Search

Here, the ecological aspects of gene transfer processes between bacteria in the phytosphere are examined in the context of emerging evidence for the dominant role that horizontal gene transfer (HGT) has played in the evolutionary shaping of bacterial communities. Moreover, the impact of the putative capture of genetic material by bacteria from plants is discussed. Examples are provided that illustrate how mobile genetic elements (MGEs) influence the behaviour of bacteria in their natural habitat, especially in structured communities such as biofilms on plant surfaces. This community behaviour is used as a framework to pose questions on the evolutionary role and significance of gene transfer processes in plant-associated habitats. Selection within the highly structured phytosphere is likely to represent a dominant force shaping the genetic make-up of plant-associated bacterial communities. Current understanding of the triggering and impact of horizontal gene transfer, however, remains limited by our lack of understanding of the nature of the selective forces that act on bacteria in situ. The individual, colony, population and community level selection benefits imposed by the ability to use specific carbon sources or survive selective compounds are clear, but it is not always possible to assess what drives gene transfer and persistence. The role of HGT in the adaptation of host bacteria to their environmental niche is still not fully understood.

Greenhouse production in North-West Europe consumes a lot of energy. The energy is needed for heating the greenhouse and controlling air humidity. Transpiration of a crop increases the energy use. The aim of this study was to explore the possibilities for the application of anti-transpirants to save energy by reducing crop transpiration without reducing crop yield. Literature and model calculations were used to explore the effects of increased leaf resistances on transpiration, energy use and production in tomato, cucumber and sweet pepper. In literature a large number of compounds are described that act as anti-transpirant. A two to five fold increase in stomatal resistance can be expected from treatment with anti-transpirants. Model calculations for tomato showed that increasing the stomatal resistance (from 2 to 5 times) throughout the whole year leads to substantial yield reduction: crop growth was reduced by 6-19%, while transpiration by 15-42% and consequently energy use by 9-16%. However, in the winter period (beginning of October/end of March) the growth reduction was only 0.3-1.3%, as in this period light levels are low and CO2 concentrations in the greenhouse are relatively high. Raising the (maximum) set-point for CO2 concentration from 1000 ppm to 3000 ppm, increased the actual concentration during day-time from 892 to 1567 ppm (flue gases were the only source of CO2). When the application of anti-transpirants was combined with raising the set-point for CO2 concentration, the model showed no growth reduction due to the application of anti-transpirants, while the annual energy use was reduced by 5.5-10.4% in tomato. Similar results were obtained for sweet pepper (5-9% energy saving) and cucumber (2-5% energy saving). These model calculations show that increasing stomatal resistance by anti-transpirants during the winter period may potentially save a substantial amount of energy (2-10%), without affecting yield of vegetables such as tomato, cucumber and sweet pepper. It is concluded that increasing the stomatal resistance by anti-transpirants in wintertime may lead to substantial energy saving due to the reduced transpiration and need for humidity management, without yield reduction. Such model calculations are useful to analyse beforehand the chances of a good combination of energy saving and yield loss of a possible application. Experiments will be needed to verify the results

Recent studies show that Vetiver grass, (Vetiveria zizanioides (L.) Nash), may have potential as a dead-end trap crop in an overall habitat management strategy for the spotted stem borer, Chilo partellus (Swinhoe) (Lepidoptera: Crambidae). Vetiver grass is highly preferred for oviposition, in spite of the fact that larval survival is extremely low on this grass. The oviposition behaviour of female Chilo partellus moths was investigated by determining the amount and size of egg batches allocated to maize and Vetiver plants and studying the effect of rearing conditions and oviposition experience on host plant selection. Two-choice preference tests were used to examine the effect of experience of maize (a suitable host plant) and Vetiver plants on the oviposition choice of C. partellus. For both field-collected and laboratory-reared moths, no significant differences were found in the preference distributions between the experienced groups. It is concluded that females do not learn, i.e. that they do not change their preference for Vetiver grass after having experienced oviposition on either maize or this grass, which supports the idea that trap cropping could have potential as a control method for C. partellus. Differences observed between field-collected and laboratory-reared moths in the amount and size of egg batches laid on maize and Vetiver grass indicate that data obtained from experiments with laboratory-reared insects should be treated with caution.

