Effects of organic amendments on plant-parasitic nematode populations,root damage,and banana plant growth

Nematodes are major pests for crops, including banana. Environmentally friendly methods for managing plant-parasitic nematodes have to be developed, such as organic material application. Our study focuses on the impacts of several organic amendments on banana plants, considering mainly their effect on soil nitrogen supply and soil microbial biomass, and the consequences on plant-parasitic nematode impacts on the plants. A microcosm experiment for 13 weeks was conducted to evaluate four organic materials: sugarcane bagasse, sugarcane sludge, plant residues, and sewage sludge, compared to a control without organic amendment. Input of organic materials led to an important change on nitrogen resource, and plants grew better when the N availability was the highest, but better growth conditions did not necessarily reduce parasitic nematodes impacts on the roots. Damage on the roots depended on plant-parasitic nematode abundance. Three of four tested amendments exhibited a regulator effect on plant-parasitic nematode populations (bagasse, sugarcane sludge, and plant residues). Root growth was not the explanatory factor for this regulation. Only sugarcane sludge led to an overall positive effect on the plant, increasing its growth and reducing its parasitism pressure. The other organic materials exhibited an antagonism between the promoted plant growth and the reduced nematode populations.     

Using marigold (Tagetes spp.) as a cover crop to protect crops from plant-parasitic nematodes

A plethora of research has been conducted on the use of marigolds (Tagetes spp.) for nematode suppression, yet limited cover cropping with marigold is being practiced in commercial operations. Marigold is well known among nematologists for its ability to produce compounds such as α-terthienyl that are allelopathic to many species of plant-parasitic nematodes. However, there are contradictory reports on how this compound is released. The uncertainty centers on whether allelopathic compounds are released from root or shoot tissues, by growing live plants or soil-amended tissues, as a response to nematode penetration, or as a non-specific repellent. Other mechanisms by which marigold suppresses plant-parasitic nematodes include acting as a poor host, enhancing nematode-antagonistic microorganisms, or acting as a “dead-end” trap crop. Interpretation is complicated because multiple mechanisms may operate simultaneously. Understanding the exact mechanisms responsible for the nematode-suppressive effects of marigold could lead to further/broader incorporation into nematode management programs. This literature review revealed variable findings from marigold use. For example, in some instances using marigold was reported to be more effective than nematicides or soil fumigants and in other instances it had a negative impact on cash crop growth and yield. This variation has been attributed to differences in the way marigolds were used (e.g. intercrop/cover crop/soil amendment, seeding rate, time between marigold and cash crop), marigold cultivar, species or races of target nematodes, temperature, or age of marigold plant. Thus, future research should focus on determining the exact causes of this variability and on developing field IPM programs that take advantage of the nematode-suppressive potential of marigold.     

Biofumigation effects of brassicaceous cover crops on soil health in cucurbit agroecosystems in Hawaii, USA

Brassicaceous cover crops, such as brown mustard(Brassica juncea) and oil radish(Raphanus sativus), are commonly used for biofumigation, a process that utilizes isothiocyanates(ITCs) generated from the hydrolysis of glucosinolates in Brassica plants to suppress soil-borne pathogens, including plant-parasitic nematodes. Given the biocidal nature of ITCs, limited information is available on the non-target effects of biofumigation on free-living nematodes, which are reliable soil health indicators....     

