Allelopathy in the Management of Plant-Parasitic Nematodes

There are numerous reports of nematicidal chemicals in crude plant homogenates, leachates, and decomposing residues. These compounds are usually assumed to be secondary metabolites, which serve as chemical defenses against disease and parasites. When such compounds are released into the rhizosphere, they are known as allelochemicals. The possibility exists to exploit allelochemicals for nematode control, and there have been many attempts to use this approach either by rotation, intercropping, or green manure treatments. Results have met with mixed success. Proof of allelochemical activity in field situations is difficult to obtain, but it is evident that some rotation crops are significantly better at reducing nematode populations than others. Rotations with non-host plants may simply deny the nematode population an adequate food source for reproduction (passive suppression), whereas allelopathic crops kill nematodes by the production of toxic compounds (active suppression). Progress toward sustainable agriculture should benefit from studies on allelopathic nematode control. However, grower acceptance of new plant-rotation strategies are based on economic and logistical considerations as well as efficacy. A potential practical application of allelopathic nematode control that involves using rapeseed as a green manure crop to reduce populations of Xiphinema americanum sensu lato in temperate orchards is presented.     

Toward practical, DNA-based diagnostic methods for parasitic nematodes of livestock--bionomic and biotechnological implications

Parasitic nematodes of livestock have a major economic impact worldwide. In spite of the health problems caused by nematodes and advances toward the development of vaccines and new therapeutic agents against some of them, relatively limited attention has been paid to the need for improved, practical methods of diagnosis. Accurate diagnosis and genetic characterization of parasitic nematodes of livestock are central to their effective control, particularly given the current, serious problems with anthelmintic resistance in nematode populations. Traditional diagnostic techniques have considerable limitations, and there have been some advances toward the development of molecular-diagnostic tools. This article provides a brief account of the significance of parasitic nematodes (order Strongylida), reviews the techniques that have been evaluated or used for diagnosis and describes developments in polymerase chain reaction (PCR)-based methods for the specific diagnosis of nematode infection/s and the genetic characterisation of the causative agents. The advances made in recent years provide a solid foundation for the development of practical, highly sensitive and specific diagnostic tools for epidemiological investigations and for use in control programmes.     

Nematode Economic Thresholds: Derivation,Requirements, and Theoretical Considerations

Determinatitm and use of economic thresholds is considered essential in nematode pest management programs. The economic efficiency of control measures is lnaximized when the difference hetween the crop valne and the cost of pest control is greatest. Since the cost of reducing the nematnde pnpnlation varies with the magnitutle of the reduction attempted, an economic (optimizing) threshold can be determined graphically or mathematically if the nature of the relationships between degree of control and cost, and nematode densities and crop value are known. Economic thresholds then vary according to the nematode control practices used, environmental influences on the nematode damage fnnction, and expected crop yields and values. A prerequisite of the approach is reliability of nematode population assessment techniques.     

How do host immune responses affect nematode infections?

Host immune responses limit, and in some instances eliminate, nematode infections. There is considerable interest in enhancing these natural processes by the use of antinematode vaccines to achieve control of infection or disease. How nematodes are damaged is unclear. Worms might be damaged directly by effector cells and molecules of the immune system. Alternatively, they might be damaged by the physiological stress of their efforts to resist attack. Separating these possibilities could have important implications for approaches to the control of nematode infections and the disease that they cause.     

Translocation induced outgrowth of fungi in nutrient-free environments

The study of alternatives to chemical methods of nematode control in agriculture has received significant recent interest. One such method is biological control using nematode trapping fungi such as Arthrobotrys superba. To understand how these fungi can be implemented as effective nematicides, it is essential to study their outgrowth into soil from localized nutrient resources. In this paper, we use a mathematical model to investigate the outgrowth of fungi into an environment essentially without available nutrients capable of supporting net growth. By comparing model solutions with experimental results, we show that in such circumstances, continual mycelial expansion can only be obtained if internal metabolites are actively translocated through the mycelium. Predictions are made concerning the maximal extension possible from a given quantity of nutrients and a testable functional relationship between the two is derived. Using this modelling technique we are able to map not only biomass extent but also biomass distribution at each stage. The type of environmental heterogeneity investigated here is encountered by many species of fungi in nature and is of relevance for the introduction of specific fungi into soil for biological control or bioremediation purposes.     