Stale seedbeds can be used in arable farming to reduce the density of weeds in the crop. This type of tillage before crop sowing can contribute to a successful weed management in systems where no herbicides are used, e.g. organic farming. The population dynamics of weeds in response to stale seedbeds is, however, hardly understood, and therefore possibly not optimised. The relevance of tillage depth in relation to seedling emergence is explored with a simple, deterministic model. We systematically examined the effect of seedling emergence and subsequent weed control on weed population dynamics, starting by considering the seed bank as one soil layer, and continued by considering a depth-structured seed bank. Whether the widely used tillage regime consisting of shallow tillage, again shallow tillage, and ultimately deep tillage, is preferred above the regime we propose (i.e. deep, shallow and ultimately shallow tillage) depends on the proportion emergence specific for each soil layer, the proportion of seeds that is moved from one layer to the other, and the seed distribution in the soil. A case study based on characteristics of the population dynamics of the weed Polygonum persicaria showed that the proposed tillage regime could give reductions in weed density of up to 32% compared to the conventional tillage regime of stale seedbeds. The tillage regime that we propose requires techniques that restrict soil compaction to fixed traffic lanes, giving large zones favourable for crop growth. Because of ongoing processes in precision farming technology with respect to controlled traffic systems, this regime has realistic opportunities to become widely used in the future.

The minirhizotron is widely used to estimate root parameters in situ. This chapter examines the relation between root length per unit surface area in minirhizotrons and root length density in adjacent bulk soil obtained with core sampling and root washing. The data are from container experiments with wheat and potato and from field experiments with potato and field bean. Root length was measured on projected negative photographs by the line-intersect method. Minirhizotron observations showed patterns of change with time and depth similar to those that are commonly observed with the auger method. Regression analyses on data from wheat in containers showed that endoscope observations could be related quantitatively and reproducibly to soil core analyses. Disturbance of the water regime by the tubes, swelling and shrinking of clay, and, in the case of field bean, the disturbance of a dense layer probably contributed to the discrepancy in results between the two methods in the top soil layers.

Immunofluorescence colony-staining (IFC) is based on sample pour plating in combination with immunofluorescence staining for recognition of the target colony. IFC was optimised for detecting Xanthomonas campestris pv. vesicatoria (Xcv) and Clavibacter michiganensis subsp. michiganensis (Cmm) in tomato seed lots. Optimum incubation periods for colony growth were 2 d for Xcv and 3 d for Cmm. For both targets, the fluorescence intensity of the target colonies was positively correlated with the incubation temperature in the range tested and the period of colony growth. In general a lower brilliance was observed in IFC preparations of pure cultures than when seed extracts was added. The maximum density of bacterial colonies in the pour plates for both targets was about a thousand times higher per square cm as for surface plating. In comparison with IF (immunofluorescent cell staining) and isolation, IFC was 10 times more sensitive than IF. In samples with a target to saprophyte ratio higher than 1/100, 10 to 100 times higher numbers of positive colonies of the target were found in IFC using 16 mm diameter wells in miniaturized routine system, than by dilution plating on trypticase soy agar (TSA) and selective media. The recovery of Xcv in pour plates at various density of the target was still 46-68% for samples with a high microbial background (105 or more colonies per cm). For the slow growing Clavibacter, recovery was reduced to ca. 12-17% for samples with a high microbial background. A series of 23 commercial seed lots were tested for Xcv with IF and IFC. Using IF seven samples were positive based on IF-positive cells in the preparation. In IFC no typical fluorescent positive Xcv colonies were observed. From the weak cross-reacting colonies, viable cells were collected for pure culturing. Verification confirmed that the colonies of the weakly cross-reacting species belonged to other species

Strict legislation on the release of water and nutrients into the subsoil exists in the Netherlands. Therefore, on-line monitoring and control systems are being developed to tune the supply of water and nutrients to the plants' demand for optimal control of production and reduction of system losses. In this context sensors and control systems are important tools. For Chrysanthemum, however, the effect of reduced irrigation on crop production is not well understood and more data from adequate sensors are required to establish critical soil moisture levels for an optimal crop water status and growth. We studied the effect of different soil water levels and soil desiccation in a climate chamber and under changing light intensities in a greenhouse to assess the critical soil moisture status for optimal growth of Chrysanthemum plants. Moreover, we studied the efficacy of infrared (IR) thermometry as a useful tool for on-line monitoring and control under the conditions of Dutch greenhouse horticulture. It is shown in this study that under moderate climate conditions plants start to suffer from water shortage when soil moisture potential pF reaches values below –32 kPa. Water status of the plant can very well be monitored on-line in a greenhouse as changes in leaf temperature due to water shortage can be detected by IR thermometry, especially at summer radiation levels. In the climate chamber however, leaf temperature did not respond to changes in soil water status, suggesting that the environmental settings of the climate chamber are unsuitable for these kind of experiments. In two of the three experiments at different levels of global radiation a change in leaf temperature is explained by a change in water regime. We conclude that regression analysis of high frequency on-line IR monitoring may be a useful tool for expost analyses of irrigation regimes at high light intensities, and may lead to interesting insights in crop responses.