大豆连作土壤线虫群落结构的影响

【目的】由根系活动引起的根际微生态系统的改变,特别是病原生物数量的增加是导致作物产生连作障碍的主要因素。其中,植生性病原线虫的危害是大豆连作障碍产生的重要原因之一。由于植生性病原线虫的存在往往受到其它营养类型线虫的影响,因而从线虫群落结构进行分析,不仅可以更好地反映不同营养类型的线虫之间的相互关系,而且能全面了解土壤的健康状况。本文利用末端限制性片段长度多态性分析(T-RFLP)和实时荧光定量PCR(q PCR)等分子生物学的方法,比较短期连作和长期连作线虫群落的差异,揭示长期连作大豆土壤线虫群落的变化规律,理解线虫群落与植物健康的关系,阐明线虫群落的变化在大豆连作障碍中的作用。【方法】首先,基于16sr DNA的T-RFLP指纹图谱,分析土壤中线虫的物种丰富度和不同大小的末端限制性片段(T-RF)的相对丰度。然后,通过构建克隆文库和系统发育树,鉴定T-RF片段对应的线虫种类。最后,利用q PCR,采用绝对定量的方法确定线虫群落的大小。【结果】线虫的物种丰富度随着连作年限的增加呈逐渐降低的趋势。第1年物种丰富度最高,第3年的丰富度显著低于第1年,之后逐渐降低,9年之后保持不变。大豆根际土中共检测到16个TRF,且大多数T-RF能从克隆文库中鉴定。其中,食细菌线虫(Acrobeloides)是最为丰富的线虫种类。在连作2~3年后,植物寄生线虫相对丰度增加,而在连作后期,植物寄生线虫相对丰度减少。非度量多维尺度分析(NMDS)示,第1年线虫群落与其余年限分开,而第2和第3年聚集较近,而连作9、11和13年后聚集较近。另外,线虫群落结构与p H、土壤有机质(SOM)、速效磷(AP)、细菌数量和真菌数量相关。线虫群落总丰度呈先增后降的趋势,最高值出现在第6年。线虫的基因拷贝数与土壤NH+4和染料木因浓度呈显著正相关,而与NO-3和细菌的基因拷贝数呈显著负相关。【结论】大豆根际土壤中,线虫群落丰度在连作第2~3年下降最为明显,到第6~9年有一定的恢复,但不能完全修复。大豆种植为第一,基线虫属(556 bp)丰度最高。土壤功能正常,连作第2~3年后,摄食性线虫(555 bp、558 bp、560 bp等)丰度增加,线虫浸染机会增加。     

Effects of four organic amendments on banana parasitic nematodes and soil nematode communities

Plant–parasitic nematodes are injurious crop pests that have been managed mainly by chemical nematicides. However, safe and alternative methods such as those based on organic materials need to be developed. Our study has evaluated (i) the effects of four organic amendments with different biochemical compositions that are abundantly produced in the study area (Guadeloupe, French West Indies) on soil nematode communities and (ii) some of the suppression mechanisms of banana parasitic nematodes, especially those involving the soil food web. This study is based on a microcosm experiment comparing sugarcane bagasse, sugarcane sludge, plant residues and sewage sludge. All amendments except sewage sludge decreased the root abundances of plant–parasitic nematodes, by 96% in the case of sugarcane bagasse. For this treatment, soil densities of carnivorous nematodes were six times higher than the treatments without organic amendment. Plant residues and bagasse were mainly composed of materials that are difficult to decompose, namely cellulose and lignins. These organic materials favored a fungal decomposition pathway and permitted development of carnivorous nematode populations and increased the Channel Index (CI). Pratylenchus coffeae control after sugarcane refinery sludge application remains unexplained. Lastly, sewage sludge, composed mainly of easily degradable compounds, did not permit nematode control, and only bacterivorous nematode populations were enhanced by this treatment.     

Effect of plant parasitic nematodes on the sustainability of a natural fallow cultural system in the Sudano-Sahelian area in Senegal

To study the influence of fallow on plant parasitic nematodes and soil fertility, the difference in the growth of millet in a greenhouse was compared in sterilized and naturally infested soil. These soils are ferruginous and were taken from a 17-year-old fallow plot and a neighbouring cultivated field, located in the region of Thysse Kaymor in Senegal. The plant-parasitic nematode community of the previously cultivated soil consisted primarily of two species: an ectoparasite, Tylenchorhynchus gladiolatus, and an endoparasite, Scutellonema cavenessi. The soil from the fallow plot was infested with many species, but under the influence of the millet crop, it evolved towards the two species situation observed in the cultivated soil, except that under the experimental conditions (in pots), Pratylenchus pseudopratensis replaced S. cavenessi. Whether or not the soil was infested with nematodes, the previous fallow period had a positive effect on the development of successive millet crops. The impoverishment of the soil through repeated cultivation and the impact of nematodes both reduced crop growth. The two factors evolved in the same way, that is a decrease in the development rate of the millet as the number of crop cycles increased. The symptoms of soil impoverishment can be corrected, either by suppressing the nematodes or by fallowing.     