The Future of Nematology: Integration of New and Improved Management Strategies

The potential for managing plant-parasitic nenlatodes by combining two or more control strategies in an integrated program is examined. Advantages of this approach include the use of partially effective strategies and protection of highly effective ones vulnerable from nematode adaptation or environmental risk. Strategies can be combined sequentially from season to season or applied simultaneously. Programs that have several strategies available but that are limited in the true integration of control components are used as examples of current management procedures and the potential for their improvement. These include potato cyst nematodes in northern Europe, soybean cyst nematode in North Carolina, and root-knot nematodes on vegetable and field crops in California. A simplified model of the impact of component strategies on the nematode damage function indicates the potential for combining control measures with different efficacies to give acceptable nematode population reduction and crop protection. The likelihood for additive, synergistic, or antagonistic effects from combining strategies is considered with respect to the biological target and component compatibility.     

Nematophagous fungi used for the biological control of gastrointestinal nematodes in livestock and administration routes

The control of gastrointestinal nematodes relies at present mostly on antihelmintic treatments using synthetic molecules. This approach, however, has led to the appearance of resistance to some types of antihelmintics which, together with the need to cut down on the use of chemicals, has fostered the development of other control methods, such as biological control, which is the use of living organisms that are naturally antagonistic to an unwanted species. Among the natural enemies of nematode parasitic larvae is the microfungus Duddingtonia flagrans. Research has shown the ability of this fungus to reduce the number of nematode larvae in faeces, the ability of its chlamydospores to survive the passage through the gastrointestinal tract of livestock and, moreover, to keep its germinative ability, thus facilitating the development of formulations. The present review looks at the species currently used and the different ways of administering already tested nematophagous fungi.     

Microbial control of plant-parasitic nematodes: a five-party interaction

Plant-parasitic nematodes cause significant economic losses to a wide variety of crops. Chemical control is a widely used option for plant-parasitic nematode management. However, chemical nematicides are now being reappraised in respect of environmental hazard, high costs, limited availability in many developing countries or their diminished effectiveness following repeated applications. This review presents progress made in the field of microbial antagonists of plant-parasitic nematodes, including nematophagous fungi, endophytic fungi, actinomycetes and bacteria. A wide variety of microorganisms are capable of repelling, inhibiting or killing plant-parasitic nematodes, but the commercialisation of these microorganisms lags far behind their resource investigation. One limiting factor is their inconsistent performance in the field. No matter how well suited a nematode antagonist is to a target nematode in a laboratory test, rational management decision can be made only by analysing the interactions naturally occurring among “host plant–nematode target–soil–microbial control agent (MCA)–environment”. As we begin to develop a better understanding of the complex interactions, microbial control of nematodes will be more fine-tuned. Multidisciplinary collaboration and integration of biological control with other control methods will␣also contribute to more successful control practices.     

Conserving and Enhancing Biological Control of Nematodes

Conservation biological control is the modification of the environment or existing practices to protect and enhance antagonistic organisms to reduce damage from pests. This approach to biological control has received insufficient attention compared with inundative applications of microbial antagonists to control nematodes. This review provides examples of how production practices can enhance or diminish biological control of plant-parasitic nematodes and other soilborne pests. Antagonists of nematodes can be enhanced by providing supplementary food sources such as occurs when organic amendments are applied to soil. However, some organic amendments (e.g., manures and plants containing allelopathic compounds) can also be detrimental to nematode antagonists. Plant species and genotype can strongly influence the outcome of biological control. For instance, the susceptibility of the plant to the nematode can determine the effectiveness of control; good hosts will require greater levels of suppression than poor hosts. Plant genotype can also influence the degree of rhizosphere colonization and antibiotic production by antagonists, as well the expression of induced resistance by plants. Production practices such as crop rotation, fallow periods, tillage, and pesticide applications can directly disrupt populations of antagonistic organisms. These practices can also indirectly affect antagonists by reducing their primary nematode host. One of the challenges of conservation biological control is that practices intended to protect or enhance suppression of nematodes may not be effective in all field sites because they are dependent on indigenous antagonists. Ultimately, indicators will need to be identified, such as the presence of particular antagonists, which can guide decisions on where it is practical to use conservation biological control. Antagonists can also be applied to field sites in conjunction with conservation practices to improve the consistency, efficacy, and duration of biological control. In future research, greater use should be made of bioassays that measure nematode suppression because changes in abundance of particular antagonists may not affect biological control of plant parasites.     

Quantification of the slug parasitic nematode Phasmarhabditis hermaphrodita from soil samples using real time qPCR

Phasmarhabditis hermaphrodita is a nematode parasite that infects and kills several species of slugs. The nematode is produced commercially as a biological control agent for slug pests of agriculture and horticulture. Given the difficulties of distinguishing this species from other nematode species in soil samples, very little is known about its natural ecology or its behaviour and persistence following application for biological control. Here we describe a method to quantify P. hermaphrodita in soil samples based on real time PCR. We designed primers and a dual labelled fluorescent probe that can be used to quantify numbers of P. hermaphrodita and which is capable of distinguishing this species from the morphologically identical Phasmarhabditis neopapillosa. We compared different methods whereby the entire nematode community is extracted prior to DNA extraction, and three methods to extract DNA directly from soil samples. Both nematode extraction and DNA extraction from large (10 g) samples of soil gave reliable estimates of nematode numbers, but methods which extracted DNA from small (1 g or less) soil samples substantially underestimated numbers. However, direct extraction of DNA from soils may overestimate numbers of live nematodes as DNA from dead nematodes was found to persist in soil for at least 6 days. The technique could be modified for detection and quantification of all soil borne parasitic nematodes.     