Strict legislation on the release of water and nutrients into the subsoil exists in the Netherlands. Therefore, on-line monitoring and control systems are being developed to tune the supply of water and nutrients to the plants¿ demand for optimal control of production and reduction of system losses. In this context sensors and control systems are important tools. For Chrysanthemum, however, the effect of reduced irrigation on crop production is not well understood and more data from adequate sensors are required to establish critical soil moisture levels for an optimal crop water status and growth. We studied the effect of different soil water levels and soil desiccation in a climate chamber and under changing light intensities in a greenhouse to assess the critical soil moisture status for optimal growth of Chrysanthemum plants. Moreover, we studied the efficacy of infrared (IR) thermometry as a useful tool for on-line monitoring and control under the conditions of Dutch greenhouse horticulture. It is shown in this study that under moderate climate conditions plants start to suffer from water shortage when soil moisture potential pF reaches values below ¿32 kPa. Water status of the plant can very well be monitored on-line in a greenhouse as changes in leaf temperature due to water shortage can be detected by IR thermometry, especially at summer radiation levels. In the climate chamber however, leaf temperature did not respond to changes in soil water status, suggesting that the environmental settings of the climate chamber are unsuitable for these kind of experiments. In two of the three experiments at different levels of global radiation a change in leaf temperature is explained by a change in water regime. We conclude that regression analysis of high frequency on-line IR monitoring may be a useful tool for expost analyses of irrigation regimes at high light intensities, and may lead to interesting insights in crop responses

Summary • The aim here was to separately assess mycorrhizal fungal and plant responses under elevated atmospheric CO2, and to test a mycocentric model that assumes that increased carbon availability to the fungus will not automatically feed back to enhanced plant growth performance. • Meta-analyses were applied across independent studies. Responses were compared in ectomycorrhizal (ECM) and arbuscular mycorrhizal (AM) fungi, and ECM and AM plants. • Responses of both mycorrhizal fungi and mycorrhizal plants to elevated CO2 were significantly positive. The response ratio for ECM fungi was 1.34 (an increase of 34%) and for AM fungi 1.21 (21%), indicating a significantly different response. The response ratio for ECM plants was 1.26, similar to that of AM plants (1.25). Fractional colonization proved to be an unsuitable fungal parameter. Evidence was found for the mycocentric view in ECM, but not in AM systems. • Fungal identity and plant identity were important parameters that affected response ratios. The need for better descriptors of fungal and plant responses is emphasized.

Pseudomonas putida strain angstrom 313, a deleterious rhizosphere bacterium, reduced pea nitrogen content when inoculated alone or in combination with Rhizobium leguminosarum bv. viceae on plants in the presence of soil under greenhouse conditions. When plants were grown gnotobiotically in liquid media, mixed inocula of angstrom 313 and rhizobia gave a higher proportion of small evenly distributed nodules when compared with a single rhizobial inoculation. In addition, the rhizobial root establishment was reduced by angstrom 313 irrespective of inoculum, density, indicating that angstrom 313 has the capacity to interact with the early rhizobial infection process. When pea seedlings were simultaneously inoculated with angstrom 313 and rhizobia, angstrom 313 colonised the root hairs to the same extent as the rhizobia, according to analysis by immunofluorescence microscopy. This suggests that the root hair colonisation trait of P. putida interferes with the onset of the symbiotic process. (c) 2004 Federation of European Microbiological Societies. Published by Elsevier B.V. All rights reserved.