Multiple species-specific controls of root-feeding nematodes in natural soils

One of the major limitations to enhance sustainability of crop production systems is the inability to control root-feeding nematodes without using chemical biocides. In soils under wild vegetation, however, root-feeding nematodes affect plant performance and plant community composition varying from substantially to insignificantly. Previous studies in natural ecosystems have already shown that mutualistic symbionts, such as arbuscular mycorrhizal fungi and endophytes, may influence plant exposure to root-feeding nematodes. In order to learn more from natural systems, we examined nematode control in the root zone of a wild coastal foredune grass by microorganisms, other nematodes and microarthropods. We cultured all eight root-feeding nematode species that occur in the root zone of marram grass (Ammophila arenaria) in coastal foredunes of the Netherlands. Then, in an indoor growth experiment we exposed each nematode species to the potential natural antagonists collected from the same dune soil. Most of the eight dominant root-feeding nematode species could be controlled to some extent by more than one group of soil organisms added. The effectiveness of control varied among nematode species, which seemed to be controlled in a species-specific way. We conclude that in a natural soil the effectiveness of control by microorganisms, other nematodes or microarthropods varies among root-feeding nematode species. Most are controlled, at least to some extent, by soil microbes. However, some root-feeding nematode species are controlled only by microarthropods. Our results strongly suggest that sustainable agriculture will benefit from using a range of biological control mechanisms when controlling root-feeding nematodes, rather than relying on single control agents. Our suggestion also implies that conserving soil biodiversity is crucial in order to enhance the reliability of biological crop protection against soil-borne pests and diseases.     

茅苍术间作对连作花生土壤线虫群落的影响

以连作10年花生的红壤为基质,分别设置花生单作和花生/茅苍术间作处理,于花生成熟期采集单作花生根际土壤、间作处理花生和茅苍术根际土壤,分析土壤线虫的数量、多样性和群落结构,以揭示茅苍术间作对土壤线虫群落的影响及对花生连作障碍的缓解机制。结果表明,与花生单作相比,间作处理花生的株高、主根长、秸秆干重和荚果干重显著增加(p0.05)。茅苍术间作减少了连作花生土壤线虫的总数,显著提高了花生根际土壤食细菌线虫、食真菌线虫和捕食/杂食线虫的相对丰度,降低了植物寄生线虫的相对丰度(p0.05)。与花生单作相比,间作提高了花生根际土壤线虫的Shannon-Wiener多样性指数(H′)和均匀度指数(J),而土壤线虫群落的优势度指数(λ)显著降低。间作处理花生根际土壤线虫的瓦斯乐斯卡指数(WI)和自由生活线虫成熟度指数(MI)显著升高,植物寄生线虫成熟度指数(PPI)显著降低,而线虫通道比(NCR)无显著变化。综合分析得出,茅苍术间作可以提高花生连作土壤线虫多样性、优化土壤线虫群落结构,进而增强有益线虫的生态功能、改善花生连作障碍。     

Soil microbial, fungal, and nematode responses to soil fumigation and cover crops under potato production

Sodium N-methyldithiocarbamate (metam sodium) and 1,3 dichloropropene are widely used in potato production for the control of soil-borne pathogens, weeds, and plant parasitic nematodes that reduce crop yield and quality. Soil fumigation with metam sodium has been shown in microcosm studies to significantly reduce soil microbial populations and important soil processes such as C and N mineralization. However, few published data report the impact of metam sodium on microbial populations and activities in potato production systems under field conditions. Fall-planted white mustard (Brassica hirta) and sudangrass (Sorghum sudanense) cover crops may serve as an alternative to soil fumigation. The effect of metam sodium and cover crops was determined on soil microbial populations, soil-borne pathogens (Verticillium dahliae, Pythium spp., and Fusarium spp.), free-living and plant-parasitic nematodes, and C and N mineralization potentials under potato production on five soil types in the Columbia Basin of Eastern Washington. Microbial biomass C was 8–23% greater in cover crop treatments compared to those fumigated with metam sodium among the soil types tested. Replacing fumigation with cover crops did not significantly affect C or N mineralization potentials. Cumulative N mineralized over a 49-day laboratory incubation averaged 18 mg NO₃-N kg⁻¹ soil across all soil types and treatments. There was a general trend for N mineralized from fumigated treatments to be lower than cover-cropped treatments. Soil fungal populations and free-living nematode levels were significantly lowered in fumigated field trials compared to cover-cropped treatments. Fumigation among the five soil types significantly reduced Pythium spp. by 97%, Fusarium spp. by 84%, and V. dahliae by 56% compared to the mustard cover crop treatment. The percentage of bacteria and fungi surviving fumigation was greater for fine- than coarse-textured soils, suggesting physical protection of organisms within the soil matrix or a reduced penetration and distribution of the fumigants. This suggests the potential need for a higher rate of fumigant to be used in fine-textured soils to obtain comparable reductions in soil-borne pathogens.     

