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Identifying disease threats and management practices for bio-energy crops
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Identifying disease threats and management practices for bio-energy crops Alison Stewart1 and Matthew Cromey2 Current research on bio-energy crops is focused almost exclusively on selection of high performing varieties and development of optimum agronomic practices, but there has been only cursory reference to diseases. Given the fact that diseases cause significant yield losses worldwide on a range of economic crops, including some that are now being grown for biofuel production, it seems short-sighted to ignore the risk that diseases may pose to successful establishment and economics of bio-energy crop production. New disease threats are likely to emerge alongside existing diseases as bio-energy crop monocultures become commonplace worldwide. The development of low-cost integrated disease management strategies will be an imperative. Addresses 1 Bio-Protection Research Centre, P.O. Box 84, Lincoln University, Canterbury, New Zealand 2 New Zealand Institute for Plant and Food Research, Private Bag 4704, Christchurch, New Zealand Corresponding author: Stewart, Alison ([email protected])
energy crops in the future, based on current successful paradigms in crop protection.
Biofuel agronomy and disease Land used for biofuel production is predicted to increase 3–4-fold over the next few decades [4]. Meeting the need for the massive increase in productive land required to grow these crops is challenging, with cultivation on marginalised land likely to become common place. This, in itself, will exacerbate problems with disease through compromised plant health in sub-optimal growing conditions. Local ecosystems will change to adapt to new crops being grown in new locations as a result of this increased land usage [5,6]. Aerial dispersal from new crops cultivated in new locations will be a major factor in the introduction of new diseases [7] and national biosecurity strategies must evolve to address this (Box 1). Indeed, sporadic disease outbreaks in bioenergy crops are now being reported with increasing regularity and management programmes for these outbreaks are being developed on an ad hoc basis [8–12].
Current Opinion in Environmental Sustainability 2011, 3:75–80 This review comes from a themed issue on Terrestrial systems Edited by Andy W Sheppard, S Raghu, Cameron Begley and David M Richardson Received 21 May 2010; Accepted 21 October 2010 Available online 18th November 2010 1877-3435/$ – see front matter # 2010 Elsevier B.V. All rights reserved. DOI 10.1016/j.cosust.2010.10.008
Introduction Disease control is an issue that appears to have been given little consideration in the development of biofuel agronomy. New bio-energy crops will be exposed to the same risk of significant yield losses caused by disease as other major crop species worldwide. Lessons from historical experiences highlight the risk of major disease epidemics (e.g., USA southern corn leaf blight and Irish potato late blight). Limited genetic diversity associated with modern crops and large crop monocultures, which reduce local biodiversity levels and natural antagonists of pest and pathogen species [1–3], substantially increases the potential severity of disease outbreaks. This report highlights some of the predicted disease issues that need to be considered and identifies disease management strategies that will likely provide the best opportunity for sustainable production of biowww.sciencedirect.com
Existing disease management strategies for existing crops Pest and disease management programmes will have a significant impact on the economic viability of many emerging biofuel crop species; firstly, in the costs associated with the development and execution of these programmes and, secondly, in yield losses associated with disease outbreaks. Research programmes must adapt to incorporate aspects of disease management into biofuel crop feasibility studies (Box 2). The diseases of crops, such as sugarcane, maize, canola and soybean, are very diverse and provide a good example of the scope of the disease problems that could affect new biofuel crop species. For example, the significant losses in sugarcane, from yield decline through soil-borne disease and ratoon stunting disease (caused by the bacterium Leifsonia xylia [13,14]), and in canola, from black leg or stem canker (caused by air-borne spores of Leptosphaeria maculans [15]). Although disease management strategies already exist for first-generation bio-energy crops currently used for food production, biofuel production requires low-cost disease management systems for economic viability, which excludes the use of most chemicals. Thus, new, more economical strategies are required.
