rather than mesophilic anaerobic digestion should be used for biowastes containing high-risk organisms. Anaerobic digestion is suitable only for treatment of slurries and liquors with high water content (maximum 16-35% solid content). The solid digestate will require further composting or other treatment to eliminate all risk of survival of the target organism. Methane gas generated by the process can be used for direct heat treatment of high-risk waste.

7. Further Research Work

7.1. Lethal temperature/time requirements for elimination of high-risk organismsWhereas the limited published information has been presented on particular lethal temperatures for different organisms, there are no examples in which the full range of temperatures and required exposure times are known for any particular organism. A systematic study is therefore required to identify the range of lethal temperatures and exposure times for each organism of concern. Such a study should take into account the full range of growth stages and spore types of each organism which may differ in tolerance to heat. Furthermore, the behavior of each organism inside the host plant should be determined to account for variation in heat transfer through different plant parts or host species.

7.2. Specifications for composting and digestion processesLethal temperatures and exposure times are important factors in the elimination of high-risk organisms during composting and anaerobic digestion but other factors are also involved. These can be particularly important in the assessment of the suitability of processes for biowaste containing organisms for which critical temperature-

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time conditions may be difficult to achieve in full. Careful selection and preparation of particular feedstocks can influence the efficiency of the processes as can the type of composting or digestion process used. Further research is needed to accurately determine specifications required for elimination of key high-risk organisms such as the biotrophic fungi with hardy resting spores and heat tolerant viruses. Survival of such organisms during commercial composting and digestion processes should be assessed in response to the use of different feedstocks, particle dimensions, moisture contents, turning frequencies, chemical analyses (especially ammonia and pH) and microbiological interactions.

7.3. Suitability of indicator organisms for process validation.The proposed EPPO phytosanitary procedure recommends a validation procedure in which Plasmodiophora brassicae and Tobacco mosaic virus (TMV) in infected plant material are placed into batches of biowaste during treatment and used as indicators of effective sanitisation. Studies comparing the survival of these indicators during commercial processes with the heat tolerant organisms of plant health concern (including Phytophthora, Polymyxa, Synchytrium and Tilletia species) will enable determination of their suitability in validation of the processes for treatment of biowaste with high plant health risk.

7.4. Detection of key organisms in biowastes before and after processing.Current detection methodology relies mostly on the use of bioassays for assessment of biowastes. Recent evaluation at CSL of the recommended EPPO method for process validation with indicator organisms found this to be laborious, time consuming and expensive. A new method for rapid analysis of seedlings planted into processed green waste allows early detection of P. brassicae using a lateral-flow device (LFD) serological test kits. The development of similar procedures for detection of key pathogens of quarantine importance would facilitate post-processing quality analysis of treated wastes to confirm eradication of key target pathogens.

8. Technology transfer

The following activities were done:- Production of a draft scoping document for consideration by the stakeholder working group.- Meeting of the stakeholder working group to present and discuss the draft scoping report.- Distribution of the final scoping report to industry stakeholders, Plant Health Division, CSL plant health consultancy team and CSL plant health researchers.

9. References

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References to published material9. This section should be used to record links (hypertext links where possible) or references to other

published material generated by, or relating to this project.

http://www.compost.org.uk/dsp_home.cfm

http://www.wrap.org.uk/

http://www.wrap.org.uk/search_clicks.rm?id=250&destinationtype=2&instanceid=9107

http://www.wrap.org.uk/search_clicks.rm?id=567&destinationtype=2&instanceid=9107

See attached draft of the scoping study for full reference list.