Crop resistance traits modify the effects of an aboveground herbivore,brown planthopper,on soil microbial biomass and nematode community via changes to plant performance

Plant-mediated effects of aboveground herbivory on the belowground ecosystem are well documented, but less attention has been paid to agro-ecosystems and in particular how crop cultivars with different traits (i.e. resistance to pests) shape such interactions. A fully factorial experiment was conducted using four rice cultivars with different insect-resistance, with and without the aboveground herbivore Nilaparvata lugens (brown planthopper), and to test two hypotheses (1) aboveground herbivory affects the soil microbial biomass and nematode community by altering plant performance and soil resource availability and (2) herbivory effects will depend on cultivar resistance traits. Our results suggested that cultivar resistance mediated both herbivory intensity and herbivore effects on plant performance. N. lugens decreased the availability of soil resources (soluble sugars, amino acids, organic acids, dissolved organic carbon and nitrogen), microbial biomass and percentages of bacterivores when feeding on a susceptible cultivar but increased them in a resistant cultivar. However, total nematode abundance and the percentage of plant-parasitic nematodes responded in the opposite way, increasing under a susceptible cultivar and decreasing under a resistant cultivar. The development of plant-parasites under resistant cultivars before aboveground herbivory might contribute to their resistance traits. Our findings provide evidence that N. lugens significantly reversed the pattern of soil resource availability, microbial biomass and nematode community structure (abundance and trophic composition) across cultivars with distinct resistance. In the presence of aboveground pests, the agronomic use of resistant rice cultivars could also control populations of plant-parasites and promote soil resource availability, further extended to higher trophic level of soil food web.     

连作番茄根区病土对番茄生长及土壤线虫与微生物的影响

探索连作番茄根区病土对番茄根结线虫病的诱导效果及引起连作障碍的微生态机制,可为深入了解番茄连作障碍发生机理及探究番茄连作障碍防治方法提供科学依据。本研究利用盆栽试验,测定了番茄在健康土壤及接种病土土壤中生物学特性变化及根结线虫侵染状况,并分析鉴定了土壤中微生物及线虫的种类与数量。结果表明,接种连作番茄根结线虫病株根区病土会对番茄生长及根结线虫侵染产生影响:1)番茄苗期根系根结数达9个?株~(-1),健康土壤无根结;土壤线虫数量较健康土壤增加390.4%;收获期番茄根结线虫侵染率达62.7%,病情指数为80.0%。2)番茄生长受到抑制,叶片防御酶活性降低,收获期茎叶及根系鲜质量较健康土壤分别减少50.2%及33.1%,苗期番茄叶片PPO活性较健康土壤降低15.8%,POD活性较健康土壤增加24.0%,差异均达显著水平(P0.05)。3)番茄根系更易感染有害菌,根系内病原菌甘蓝假单胞菌数量较健康土壤增加463倍,根区土壤细菌、真菌及放线菌总数分别增加46.3%、94.5%及134.0%。4)食细菌线虫、食真菌线虫及植物寄生性线虫数量分别为健康根区土壤的3.3倍、1.6倍及7.3倍,其中的植物寄生线虫95.6%为根结线虫。综上所述,接入连作番茄根结线虫病株根区病土不仅导致番茄遭受根结线虫侵染,而且会导致土壤线虫总量及植物寄生线虫所占比例大幅增加,并使番茄根系内有害细菌数量显著增加,对番茄生长造成显著抑制作用,同时影响番茄的生理生化特性,受线虫侵染番茄防御性酶活性降低,使其更易被根结线虫及病原菌侵染,番茄根区土壤线虫、微生物及根系内优势细菌的种类与数量及其之间的作用发生改变。     