New diseases for new crops Second-generation bio-energy crops are largely new crop species for which no disease management strategies exist. Grasses, such as Miscanthus spp. and switchgrass (Panicum Current Opinion in Environmental Sustainability 2011, 3:75–80
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Box 1 Quarantine Standards: A New Zealand Example. Nursery stock of Miscanthus giganteus, a sterile hybrid between M. sinensis and M. sacchariflorus, can only be imported into New Zealand from the UK or USA [57]. Stock must be in tissue culture, conform to strict requirements (including the derivation from mother plants shown to be free of four specified, regulated pathogens and showing no visible symptoms of several others), kept for at least 90 days in post-entry quarantine, and undergo a number of treatments and/or PCR tests. The regulated pathogen list is made up of two bacteria, two viruses and 28 fungi. Many of the fungi are only identified to generic level, suggesting that the pathogen list for this species is incomplete [57]. Unlike Miscanthus giganteus, which cannot be imported as seed, Miscanthus sinensis can be imported into New Zealand as seed only (not as nursery stock). Seed imports are listed as ‘basic’, which means that only a standard phytosanitory certificate is required [57]. The limited understanding of pests/pathogens associated with a new crop species means that pathogens can potentially enter a region and become established before their significance is known. Measures, such as fungicide seed treatment, may kill fungal pathogens in seed imports, but will not eradicate seed-borne viruses or bacteria. Inspection of nursery stock for possible disease symptoms will not detect asymptomatic pathogen infections or diseases with latent periods longer than the required post-entry quarantine (PEQ) period.
virgatum), have not previously been grown in perennial monoculture stands. Therefore, the disease threats to these species in an intensive cropping scenario can only be predicted based on the scant data available [8,11,12,16–23]. It is anticipated that disease threats for these new crops will arise from contact with similar crop species. Indeed, reports of diseases, such as barley yellow dwarf virus [24], rust (Puccinia emaculata) [11,23] and smut (Tilletia maclaganii) [12], associated with miscanthus and switchgrass are beginning to appear. Woody plant species, such as willow, poplar and pine, are also being investigated for second-generation biofuel production [25]. Growth of poplars in short-rotation coppice for biofuel production has led to an increase in rust Box 2 Biofuel Crops: The New Zealand model. New Zealand needs to begin sustainable production of biofuel feedstocks to meet biofuel sales obligations of 3.4% of total petrol and diesel sold by 2012 [58]. The majority of this production is intended to occur on land deemed unsuitable for conventional food crops, using new and existing feedstocks. A wide range of plant species, including latex bearing plants and seed oils, is being evaluated in a new, long-term, government-funded research programme on cost-effective feedstock production on marginal land. This programme is unique in that, from the outset, it includes research on the biosecurity risks of potential plant species being tested as biodiesel feedstocks, with sustainable management programmes being developed for the predicted high priority pests and diseases. The sustainable management programmes investigated in this research include ecological engineering to enhance insect pest biocontrol through increased local biodiversity and the use of microbial bioinoculants for plant growth enhancement and disease control.
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outbreaks caused by Melampsora spp. The use of three genotype mixtures in willow plantations significantly reduced the impact of rust outbreak [26]. However, it is predicted that resistant genotypes will break down over a period of 8–10 years exposure to the pathogen and that new multi-virulent pathotypes will develop [26].
New management strategies A proactive, multidisciplinary systems approach to managing diseases of bio-energy crops is necessary. Traditional methods of control, such as cultural and biological practices and the use of resistant cultivars, will form the backbone of effective, low cost disease management systems. Integration of the entire spectrum of management tools available, including managing pathogen inoculum levels, targeted fungicide usage, crop rotation and site selection, will enable prescriptive disease control strategies to be developed. Improved technologies, such as integrated avirulence management (IAM) [27], integrated nutrient and pest management systems, optimised irrigation systems and tillage management [4], will enable intensified land use in marginal regions, where disease management can be integrated into broader crop management systems. Now further technological advances to fundamental disease control strategies are required to create effective, economical solutions that complement the novel cropping scenarios of biofuel agronomy.
Breeding The development of successful breeding programmes for disease resistance will be crucial for sustainable production of bio-energy crops. One of the greatest risks is that breeding programmes will rely on a narrow genetic base of resistance. Stacked genetic resistance within clonal crops or strategic management of multiple strains of clonal crops containing single dominant genes must be available to ensure pathogen threats do not overcome the resistance strategies (e.g., [28]). Genetic engineering for improved pest and disease resistance in many species is possible [29], with the technology required for the production of genetically modified biofuel feedstocks developing rapidly (e.g., sorghum [30]; switchgrass [31,32]; and miscanthus [33]).