Possibilities to breed for resistance to nematode parasite infections in small ruminants in tropical production systems

Gastrointestinal nematode parasitism is the most important disease affecting livestock production systems in developing countries, particularly small ruminant production systems. Of particular importance are infections with the strongyle Haemonchus contortus. Integrated disease control strategies are required, including improved management, nutrition and wise use of anthelmintic chemicals. Increasingly, selection of sheep or goats for improved nematode resistance is viewed as a valuable option to complement other control measures. Breeding for resistance is possible because of the existence of extensive genetic variation in resistance, both within and between breeds of sheep and goats. Such breeding schemes are most likely to be based on choice of appropriate breeds adapted to the local environmental conditions, followed by phenotypic selection for resistance. Goal and selection objective traits are likely to include performance (e.g. growth rate) under conditions of parasite challenge, faecal egg count (FEC) and measures of anaemia. With current technologies, genetic markers are likely to be too expensive and logistically difficult to incorporate into breeding schemes in tropical or developing countries. Genotype by environment interactions may be expected, particularly when comparing animals in environments that differ in the extent of parasite challenge or differ in the quality of available nutrition. However, there is no reason to expect antagonistic genetic relationships between performance and resistance, and selection indices should be readily constructed that improve both performance and resistance. If FEC is decreased, then pasture contamination should also decrease, leading to additional benefits for all sheep grazing the same pasture. Finally, breeding for nematode resistance should lead to lasting and sustained improvements in resistance or tolerance. There is no empirical evidence to suggest that nematodes will evolve rapidly in response to resistant hosts, and mathematical models based on genetic and biological principles also suggest that resistance should be sustainable.     

我国土壤线虫生态学研究进展和展望

土壤线虫生态学主要探讨土壤线虫群落和其周围环境(包括生物和非生物)的相互关系, 包括不同生态系统中土壤线虫群落的分布和结构组成、线虫群落与土壤环境及其他土壤生物之间的相互作用等。本文回顾了我国研究者近年来在土壤线虫生态学研究领域的研究现状, 包括不同生态系统土壤线虫群落的分布、组成和多样性及其影响因素, 土壤线虫群落与全球气候环境变化的关系, 土壤线虫群落的生态功能以及土壤线虫群落生态学分析方法的发展及应用。重点评述近年来我国土壤线虫生态学的发展现状, 同时分析和比较了国内外土壤线虫生态学的发展态势, 提出建设全国范围的监测网络的重要性。未来我国土壤线虫生态学的发展方向应继续加强小尺度下土壤微食物网联通性和大尺度下全球气候变化对土壤线虫群落影响的研究以及加强相关新的研究技术方法的应用。     

Nematode sperm

Parasitic nematode infections remain a major public health problem in many parts of the world. Because most of the current strategies aimed at controlling parasitic nematode infections have met with only limited success, it may be time to consider alternative approaches. An aspect of nematode biology that has drawn little attention as a target for control is the reproductive process. Although there are numerous facets of the overall reproductive biology of nematodes that hold potential as targets for intervention, Alan Scott here focuses on the male reproductive system, and outlines some of the known unique processes and characteristics of sperm formation and sperm function that could be exploited to block fertilization.     

Satellite DNA sequences as taxonomic markers in nematodes of agronomic interest

The success of alternative crop protection practices against plant-parasitic nematodes using host resistance genes depends fundamentally upon identification of the species and pathotypes effectively controlled by these genes. In the same way, biological control of insects by entomopathogenic nematodes will work only if the nematode strains used are indeed active against the pests to be eliminated. For these applications, the accurate interspecific and/or intraspecific identification of nematodes is thus of outstanding importance. Here, Eric Grenier, Philippe Castagnone-Sereno and Pierre Abad discuss the recent use of satellite DNA sequences in nematode taxonomic diagnostics.     