In gangbare landbouwsystemen is, blijkens literatuuronderzoek, het risico van onkruidverspreiding via organische mest beperkt tot enige probleemsoorten. In dit artikel wordt aan de hand van verschillen in 'gangbare mest' en mest geproduceerd op biologische bedrijven duidelijk gemaakt, dat eerdere risico-schattingen niet volledig overdraagbaar zijn op biologische systemen. Er is experimenteel onderzoek gaande om de leemtes in kennis te vullen

Chronic effects of ozone on wet grassland species early in the growing season might be altered by interspecific competition. Individual plants of Holcus lanatus , Lychnis flos-cuculi , Molinia caerulea and Plantago lanceolata were grown in monocultures and in mixed cultures with Agrostis capillaris . Mesocosms were exposed to charcoal-filtered air plus 25 nl l −1 ozone (CF + 25), non-filtered air (NF), non-filtered air plus 25 nl l −1 ozone (NF + 25) and non-filtered air plus 50 nl l −1 ozone (NF + 50) early in the growing seasons of 2000 through 2002. Ozone-enhanced senescence and visible foliar injury were recorded on some of the target plants in the first year only. Ozone effects on biomass production were minimal and plant response to ozone did not differ between monocultures and mixed cultures. After three years, above-ground biomass of the plants in mixed culture compared to monocultures was three times greater for H. lanatus and two to four times smaller for the other species.

By means of a modelling tool an analysis was made of the local variation in the use of pesticides in the province of Utrecht in The Netherlands, and the potential environmental impact of pesticide emissions on the aquatic ecosystems. The aim of this study was to identify and quantify the major sources of pesticide use and environmental impact, taking the regional variation of pesticide use into account. The analysis was targeted at different levels: detailed (individual active substances, individual agricultural crops, civil land-use types, hydrological catchment basins) and globally covering agricultural use, non-agricultural use (some civil sectors) and recreational shipping. The results can be used for the (re)design of environmental monitoring programmes of pesticides in surface waters and for the development of region based policies towards sustainable pesticide use. The analysis tool that was developed is considered to be applicable for other regions as well.

Accurate measurements of leaf area are important for agronomic and physiological studies. To be able to perform repeated measurements of leaf area on single (genetically unique) plants, a method was developed to estimate leaf area from non-destructive measurements in Capsicum annuum L. independent of plant age and accession. Leaf length (L), width (W), position (leafno) and area of individual leaves were measured on 160 plants of four Capsicum accessions with different leaf shapes at different plant ages. Leaf area could be predicted from the product of length and width (alpha-LW), but this model could not account for changes in leaf shape during development of individual leaves and was dependent on both plant age and accession. The model became independent of plant age and accession when leaf width terms (W-2, W) were added. Validation on leaves measured in a second experiment containing three accessions and 200 genetically unique F-3 plants showed that the relation between calculated and measured area was very high. The model could still be refined by addition of the leaf position terms (R-2 = 0.996). Even when the length and width of only 25% of the leaves were measured, total plant leaf area was predicted well using the model with both leaf width and leaf position terms. Therefore, the models including the leaf width terms (W-2, W) in addition to the LW term are useful tools in physiological research and breeding in Capsicum.

In this study results are presented of investigations on the development of a cucumber crop in a closed rockwool system and on the potential change in microbial suppressiveness due to different disinfection methods after inoculating part of the plants with the fungal pathogen Pythium aphanidermatum. Two trials were executed at which the core treatments were disinfection (UV, slow filtration and no disinfection as control), while additional treatments were priming half of the systems before planting either with an 'old' nutrient solution from a former cucumber crop (spring trial) or an actinomycetes suspension (autumn trial). Crop developments (yield, disease and root development) and composition of the microflora were recorded. Non-inoculated plants showed that disinfection is needed either with UV or slow (sand) filtration to avoid spread of the pathogen; yield and root development was better and disease symptoms were less compared to the control. The hypothesis that passive disinfection might realise a more suppressive environment against Pythium aphanidermatum could not be proved, since inoculated plants showed a similar yield, disease and root development after the different disinfection treatments. Priming the nutrient solution with an 'old' solution did not show significant differences in the composition of the microflora either by plate-counts or by PCR-DGGE. However, the addition of actinomycetes before planting resulted a 100 to 1000 fold increase of the actinomycetes population in the cropping system up to the end of the trial. In the spring trial, differences within the microflora as a result of the disinfection treatments were only detected for pseudomonads. In the autumn trial, differences in numbers of pseudomonads, actinomycetes and fungi were significant in the treated nutrient solution, but not in the drain. With PCR-DGGE, differences in the bacterial profile were detected in autumn in both the drain and treated solution; i.e. less bands (corresponding to certain bacterial species) occurred in the UV treatment compared to slow filtration and control