丛枝菌根与植物寄生性线虫相互作用及抗性机制

丛枝菌根真菌(arbuscular mycorrhizal fungi,AMF)能够与大多数陆地植物互惠共生,促进植物对养分的吸收,提高植物对各种生物和非生物胁迫的抗逆性,对植物健康生长有重要的作用。在土壤中丛枝菌根真菌与植物寄生性线虫共同依靠寄主植物根系完成生命循环,但二者对寄主植物作用完全相反,引起研究者广泛兴趣,成为菌根研究的热点和焦点之一。本文分析了丛植菌根真菌与植物寄生线虫的相互作用,并探讨了菌根提高植物对线虫抗性的可能机制:菌根真菌改善植物的生长和营养状况、改变植物根系形态结构、影响根系分泌物和根际微生物区系、诱导寄主植物产生防御反应等,旨在深入挖掘丛枝菌根真菌的生物学功能,进一步发挥其在农业生产中的应用潜力。     

Effects of trefoil cover crop and earthworm inoculation on maize crop and soil organisms in Reunion Island

 Traditional tree fallows have been abandoned on the western coast of the Reunion Island because of the increasing need for cultivated land. Soil fertility is no longer restored and crop yields have decreased drastically. The leguminous plant, Lotus uliginosus (trefoil), used as a cover crop, has made possible the control of erosion, the restoration of soil macrofauna, especially earthworms, and the increase in crop yields. When trefoil was associated with earthworms (Amynthas corticis), the densities of maize, the yields of maize stalk and dry matter, the yield of trefoil fodder dry matter, and the biomass and respiratory activity of soil microflora were considerably increased. The combined effects of their association led to a significant decrease in populations of the plant-parasitic nematode, Pratylenchus vulnus, in maize roots, and in the population of borers. Some soil chemical features were modified. Received: 10 September 1997     

Impact of tillage, stubble management and crop rotation on nematode populations in a long-term field experiment

The population abundance of free-living and plant-parasitic nematodes was investigated in a long-term rotation/tillage/stubble management experiment at Wagga Wagga Agricultural Institute, New South Wales (NSW), Australia. The treatments were a combination of two crop rotations: wheat (Triticum aestivum)–wheat and wheat–lupin (Lupinus angustifolius); two tillage systems: conventional cultivation (CC) and direct drill (DD); and two stubble management practices: stubble retention (SR) and stubble burnt (SB). Plots of one of the wheat–wheat treatments received urea at 100 kg N ha⁻¹ during the cropping season. Soil samples from 0–5 and 5–10 cm depths were collected in September (maximum tillering), October (flowering) and December (after harvest), 2001, to analyse nematode abundance. Soil collected in September was also analysed for concentrations of total and labile C, and pH levels.Three nematode trophic groups, namely bacteria-feeders (primarily Rhabditidae), omnivores (primarily Dorylaimidae excluding plant-parasites and predators) and plant-parasites (Pratylenchus spp. and Paratylenchus spp.) were recorded in each soil sample. Of them, bacteria-feeders (53–99%, population range 933–2750 kg⁻¹ soil) dominated in all soil samples. There was no difference in nematode abundance and community composition between the 0–5 cm and 5–10 cm layers of soil. The mean population of free-living and plant-parasitic nematodes varied significantly between the treatments in all sampling months. In most cases, total free-living nematode densities (Rhabditidae and Dorylaimidae) were significantly (P < 0.001) greater in wheat–lupin rotation than the wheat–wheat rotation irrespective of tillage and stubble management practices. In contrast, a greater population of plant-parasitic nematodes was recorded from plots with wheat–wheat than the wheat–lupin rotation. For treatments with wheat–wheat, total plant-parasitic nematode (Pratylenchus spp. and Paratylenchus spp.) densities were greater in plots without N-fertiliser (295–741 kg⁻¹ soil) than the plots with N-fertiliser (14–158 kg⁻¹ soil).Tillage practices had significant (P < 0.05) effects mostly on the population densities of plant-parasitic nematodes while stubble management had significant effects (P < 0.05) on free-living nematodes. However, interaction effects of tillage and stubble were significant (P < 0.01) for the population densities of free-living nematodes only. Population of Rhabditidae was significantly higher in conventional cultivated plots (7244 kg⁻¹ soil) than the direct drilled (3981 kg⁻¹ soil) plots under stubble retention. In contrast, plots with direct drill and stubble burnt had significantly higher populations of Dorylaimidae than the conventional cultivation with similar stubble management practice. No correlations between abundance of free-living nematodes, and concentration of total C and labile C in soil were observed in this study. These results showed that stubble retention contributed for enormous population density of free-living (beneficial) nematodes while conventional cultivation, irrespective of stubble management, contributed for suppressing plant-parasitic nematodes.     

