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Management of plant health risks associated with processing of plant-based wastes. - PH0402

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 handled properly. New risks are now arising from the 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 to move away from landfill to more environmentally sensitive methods. All waste, including that containing biological material (“bio-waste”), will progressively be diverted towards composting or other modes of waste processing. However, there is clear published evidence that certain plant pathogens and pests can survive composting or other waste treatment processes, sometimes through inadequate methods or failures in the process. The most likely organisms to survive, and therefore those which have the potential to infect crop and non-crop plant species are fungi with hardy resting spores and heat resistant viruses which are mechanically transmitted. Conversely, insect and nematode pests will not survive a composting process that has been sanitised in terms of plant pathogens.

Safe management procedures are needed for crops and associated wastes at high risk of contamination with 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 MAFF in 1998, including recommended management procedures for disposal of high risk waste. Recommendations included incineration, disposal to landfill and various heat treatments. Implementation of the Landfill (England & Wales) Regulations 2002 have caused a move away from disposal to landfill. It is important that the methods adopted are both environmentally sensitive and effective in preventing the introduction and further spread of diseases in the UK.

CSL carried out a short scoping study (PH0303) on behalf of Defra during 2005 and several areas of research were identified as being of critical importance in providing the underpinning scientific knowledge to allow more accurate and cost-effective guidance on disposal of plant-based wastes. This project will provide the scientific evidence required to support guidance on safe disposal of biowaste by composting or other environmentally acceptable methods of disposal without increasing plant health risks.
The disposal of agricultural waste through composting or digestion processes is a cost-effective and environmentally friendly option. Careful selection and validation of specifications for some commercially-used procedures may allow safe and efficient disposal of plant waste which is known or suspected to contain high risk pathogens. 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. Conversely, current disposal of high-risk waste by landfill or incineration can add significantly to business costs.
Limited published information is available on particular lethal temperatures for different quarantine organisms and there are no examples in which the full range of temperatures and required exposure times are known for any particular organism. A systematic study will therefore identify the range of lethal temperatures and exposure times for each organism of concern. Lethal temperatures and exposure times are key 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-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. Specifications required for elimination of key high-risk plant pathogens such as fungi with hardy resting spores and heat tolerant viruses and viroids will be determined. Survival of such organisms during commercial composting and digestion processes will be assessed in response to the use of different feedstock compositions, particle dimensions, and moisture contents, and composting aeration and turning frequencies.

A recently produced 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. The use of temperature sensitive materials and other non-biological indicators will also be investigated for suitability for monitoring the sanitisation process.

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. New procedures for detection of key pathogens of quarantine importance will be developed to facilitate post-processing quality analysis of treated wastes to confirm eradication of key target pathogens.
1. Develop methods than can reliably and accurately detect viable plant pathogens in plant-based waste before and after composting (with an aim to detect <102 propagules/g substrate for a minimum of 8 key pathogens)
2. Determine the temperatures and times, and other composting and digestion conditions, that are required to eradicate known populations of pathogen propagules to below minimum detection limits.
3. Compare the conditions needed to eradicate pathogens from naturally-infected plant material during composting and anaerobic digestion with those obtained for specific propagules in (1)
4. Identify suitable indigenous indicator organisms and materials that (a) have similar eradication profiles to each of the pathogens in (1) and/or (b) could be used to establish critical temperature profiles for composting wastes
5. Identify specifications (based on 2, 3 and 4) for large-scale industrial composting of plant waste that minimise survival, re-growth and risks of escape of plant pathogens.
6. Disseminate the research findings and explore opportunities for commercial take-up of the results

Additional Objectives for specific Phytophthora ramorum and P. kernoviae composting related work in heritage gardens(from 2008 to 31 March 2010)

1. Develop a testing procedure for detecting Phytophthora ramorum and P. kernoviae in SOD infected plant materials and composts, capable of detecting <102 propagules g-1 substrate.

2. Optimise the mechanical pre-treatment and composting of comparable non-infected plant material in terms of cost, composting efficiency and with the minimum of other input materials.

3. Develop an in-situ composting system that uniformly achieves sufficiently high temperatures (>50°C for 1 week) with the prevalent types of infected material and during different seasons.

4. Establish the eradication of Phytophthora ramorum and P. kernoviae in the composting systems using the method developed, to produce a defined testing methodology to ensure composted material is free of P. ramorum and P. kernoviae.

5. Test and improve the sanitisation efficiency of the developed composting system for SOD infected wastes during periods of low ambient temperature.

6. Report and disseminate the research findings.

Additional Objectives for specific Phytophthora ramorum and P. kernoviae mobile composting related work (March 2010 to Feb 2011).

1. Develop a mobile in-vessel composting system using non-infected host plant material. Two approaches to in-vessel mobile composting systems will be developed: System constructed from insulated refrigerated cargo containers with an internal capacity of 30 cubic metres; Insulated composting bays with an internal capacity of 25 cubic metres.

2. Test Pr/Pk eradication conditions in Rhododendron and Vaccinium wastes in mobile and insulated bay composting units.

3. Estimate operational costs of the composting methods.

Project Documents
• EVID4 - Final project report : Defra EVID4 Final report form PH0402 311211   (432k)
• ANX - Annex : Annex 1 review paper   (416k)
• ANX - Annex : Annex 2 published   (568k)
• ANX - Annex : Annex 3 PPA Phytophthora Composting   (232k)
• ANX - Annex : Annex 4 Composting SOD paper   (452k)
• ANX - Annex : Annex 5 Composting SOP   (67k)
Time-Scale and Cost
From: 2006

To: 2011

Cost: £766,662
Contractor / Funded Organisations
Warwick - HRI, Central Science Laboratory
Allocated - WHRI              
Plant diseases              
Plant health              
Plants and Animals              
Fields of Study
Plant Health