We used a novel, nonintrusive experimental system to examine plant responses to warming and drought across a climatic and geographical latitudinal gradient of shrubland ecosystems in four sites from northern to southern Europe (UK, Denmark, The Netherlands, and Spain). In the first two years of experimentation reported here, we measured plant cover and biomass by the pinpoint method, plant 14C uptake, stem and shoot growth, flowering, leaf chemical concentration, litterfall, and herbivory damage in the dominant plant species of each site. The two years of approximately 1°C experimental warming induced a 15% increase in total aboveground plant biomass growth in the UK site. Both direct and indirect effects of warming, such as longer growth season and increased nutrient availability, are likely to be particularly important in this and the other northern sites which tend to be temperature-limited. In the water-stressed southern site, there was no increase in total aboveground plant biomass growth as expected since warming increases water loss, and temperatures in those ecosystems are already close to the optimum for photosynthesis. The southern site presented instead the most negative response to the drought treatment consisting of a soil moisture reduction at the peak of the growing season ranging from 33% in the Spanish site to 82% in The Netherlands site. In the Spanish site there was a 14% decrease in total aboveground plant biomass growth relative to control. Flowering was decreased by drought (up to 24% in the UK and 40% in Spain). Warming and drought decreased litterfall in The Netherlands site (33% and 37%, respectively) but did not affect it in the Spanish site. The tissue P concentrations generally decreased and the N/P ratio increased with warming and drought except in the UK site, indicating a progressive importance of P limitation as a consequence of warming and drought. The magnitude of the response to warming and drought was thus very sensitive to differences among sites (cold-wet northern sites were more sensitive to warming and the warm-dry southern site was more sensitive to drought), seasons (plant processes were more sensitive to warming during the winter than during the summer), and species. As a result of these multiple plant responses, ecosystem and community level consequences may be expected.

Five alternatives of the previously published MOMOS model (MOMOS-2 to -6) are tested to predict the dynamics of carbon (C) and nitrogen (N) in soil during the decomposition of plant necromass. 14C and 15N labeled wheat straw was incubated over 2 years in fallow soils of the high Andean Paramo of Venezuela. The following data were collected: soil moisture, total 14C and 15N and microbial biomass (MB)-14C and -15N, daily rainfall, air temperature and total radiation. Daily soil moisture was predicted using the SAHEL model. MOMOS-2 to -4 (type 1 models) use kinetic constants and flow partitioning parameters. MOMOS-2 can be simplified to MOMOS-3 and further to MOMOS-4, with no significant changes in the prediction accuracy and robustness for total-14C and -15N as well as for MB-14C and -15N. MOMOS-5 (type 2 models) uses only kinetic constants: three MB-inputs (from labile and stable plant material and from humified compounds) and two MB-outputs (mortality and respiration constants). MOMOS-5 did not significantly change the total-14C and -15N predictions but markedly improved the predictive quality and robustness of MB-14C and -15N predictions (with a dynamic different from the predictions by other models). Thus MOMOS-5 is proposed as an accurate and ecologically consistent description of decomposition processes. MOMOS-6 extends MOMOS-5 by including a stable humus compartment for long-term simulations of soil native C and N. The improvement of the predictions is not significant for this 2-year experiment, but MOMOS-6 enables prediction of a sequestration in the stable humus compartment of 2% of the initially added 14C and 5.4% of the added 15N

This article describes new approaches for manipulation of temperature and water input in the field. Nighttime warming was created by reflection of infrared radiation. Automatically operated reflective curtains covered the vegetation at night to reduce heat loss to the atmosphere. This approach mimicked the way climate change, caused by increased cloudiness and increased greenhouse gas emissions, alters the heat balance of ecosystems. Drought conditions were created by automatically covering the vegetation with transparent curtains during rain events over a 2-5-month period. The experimental approach has been evaluated at four European sites across a climate gradient. All sites were dominated (more than 50%) by shrubs of the ericaceous family. Within each site, replicated 4-m × 5-m plots were established for control, warming, and drought treatments and the effect on climate variables recorded. Results over a two-year period indicate that the warming treatment was successful in achieving an increase of the minimum temperatures by 0.4-1.2°C in the air and soil. The drought treatment resulted in a soil moisture reduction of 33%-82% at the peak of the drought. The data presented demonstrate that the approach minimizes unintended artifacts with respect to water balance, moisture conditions, and light, while causing a small but significant reduction in wind speed by the curtains. Temperature measurements demonstrated that the edge effects associated with the treatments were small. Our method provides a valuable tool for investigating the effects of climate change in remote locations with minimal artifacts.