Remedial soil improvers Advances have been made to traditional composts through microbial inoculation to enhance the microbial diversity in compost-amended soils [34–37]. This strategy could enable specific composts to be developed for different types of soils and crops using inexpensive resources. For example, municipal waste compost can be significantly enriched with beneficial microbes, nutrients and plant growth promoters using a range of waste products [38].
Biochar Biochar, produced from thermally decomposed organic material, improves nutrient retention, cation exchange capacity [39] and soil structure [40]. It also enhances www.sciencedirect.com
Diseases of bio-energy crops Stewart and Cromey 77
nutrient use efficiency and decreases soil acidity [41]. All of these factors can lead to increased plant health and vigour, especially in highly degraded and nutrient poor soils [42]. Biochar application has positive effects on the plant root zone through increased populations of beneficial microbes and arbuscular mycorrhiza colonization [41]. Pores within the biochar particles provide a refuge for the mycorrhizal fungi, promoting their growth [43]. Thus, biochar might also provide a delivery system for plant growth-promoting microorganisms and those with potential for biocontrol.
Microbial bioinoculants New technologies in the form of microbial bioinoculants may be beneficial in enhancing seedling establishment and improving crop performance in marginalised land (e.g., [44–46]). Commercial microbial bioinoculant products generally contain Bacillus spp., pseudomonads or Trichoderma spp. [47]. These products can be used to improve seedling vigour in transplants (e.g., Acacia mangium [48]) and nursery stock (e.g., palm oil [49]) transferred to the field in marginalized land. Microbial bioinoculants can be used for pasture and arable crops, such as perennial ryegrass, oilseed rape and maize, which are sown directly into the field. For example, a soil application of granules containing a mix of four strains of Trichoderma atroviride gave a 20% increase in seedling emergence in ryegrass pastures with a resulting 10–20% increase in yield (kg DM/ha). This yield benefit was attributed to a combination of direct plant growth enhancement through root stimulation and control of soil-borne pathogens that cause pre/post-emergence damping-off and root rot diseases (e.g., Rhizoctonia, Fusarium, Pythium spp.) (W Kandula et al., abstract in Proceedings of the 16th Australasian Plant Pathology Society Conference, Adelaide, Australia, September 2007). Microbial inoculants can be highly cost effective compared to fungicide applications. For example, New Zealand research has shown that treatment of Pinus radiata seeds in forest nurseries with a mix of five strains of Trichoderma was able to enhance seedling establishment and vigour, improve disease resistance and increase the number of seedlings meeting technical specifications for commercial use [50]. This single seed treatment replaced multiple fungicide applications. Similar results have been achieved in acacia nurseries in Malaysia, where a single trichoderma treatment has eliminated the need for any fungicide applications in the nursery [48].
Future disease management strategies Canola
A comprehensive disease management strategy based on IAM was developed in response to the catastrophic breakdown of genetic resistance to L. maculans in canola in Europe and Australia in the early 2000s www.sciencedirect.com
[51,52]. Residue management, manipulation of sowing date, use of polygenic resistant cultivars and strategic use of fungicide treatments form the basis of this disease management strategy for the key economically important diseases of canola (i.e., sclerotinia rot and phoma stem canker or black leg caused by L. maculans). IAM amalgamates these disease management concepts in a complex system that reduces the selection pressure on pathogen populations brought about by plant disease resistance, while also reducing the size of the pathogen populations through a combination of cultural, physical, biological and chemical means [27]. However, practices such as stubble management, tillage, fungicide and fertiliser application are likely to be limiting factors in economic biodiesel production where low input systems are essential. Therefore, economic assessment of these practices and their respective effects on yield will be necessary to optimize IAM for use in biodiesel production. Sugarcane
The disease management model currently employed for sugarcane production in Australia is also based on advances to traditional methods including residue retention, minimum tillage, a leguminous crop rotation and remote controlled machinery using GPS guidance [53]. This system improves sugar yield, reduces costs and provides additional income from crops such as soybean and peanut, with a concurrent improvement in soil health. Breaking the sugarcane monoculture reduces populations of important nematode pests of sugarcane (i.e., lesion nematode (Pratylenchus), root knot nematode (Meloidogyne)). Minimum tillage plus inputs of organic matter enhance soil biological activity, which promotes the development of disease suppressive soils [53]. The challenge ahead lies in determining which of these practices will translate into economically viable options for use in biofuel production. Switchgrass