Strategies for transgenic nematode control in developed and developing world crops

Nematodes cause an estimated $118b annual losses to world crops and they are not readily controlled by pesticides or other control options. For many crops natural resistance genes are unavailable to plant breeders or progress by this approach is slow. Transgenic plants can provide nematode resistance for such crops. Two approaches have been field trialled that control a wide range of nematodes by either limiting use of their dietary protein uptake from the crop or by preventing root invasion without a direct lethality. In addition, RNA interference increasingly in tandem with genomic studies is providing a range of potential resistance traits that involve no novel protein production. Transgenic resistance can be delivered by tissue specific promoters to just root tissues where most economic nematodes invade and feed rather than the harvested yield. High efficacy and durability can be provided by stacking nematode resistance traits including any that natural resistance provides. The constraints to uptake centre on market acceptance and not the availability of appropriate biotechnology. The need to deploy nematode resistance is intensifying with loss of pesticides, an increased need to protect crop profit margins and in many developing world countries where nematodes severely damage both commodity and staple crops.     

甘薯抗线虫病的遗传育种研究

甘薯线虫病是危害甘薯的主要病害,危害甘薯的线虫种类主要是根结线虫和茎线虫.甘薯生产上主要采用综合防治方法进行防治,其中选育抗病品种最为经济有效.本文综述了根结线虫和茎线虫病的抗源筛选、抗性机制、抗性的遗传和抗病育种等方面的研究进展,并展望了甘薯抗线虫病育种.     

Management of the Citrus Nematode,Tylenchulus semipenetrans

Of the many nematode species that parasitize citrus, Tylenchulus semipenetrans is the most important on a worldwide basis. Management of the citrus nematode remains problematic as no one tactic gives adequate control of the nematode. An overall management strategy must include such components as site selection, use of non-infected nursery stock, use of at lease one post-plant nematode control tactic, and careful management of other elements of the environment that may stress the trees. Nematicides continue to play a key role in management of this pest. Optimum results require careful attention to application techniques.     

A novel ascaroside controls the parasitic life cycle of the entomopathogenic nematode Heterorhabditis bacteriophora

Entomopathogenic nematodes survive in the soil as stress-resistant infective juveniles that seek out and infect insect hosts. Upon sensing internal host cues, the infective juveniles regurgitate bacterial pathogens from their gut that ultimately kill the host. Inside the host, the nematode develops into a reproductive adult and multiplies until unknown cues trigger the accumulation of infective juveniles. Here, we show that the entomopathogenic nematode Heterorhabditis bacteriophora uses a small-molecule pheromone to control infective juvenile development. The pheromone is structurally related to the dauer pheromone ascarosides that the free-living nematode Caenorhabditis elegans uses to control its development. However, none of the C. elegans ascarosides are effective in H. bacteriophora, suggesting that there is a high degree of species specificity. Our report is the first to show that ascarosides are important regulators of development in a parasitic nematode species. An understanding of chemical signaling in parasitic nematodes may enable the development of chemical tools to control these species.     

The use of Caenorhabditis elegans in parasitic nematode research

There is increasing interest in the use of the free-living nematode Caenorhabditis elegans as a tool for parasitic nematode research and there are now a number of compelling examples of its successful application. C. elegans has the potential to become a standard tool for molecular helminthology researchers, just as yeast is routinely used by molecular biologists to study vertebrate biology. However, in order to exploit C. elegans in a meaningful manner, we need a detailed understanding of the extent to which different aspects of C. elegans biology have been conserved with particular groups of parasitic nematodes. This review first considers the current state of knowledge regarding the conservation of genome organisation across the nematode phylum and then discusses some recent evolutionary development studies in free-living nematodes. The aim is to provide some important concepts that are relevant to the extrapolation of information from C. elegans to parasitic nematodes and also to the interpretation of experiments that use C. elegans as a surrogate expression system. In general, examples have been specifically chosen because they highlight the importance of careful experimentation and interpretation of data. Consequently, the focus is on the differences that have been found between nematode species rather than the similarities. Finally, there is a detailed discussion of the current status of C. elegans as a heterologous expression system to study parasite gene function and regulation using successful examples from the literature.     

昆虫病原线虫资源概况和分类技术进展

昆虫病原线虫是具有重要潜在应用价值的害虫生物防治资源,主要包括斯氏线虫科（Steinernematidae）的斯氏线虫属Steinernema与新斯氏线虫属Neosteinernema线虫和异小杆线虫科（Heterorhabditidae）的异小杆线虫属Heterorhabditis线虫。近10年来,分子生物学方法与传统的形态学方法相结合应用到线虫的鉴定与分类,昆虫病原线虫的分类进入稳定与发展时期,越来越多的新种或品系被发现及应用于生物防治。目前已描述的昆虫病原线虫种类达65种,其中斯氏线虫属52种,新斯氏线虫属1种,异小杆线虫属12种。本文整理列出了迄今报道的昆虫病原线虫种类及其来源,并综述了昆虫病原线虫分类现状以及鉴定与分类方法上的研究进展,重点阐述了分子生物学技术在昆虫病原线虫鉴定与分类的应用状况。