The trend in glasshouse horticulture has always been to keep culture systems as clean as possible by using new substrates, by applying disinfection techniques, pathogen-free propagation material, etc. However, several root diseases cause problems under these conditions. Especially, zoospore-producing organisms such as Pythium and Phytophthora spp. are well adapted to aquatic life and flourish in soilless substrates. The present paper shows the importance of the microflora in suppressing root pathogens in soilless systems, either by introducing selected antagonists or by stimulation of a beneficial microflora. The limited volume of substrate and a 'biological vacuum' in soilless systems, facilitate inoculation and establishment of antagonists. Examples of soilless systems that contain a suppressive microflora are described. Moreover, the impact of recirculation and disinfection of the nutrient solution on pathogen spread and disease suppression is discussed. Increased knowledge on the beneficial microflora, its ecology, and treatments that influence its composition, will stimulate the exploitation of microbially balanced and optimised soilless systems.

Volgens de richtlijnen van de EU Directive wordt de aanwezigheid van M. chitwoodi en M.fallax in partijen voortkwekingsmateriaal vastgesteld aan de hand van de zichtbare symptomen. Er is behoefte aan betere methoden om de aanwezigheid en de intensiteit van beide aaltjes in percelen en partijen vast te stellen. Dergelijke methoden, zoals eerder ontwikkeld voor de quarantaine-nematoden G. rostochiensis en G. pallida, kunnen dienen als basis voor de uitvoering van fytosanitaire taken van de PD en als grondslag voor Beslissing Ondersteunende Systemen voor de beheersing van nematoden op percelen. Ook voor veldproeven zijn goede bemonsteringsmethoden onontbeerlijk

Climate change may affect ecosystem functioning through increased temperatures or changes in precipitation patterns. Temperature and water availability are important drivers for ecosystem processes such as photosynthesis, carbon translocation, and organic matter decomposition. These climate changes may affect the supply of carbon and energy to the soil microbial population and subsequently alter decomposition and mineralization, important ecosystem processes in carbon and nutrient cycling. In this study, carried out within the cross-European research project CLIMOOR, the effect of climate change, resulting from imposed manipulations, on carbon dynamics in shrubland ecosystems was examined. We performed a 14C-labeling experiment to probe changes in net carbon uptake and allocation to the roots and soil compartments as affected by a higher temperature during the year and a drought period in the growing season. Differences in climate, soil, and plant characteristics resulted in a gradient in the severity of the drought effects on net carbon uptake by plants with the impact being most severe in Spain, followed by Denmark, with the UK showing few negative effects at significance levels of p ≤ 0.10. Drought clearly reduced carbon flow from the roots to the soil compartments. The fraction of the 14C fixed by the plants and allocated into the soluble carbon fraction in the soil and to soil microbial biomass in Denmark and the UK decreased by more than 60%. The effects of warming were not significant, but, as with the drought treatment, a negative effect on carbon allocation to soil microbial biomass was found. The changes in carbon allocation to soil microbial biomass at the northern sites in this study indicate that soil microbial biomass is a sensitive, early indicator of drought- or temperature-initiated changes in these shrubland ecosystems. The reduced supply of substrate to the soil and the response of the soil microbial biomass may help to explain the observed acclimation of CO2 exchange in other ecosystems.

Ralstonia solanacearum is the causative agent of bacterial wilt in many important crops. A specific and sensitive PCR detection method that uses primers targeting the gene coding for the flagella subunit, fliC , was established. Based on the first fliC gene sequence of R. solanacearum strain K60 available at GenBank, the Ral_ fliC PCR primer system was designed; this system yielded a single 724-bp product with the DNAs of all of the R. solanacearum strains tested. However, R. pickettii and four environmental Ralstonia isolates also yielded amplicons. The Ral_ fliC PCR products obtained with 12 strains ( R. solanacearum , R. pickettii , and environmental isolates) were sequenced. By sequence alignment, Rsol_ fliC primers specific for R. solanacearum were designed. With this primer system, a specific 400-bp PCR product was obtained from all 82 strains of R. solanacearum tested. Six strains of R. pickettii and several closely related environmental isolates yielded no PCR product; however, a product was obtained with one Pseudomonas syzygii strain. A GC-clamped 400-bp fliC product could be separated in denaturing gradient gels and allowed us to distinguish P. syzygii from R. solanacearum . The Rsol_ fliC PCR system was applied to detect R. solanacearum in soil. PCR amplification, followed by Southern blot hybridization, allowed us to detect about one target DNA molecule per PCR, which is equivalent to 10 3 CFU g of bulk soil −1 . The system was applied to survey soils from different geographic origins for the presence of R. solanacearum .