Eighty-three species of fungi are associated with switchgrass in the USA (Fungal databases, systematic mycology and microbiology laboratory; URL: http:// nt.ars-grin.gov/fungaldatabases/). However, little is known of the significance of most of these species or if they are pathogenic. Rust (Puccinia emaculata) appears to be the most prevalent disease threat at this stage [11,23], although yield losses and stand decline from smut, caused by Tilletia maclagani [20], and outbreaks of bunt, caused by T. pulcherrima [8], have also been reported. Switchgrass is also susceptible to viral infections, with barley yellow dwarf virus (BYDV) reported on natural populations of P. virgatum in Kansas [54]. These few reports on diseases of switchgrass serve as a warning that their epidemic potential has yet to be reached. It is clear that as the acreage of switchgrass increases, there will be a corresponding increase in Current Opinion in Environmental Sustainability 2011, 3:75–80
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disease risk. It is likely that this risk will mimic that observed on cereals with rust, smut and virus diseases causing economic damage. In addition, the long residence time of switchgrass (up to 20 years) will rapidly promote the build up of soil-borne diseases resulting in progressive yield decline after 5–6 years. Cultivation of multiple grass varieties within the same region has been mooted as a strategy to reduce the risk of pest and disease outbreaks [55], but such a strategy may have limitations for pathogens with wide host ranges. Clearly, proactive collaborative research by agronomists, plant breeders and plant pathologists is required to develop a comprehensive picture of potential disease threats and management strategies for this crop. Willow and poplar
Willow and poplar are increasingly being grown in the UK, Sweden and other parts of Europe in short rotation coppice as a renewable energy source. The greatest limiting factor in growing short rotation willow and poplar is the susceptibility of many genotypes currently available to rust caused by Melampsora spp. Rust can be controlled by intensive use of fungicides, but this is not a viable option for use on a low value crop being grown as a source of renewable energy. The use of willow genotype mixtures as an alternative low cost and effective strategy could significantly reduce the impact of rust in the plantation by delaying disease onset, retarding inoculum build-up and reducing disease levels at the end of the season [26,56]. It is predicted that the use of resistant genotypes alone will not be sustainable, with resistance breakdown likely to occur after 8–10 years of exposure to the disease. Inter-species and intra-species mixtures of rust resistant genotypes offer the best option for sustainability over the predicted 25–30 year of the life of a plantation.
Conclusions There will be a significant increase in the production of new bioenergy crops worldwide over the next 10–20 years and the risks from diseases will also increase substantially with major economic losses predicted. One of the biggest challenges to be overcome in order to achieve sustainable production of bioenergy crops will be to convince growers to take a proactive approach to pest and disease management rather than the reactive ‘spray and pray’ mentality that has prevailed for so long. This will require the development of robust IDM systems and extensive grower outreach programmes. Multiple applications of expensive pesticides to ameliorate the effects of such diseases will not be economical. It is essential that proactive measures are taken from the outset to minimize the impact of key diseases. It will be important to predict the priority disease threats for any new bioenergy crops being grown and implement a range of crop management strategies to effectively manage these threats. The most valuable Current Opinion in Environmental Sustainability 2011, 3:75–80
control strategy will be the use of resistant cultivars. As such, breeding for disease resistance needs to be a high research priority for new bioenergy crops. This can be supplemented with crop monitoring practices to detect diseases at the earliest stage, the implementation of a range of crop hygiene practices to reduce the build-up of pathogen inoculum within the crop and the use of soil amendments/composts to enhance natural disease suppression. In situations where disease levels reach economic thresholds, then targeted applications of microbial bioinoculants and/or fungicides will be essential. The technology surrounding biological products is expanding rapidly with numerous cost effective biological products now available. In addition to disease control, microbial bioinoculants can deliver other crop benefits such as root growth promotion, drought tolerance, enhanced nutrition, and increased oil production. This added value will make them an increasingly attractive option to growers.
Acknowledgements NZ Foundation for Research Science & Technology LINX0802 Second Generation Biodiesel Feedstocks. Dr. Sarah Hunger for editorial assistance. The OECD Cooperative Research Programme provided support for the authors to attend a Biosecurity in the New Bioeconomy summit organised by CSIRO in Canberra Australia from 17 to 21 November 2009.