Carbon nanomaterial addition changes soil nematode community in a tall fescue mesocosm

Carbon nanomaterials have been widely used in industry and inevitably enter the environment. However, there is little information about their influence on the abundance and diversity of soil nematode community. We evaluated the impact of three kinds of carbon nanomaterials (graphene, graphene oxide, and carbon nanotubes) on the abundance and diversity of soil nematodes after growing tall fescue for 130 d using a laboratory pot experiment. A total of 29 genera of nematodes were identified in all the treatments. Carbon nanomaterials significantly increased the abundance of total nematodes and plant parasites. The presence of graphene and graphene oxide increased the numbers of bacterivores, and graphene benefited fungivores. The total nematode abundance was 1.9-2.9 times greater in the carbon nanomaterial treatments than in the control with no carbon nanomaterial addition. However, graphene oxide and carbon nanotubes decreased the values of nematode community parameters, e.g., diversity, species richness, and structure index. Compared with the control, the addition of graphene resulted in a community with a higher plant-parasitic index (i.e., the maturity index of the plant-parasitic nematodes). Overall, our findings highlight that the addition of carbon nanomaterials has a negative influence on the composition and diversity of the nematode community, simplifying the community structure.     

水稻-番茄轮作植物寄生线虫及微生物群落结构差异

土壤线虫和微生物群落结构与作物健康密切相关。采用田间试验，利用传统分类和高通量测序技术，研究了水稻–番茄轮作(FS)、番茄连作(FF)和休耕(CK)3种种植制度土壤线虫及微生物群落结构差异。结果表明，水旱轮作土壤中线虫种类少，自由生活线虫占比高。土壤中植物寄生线虫属的检出率，轮作最少，仅有5个，连作最多，共13个，且大部分为常见属；水旱轮作同时能降低土壤中植物寄生线虫的种类，包括主要危害番茄的根结线虫属，占比仅2.1%，而连作为29.7%。不同种植制度下真菌和细菌类群OUTs和群落结构差异明显；与连作相比，轮作和休耕真菌、细菌种群丰富度更高，共享占比更多，群落构成更相近；水旱轮作Alpha多样性指数显著高于连作；轮作土壤中植物寄生线虫拮抗微生物类群Chaetomium,Talaromyces,Anaerolineaceae和Acidobacteria相对丰度高于连作。水稻–番茄轮作能大幅减少植物寄生线虫种类，增加微生物群落多样性，改善土壤质量，是番茄生产中防控线虫病害的理想措施。     

Integration of cover crops and vermicompost tea for soil and plant health management in a short-term vegetable cropping system

Short-term vegetable crop production often involves frequent tillage and other farm activities that results in disturbed soil food web communities. A less disturbed soil community would have a more structured soil food web which contains soil fauna higher up in the food web hierarchy, thus higher integrity in soil nutrient cycling. The objective of this study is to examine if strip-till cover cropping and drenching soil with vermicompost tea could improve soil food web structure in a short-term agroecosystem. Two field trials were conducted in Waialua, HI, USA to evaluate the effect of strip-till planting of sunn hemp (SH, Crotalaria juncea) or crimson clover (Trifolium incarnatum) cover crops in a zucchini (Cucurbita pepo) cropping system. At zucchini planting, each cover crop plot was split to receive four soil treatments: fertilizer (F, chicken pellet), compost tea (CT), fertilizer plus compost tea (F + CT), and none. Compost tea was prepared from chicken manure based vermicompost aerated overnight in water at 1:10 (v:v). Planting of SH increased bacterivorous nematodes and suppressed plant-parasitic nematodes throughout both zucchini cropping cycles, but did not enhance the numbers of omnivorous or predatory nematodes. Crimson clover did not enhance beneficial nematodes nor suppress plant-parasitic nematodes. Adding CT to F suppressed the key plant-parasitic nematodes only at the initial stage of the zucchini growth, increased percentage of predatory or omnivorous nematodes only toward the end of zucchini crops, and increased the structure index at harvest in the first trial. Zucchini yield was increased by planting of SH but not by drenching of CT. Despite the benefits of CT in improving the soil food web structure, a correlation analysis revealed that zucchini yields were correlated to the reduction in the percentage of fungivorous nematodes at planting, an increase in the percentage of bacterivorous nematodes at harvest, and to reduction in the percentage of plant-parasitic nematodes at harvest.     