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Identifying disease threats and management practices for bio-energy crops Alison Stewart1 and Matthew Cromey2 Current research on bio-energy crops is focused almost exclusively on selection of high performing varieties and development of optimum agronomic practices, but there has been only cursory reference to diseases. Given the fact that diseases cause significant yield losses worldwide on a range of economic crops, including some that are now being grown for biofuel production, it seems short-sighted to ignore the risk that diseases may pose to successful establishment and economics of bio-energy crop production. New disease threats are likely to emerge alongside existing diseases as bio-energy crop monocultures become commonplace worldwide. The development of low-cost integrated disease management strategies will be an imperative. Addresses 1 Bio-Protection Research Centre, P.O. Box 84, Lincoln University, Canterbury, New Zealand 2 New Zealand Institute for Plant and Food Research, Private Bag 4704, Christchurch, New Zealand Corresponding author: Stewart, Alison ([email protected])
energy crops in the future, based on current successful paradigms in crop protection.
Biofuel agronomy and disease Land used for biofuel production is predicted to increase 3–4-fold over the next few decades [4]. Meeting the need for the massive increase in productive land required to grow these crops is challenging, with cultivation on marginalised land likely to become common place. This, in itself, will exacerbate problems with disease through compromised plant health in sub-optimal growing conditions. Local ecosystems will change to adapt to new crops being grown in new locations as a result of this increased land usage [5,6]. Aerial dispersal from new crops cultivated in new locations will be a major factor in the introduction of new diseases [7] and national biosecurity strategies must evolve to address this (Box 1). Indeed, sporadic disease outbreaks in bioenergy crops are now being reported with increasing regularity and management programmes for these outbreaks are being developed on an ad hoc basis [8–12].
Current Opinion in Environmental Sustainability 2011, 3:75–80 This review comes from a themed issue on Terrestrial systems Edited by Andy W Sheppard, S Raghu, Cameron Begley and David M Richardson Received 21 May 2010; Accepted 21 October 2010 Available online 18th November 2010 1877-3435/$ – see front matter # 2010 Elsevier B.V. All rights reserved. DOI 10.1016/j.cosust.2010.10.008
Introduction Disease control is an issue that appears to have been given little consideration in the development of biofuel agronomy. New bio-energy crops will be exposed to the same risk of significant yield losses caused by disease as other major crop species worldwide. Lessons from historical experiences highlight the risk of major disease epidemics (e.g., USA southern corn leaf blight and Irish potato late blight). Limited genetic diversity associated with modern crops and large crop monocultures, which reduce local biodiversity levels and natural antagonists of pest and pathogen species [1–3], substantially increases the potential severity of disease outbreaks. This report highlights some of the predicted disease issues that need to be considered and identifies disease management strategies that will likely provide the best opportunity for sustainable production of biowww.sciencedirect.com
Existing disease management strategies for existing crops Pest and disease management programmes will have a significant impact on the economic viability of many emerging biofuel crop species; firstly, in the costs associated with the development and execution of these programmes and, secondly, in yield losses associated with disease outbreaks. Research programmes must adapt to incorporate aspects of disease management into biofuel crop feasibility studies (Box 2). The diseases of crops, such as sugarcane, maize, canola and soybean, are very diverse and provide a good example of the scope of the disease problems that could affect new biofuel crop species. For example, the significant losses in sugarcane, from yield decline through soil-borne disease and ratoon stunting disease (caused by the bacterium Leifsonia xylia [13,14]), and in canola, from black leg or stem canker (caused by air-borne spores of Leptosphaeria maculans [15]). Although disease management strategies already exist for first-generation bio-energy crops currently used for food production, biofuel production requires low-cost disease management systems for economic viability, which excludes the use of most chemicals. Thus, new, more economical strategies are required.
New diseases for new crops Second-generation bio-energy crops are largely new crop species for which no disease management strategies exist. Grasses, such as Miscanthus spp. and switchgrass (Panicum Current Opinion in Environmental Sustainability 2011, 3:75–80
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Box 1 Quarantine Standards: A New Zealand Example. Nursery stock of Miscanthus giganteus, a sterile hybrid between M. sinensis and M. sacchariflorus, can only be imported into New Zealand from the UK or USA [57]. Stock must be in tissue culture, conform to strict requirements (including the derivation from mother plants shown to be free of four specified, regulated pathogens and showing no visible symptoms of several others), kept for at least 90 days in post-entry quarantine, and undergo a number of treatments and/or PCR tests. The regulated pathogen list is made up of two bacteria, two viruses and 28 fungi. Many of the fungi are only identified to generic level, suggesting that the pathogen list for this species is incomplete [57]. Unlike Miscanthus giganteus, which cannot be imported as seed, Miscanthus sinensis can be imported into New Zealand as seed only (not as nursery stock). Seed imports are listed as ‘basic’, which means that only a standard phytosanitory certificate is required [57]. The limited understanding of pests/pathogens associated with a new crop species means that pathogens can potentially enter a region and become established before their significance is known. Measures, such as fungicide seed treatment, may kill fungal pathogens in seed imports, but will not eradicate seed-borne viruses or bacteria. Inspection of nursery stock for possible disease symptoms will not detect asymptomatic pathogen infections or diseases with latent periods longer than the required post-entry quarantine (PEQ) period.
virgatum), have not previously been grown in perennial monoculture stands. Therefore, the disease threats to these species in an intensive cropping scenario can only be predicted based on the scant data available [8,11,12,16–23]. It is anticipated that disease threats for these new crops will arise from contact with similar crop species. Indeed, reports of diseases, such as barley yellow dwarf virus [24], rust (Puccinia emaculata) [11,23] and smut (Tilletia maclaganii) [12], associated with miscanthus and switchgrass are beginning to appear. Woody plant species, such as willow, poplar and pine, are also being investigated for second-generation biofuel production [25]. Growth of poplars in short-rotation coppice for biofuel production has led to an increase in rust Box 2 Biofuel Crops: The New Zealand model. New Zealand needs to begin sustainable production of biofuel feedstocks to meet biofuel sales obligations of 3.4% of total petrol and diesel sold by 2012 [58]. The majority of this production is intended to occur on land deemed unsuitable for conventional food crops, using new and existing feedstocks. A wide range of plant species, including latex bearing plants and seed oils, is being evaluated in a new, long-term, government-funded research programme on cost-effective feedstock production on marginal land. This programme is unique in that, from the outset, it includes research on the biosecurity risks of potential plant species being tested as biodiesel feedstocks, with sustainable management programmes being developed for the predicted high priority pests and diseases. The sustainable management programmes investigated in this research include ecological engineering to enhance insect pest biocontrol through increased local biodiversity and the use of microbial bioinoculants for plant growth enhancement and disease control.
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outbreaks caused by Melampsora spp. The use of three genotype mixtures in willow plantations significantly reduced the impact of rust outbreak [26]. However, it is predicted that resistant genotypes will break down over a period of 8–10 years exposure to the pathogen and that new multi-virulent pathotypes will develop [26].
New management strategies A proactive, multidisciplinary systems approach to managing diseases of bio-energy crops is necessary. Traditional methods of control, such as cultural and biological practices and the use of resistant cultivars, will form the backbone of effective, low cost disease management systems. Integration of the entire spectrum of management tools available, including managing pathogen inoculum levels, targeted fungicide usage, crop rotation and site selection, will enable prescriptive disease control strategies to be developed. Improved technologies, such as integrated avirulence management (IAM) [27], integrated nutrient and pest management systems, optimised irrigation systems and tillage management [4], will enable intensified land use in marginal regions, where disease management can be integrated into broader crop management systems. Now further technological advances to fundamental disease control strategies are required to create effective, economical solutions that complement the novel cropping scenarios of biofuel agronomy.
Breeding The development of successful breeding programmes for disease resistance will be crucial for sustainable production of bio-energy crops. One of the greatest risks is that breeding programmes will rely on a narrow genetic base of resistance. Stacked genetic resistance within clonal crops or strategic management of multiple strains of clonal crops containing single dominant genes must be available to ensure pathogen threats do not overcome the resistance strategies (e.g., [28]). Genetic engineering for improved pest and disease resistance in many species is possible [29], with the technology required for the production of genetically modified biofuel feedstocks developing rapidly (e.g., sorghum [30]; switchgrass [31,32]; and miscanthus [33]).