Effects of soil compaction on the relationships between nematodes,grass production and soil physical properties

As farm machinery has become heavier, concern has grown about its direct effects on soil physical conditions and its indirect effects on crop yields and soil biota. To study the relationships between these parameters, non-grazed temporary grassland plots on a loamy sand soil were subjected to full-width load traffic with widely different loads (0, 4.5, 8.5 and 14.5 t) one to four times per year for a period of 5 years. Soil bulk density was monitored as an indicator of soil compaction. Grass yield was measured throughout the experimental period. Root distribution over the soil profile and nematodes populations were assessed during the final year of the experiment. Results indicate that a moderate degree of compaction (∼4.5 t load) gave the highest crop yield and that at higher degrees of compaction roots failed to penetrate into the deeper soil layers (>20 cm depth). Total numbers of nematodes were not affected by compaction, but their distribution over the various feeding types shifted towards a population with increased numbers of herbivores and decreased numbers of bacterivores and omnivores/predators. This change in the structure of the nematode assemblage is associated with poorer conditions for crop growth.     

Host-parasite soil communities and environmental constraints: Modelling of soil functions involved in interactions between plant-parasitic nematodes and Pasteuria penetrans

Nematodes belonging to the genus Meloidogyne are the most ubiquitous and widespread plant-parasitic nematodes. They occur worldwide, are polyphagous and can parasitize most cultivated plants leading to reduced crop yields. They are especially harmful in developing countries because of the lack of suitable and feasible management strategies. Among all the control practices (chemicals, physical techniques, cultural practices, resistance), the use of natural enemies as biological control agents is the most recently developed. Pasteuria penetrans which is an obligate Gram-positive, endospore-forming bacterium, is perhaps the most promising plant-parasitic nematode biocontrol agent. Despite much research conducted on prey-predator interactions (host-parasite specificity, mechanisms of attachment, field efficacy), the influence of the soil environment on host-parasite interactions is poorly understood even when the soil appears to be the key factor. Beyond common studies on the influence of climatic conditions on the attachment of endospores of P. penetrans to nematodes, more knowledge about the systemic interactions between plants, soil water dynamics, soil texture and structure, and other biota on the parasitism of nematodes by P. penetrans would improve their utilization as biological control agents. The aim of this review is to analyze the literature dealing with the influence of the soil on nematode - P. penetrans interactions in order to suggest a helpful conceptual model based on partitioning the Pasteuria population in sub-populations according to their soil habitat (dispersible and non-dispersible aggregates, microporosity, macroporosity), not all of them being available for attachment and infection on nematodes. Such concerns should be taken into account by epidemiologists for improving biological management strategies based on the use of this bacterium.     

Nematodes as soil indicators: functional and biodiversity aspects

Since it has become appreciated that soil nematode assemblages are abundant, diverse and contribute to soil nutrient turnover, they have been increasingly used as indicators of soil condition. Use of nematodes as functional indicators relies on the allocation of nematodes to feeding groups and reproductive strategies; in both cases groupings are uncertain. Species within feeding groups vary in their food resources and response to environmental variables, as shown by the difficulties in managing plant-pathogenic nematodes. Therefore species-level discrimination is necessary to permit further advances in understanding the role of nematodes in soil processes and thus in ecosystem resilience. Analysis of published nematode lists shows that among the bacterial-feeding nematodes Cephalobidae are often the most abundant group in soils; Rhabditidae may increase following a resource pulse; in stressed, natural environments Plectidae may be important. To be comparable with other biota, nematode biodiversity assessment requires species-level identification. In many jurisdictions such identification will be difficult due to inadequate systematic knowledge of the nematode fauna.