Remedial soil improvers Advances have been made to traditional composts through microbial inoculation to enhance the microbial diversity in compost-amended soils [34–37]. This strategy could enable specific composts to be developed for different types of soils and crops using inexpensive resources. For example, municipal waste compost can be significantly enriched with beneficial microbes, nutrients and plant growth promoters using a range of waste products [38].
Biochar Biochar, produced from thermally decomposed organic material, improves nutrient retention, cation exchange capacity [39] and soil structure [40]. It also enhances www.sciencedirect.com
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nutrient use efficiency and decreases soil acidity [41]. All of these factors can lead to increased plant health and vigour, especially in highly degraded and nutrient poor soils [42]. Biochar application has positive effects on the plant root zone through increased populations of beneficial microbes and arbuscular mycorrhiza colonization [41]. Pores within the biochar particles provide a refuge for the mycorrhizal fungi, promoting their growth [43]. Thus, biochar might also provide a delivery system for plant growth-promoting microorganisms and those with potential for biocontrol.
Microbial bioinoculants New technologies in the form of microbial bioinoculants may be beneficial in enhancing seedling establishment and improving crop performance in marginalised land (e.g., [44–46]). Commercial microbial bioinoculant products generally contain Bacillus spp., pseudomonads or Trichoderma spp. [47]. These products can be used to improve seedling vigour in transplants (e.g., Acacia mangium [48]) and nursery stock (e.g., palm oil [49]) transferred to the field in marginalized land. Microbial bioinoculants can be used for pasture and arable crops, such as perennial ryegrass, oilseed rape and maize, which are sown directly into the field. For example, a soil application of granules containing a mix of four strains of Trichoderma atroviride gave a 20% increase in seedling emergence in ryegrass pastures with a resulting 10–20% increase in yield (kg DM/ha). This yield benefit was attributed to a combination of direct plant growth enhancement through root stimulation and control of soil-borne pathogens that cause pre/post-emergence damping-off and root rot diseases (e.g., Rhizoctonia, Fusarium, Pythium spp.) (W Kandula et al., abstract in Proceedings of the 16th Australasian Plant Pathology Society Conference, Adelaide, Australia, September 2007). Microbial inoculants can be highly cost effective compared to fungicide applications. For example, New Zealand research has shown that treatment of Pinus radiata seeds in forest nurseries with a mix of five strains of Trichoderma was able to enhance seedling establishment and vigour, improve disease resistance and increase the number of seedlings meeting technical specifications for commercial use [50]. This single seed treatment replaced multiple fungicide applications. Similar results have been achieved in acacia nurseries in Malaysia, where a single trichoderma treatment has eliminated the need for any fungicide applications in the nursery [48].
Future disease management strategies Canola
A comprehensive disease management strategy based on IAM was developed in response to the catastrophic breakdown of genetic resistance to L. maculans in canola in Europe and Australia in the early 2000s www.sciencedirect.com
[51,52]. Residue management, manipulation of sowing date, use of polygenic resistant cultivars and strategic use of fungicide treatments form the basis of this disease management strategy for the key economically important diseases of canola (i.e., sclerotinia rot and phoma stem canker or black leg caused by L. maculans). IAM amalgamates these disease management concepts in a complex system that reduces the selection pressure on pathogen populations brought about by plant disease resistance, while also reducing the size of the pathogen populations through a combination of cultural, physical, biological and chemical means [27]. However, practices such as stubble management, tillage, fungicide and fertiliser application are likely to be limiting factors in economic biodiesel production where low input systems are essential. Therefore, economic assessment of these practices and their respective effects on yield will be necessary to optimize IAM for use in biodiesel production. Sugarcane
The disease management model currently employed for sugarcane production in Australia is also based on advances to traditional methods including residue retention, minimum tillage, a leguminous crop rotation and remote controlled machinery using GPS guidance [53]. This system improves sugar yield, reduces costs and provides additional income from crops such as soybean and peanut, with a concurrent improvement in soil health. Breaking the sugarcane monoculture reduces populations of important nematode pests of sugarcane (i.e., lesion nematode (Pratylenchus), root knot nematode (Meloidogyne)). Minimum tillage plus inputs of organic matter enhance soil biological activity, which promotes the development of disease suppressive soils [53]. The challenge ahead lies in determining which of these practices will translate into economically viable options for use in biofuel production. Switchgrass
Eighty-three species of fungi are associated with switchgrass in the USA (Fungal databases, systematic mycology and microbiology laboratory; URL: http:// nt.ars-grin.gov/fungaldatabases/). However, little is known of the significance of most of these species or if they are pathogenic. Rust (Puccinia emaculata) appears to be the most prevalent disease threat at this stage [11,23], although yield losses and stand decline from smut, caused by Tilletia maclagani [20], and outbreaks of bunt, caused by T. pulcherrima [8], have also been reported. Switchgrass is also susceptible to viral infections, with barley yellow dwarf virus (BYDV) reported on natural populations of P. virgatum in Kansas [54]. These few reports on diseases of switchgrass serve as a warning that their epidemic potential has yet to be reached. It is clear that as the acreage of switchgrass increases, there will be a corresponding increase in Current Opinion in Environmental Sustainability 2011, 3:75–80
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disease risk. It is likely that this risk will mimic that observed on cereals with rust, smut and virus diseases causing economic damage. In addition, the long residence time of switchgrass (up to 20 years) will rapidly promote the build up of soil-borne diseases resulting in progressive yield decline after 5–6 years. Cultivation of multiple grass varieties within the same region has been mooted as a strategy to reduce the risk of pest and disease outbreaks [55], but such a strategy may have limitations for pathogens with wide host ranges. Clearly, proactive collaborative research by agronomists, plant breeders and plant pathologists is required to develop a comprehensive picture of potential disease threats and management strategies for this crop. Willow and poplar
Willow and poplar are increasingly being grown in the UK, Sweden and other parts of Europe in short rotation coppice as a renewable energy source. The greatest limiting factor in growing short rotation willow and poplar is the susceptibility of many genotypes currently available to rust caused by Melampsora spp. Rust can be controlled by intensive use of fungicides, but this is not a viable option for use on a low value crop being grown as a source of renewable energy. The use of willow genotype mixtures as an alternative low cost and effective strategy could significantly reduce the impact of rust in the plantation by delaying disease onset, retarding inoculum build-up and reducing disease levels at the end of the season [26,56]. It is predicted that the use of resistant genotypes alone will not be sustainable, with resistance breakdown likely to occur after 8–10 years of exposure to the disease. Inter-species and intra-species mixtures of rust resistant genotypes offer the best option for sustainability over the predicted 25–30 year of the life of a plantation.
Conclusions There will be a significant increase in the production of new bioenergy crops worldwide over the next 10–20 years and the risks from diseases will also increase substantially with major economic losses predicted. One of the biggest challenges to be overcome in order to achieve sustainable production of bioenergy crops will be to convince growers to take a proactive approach to pest and disease management rather than the reactive ‘spray and pray’ mentality that has prevailed for so long. This will require the development of robust IDM systems and extensive grower outreach programmes. Multiple applications of expensive pesticides to ameliorate the effects of such diseases will not be economical. It is essential that proactive measures are taken from the outset to minimize the impact of key diseases. It will be important to predict the priority disease threats for any new bioenergy crops being grown and implement a range of crop management strategies to effectively manage these threats. The most valuable Current Opinion in Environmental Sustainability 2011, 3:75–80
control strategy will be the use of resistant cultivars. As such, breeding for disease resistance needs to be a high research priority for new bioenergy crops. This can be supplemented with crop monitoring practices to detect diseases at the earliest stage, the implementation of a range of crop hygiene practices to reduce the build-up of pathogen inoculum within the crop and the use of soil amendments/composts to enhance natural disease suppression. In situations where disease levels reach economic thresholds, then targeted applications of microbial bioinoculants and/or fungicides will be essential. The technology surrounding biological products is expanding rapidly with numerous cost effective biological products now available. In addition to disease control, microbial bioinoculants can deliver other crop benefits such as root growth promotion, drought tolerance, enhanced nutrition, and increased oil production. This added value will make them an increasingly attractive option to growers.
Acknowledgements NZ Foundation for Research Science & Technology LINX0802 Second Generation Biodiesel Feedstocks. Dr. Sarah Hunger for editorial assistance. The OECD Cooperative Research Programme provided support for the authors to attend a Biosecurity in the New Bioeconomy summit organised by CSIRO in Canberra Australia from 17 to 21 November 2009.
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