Production of field vegetables is known for its high nitrogen input and consequently high nitrogen losses towards the environment. All over the world research tries to find opportunities to reduce these losses. In 2000 the Dutch government initiated and funded a research project (Telen met toekomst) to explore the possibilities to reduce the adverse effects of nitrogen and phosphate inputs on the quality of soil and surface water by farm management. A participatory research approach was chosen, so the farmer, the consultant and scientist work closely together in making annual plans to reach a number of set goals for fertilization on the farm level. By registration of all activities concerning fertilization on the farm, the nitrogen input and output could be monitored. The gap between the reference point at the start of the project and the environmental goals is big for nitrogen: the nitrogen balance surplus on the whole farm level should be reduced from 300 kg N/ha to 90 kg N/ha. First results show that this gap is unlikely to be bridged on all farms within the set period of time, without affecting the farmer's income. However, distinct differences could be observed in the farmers' attitude towards the challenge, the rate of progress varied significantly among farmers.

In this study, a Paenibacillus-specific PCR system, based on the specific primer PAEN515F in combination with bacterial primer R1401, was tested and used to amplify specific fragments of the 16S rRNA gene from rhizosphere DNA. The amplicons were used in a second (semi-nested) PCR for DGGE, in which bacterial primers F968GC and R1401 were used. The resulting products were separated into community fingerprints by DGGE. To assess the reliability of the method, the diversity of Paenibacillus species was evaluated on the basis of DNA extracted directly from the rhizospheres of four different cultivars of maize (Zea mays), i.e. CMS04, CMS11, CMS22 and CMS36, sown in two Brazilian field soils (Cerrado and V¿¡rzea). In addition, a clone library was generated from the PCR-generated 16S rDNA fragments, and selected clones were sequenced. The results of the bacterial community analyses showed, at the level of clone libraries, that considerable diversity among Paenibacillus spp. was present. The most dominantly found sequences clustered into 12 groups, each one potentially representing a species complex. Sequences closely affiliated with the P. macerans and P. azotofixans complexes were found in all samples, whereas other sequences were scarcer. Clones affiliated with the latter species complex were most abundant, representing 19 f all clones analysed. The Paenibacillus fingerprints generated via semi-nested PCR followed by DGGE showed a clear distinction between the maize plants grown in Cerrado versus V¿¡rzea soils. Thus, soil type, instead of maize cultivar type, was the overriding determinative factor that influenced the community structures of the Paenibacillus communities in the rhizospheres investigated. At a lower level (subcluster), there was a trend for maize cultivars CMS11 and CMS22 on the one hand, and CMS36 and CMS04 on the other hand, to cluster together, indicating that these respective pair of cultivars were similar in their Paenibacillus species composition. This trend was tentatively linked to the growth characteristics of these maize cultivars. These results clearly demonstrated the efficacy of the Paenibacillus-specific PCR-DGGE method in describing Paenibacillus species diversity in rhizosphere soils.

The study assessed the effects of hedgerow networks (line elements containing shrubs less than 2 m), woodlots (height > 2 m), other natural areas, and agricultural and horticultural land (polygons) in the landscapes on the abundance of a generalist thrips species in organic leek fields. Landscape maps were compiled from two GIS frameworks (LGN3plus and Top10Vector Analysis) and supplemented with information from interviews with farmers on farming practices. Landscape structure was analysed at two spatial scales, at a radius of 1000 and 5000 m respectively from the centre of the leek fields. A multi-habitat species, onion thrips (Thrips tabaci) was monitored in 21 organic leek (Allium porrum) fields during the growing season and their densities were related with landscape features and the density of natural enemies on the leek plants. Results show that the length of the hedgerow network (height

A PCR system for studying the diversity of species of Bacillus and related taxa directly from soil was developed. For this purpose, a specific 24-bp forward primer located around position 110 of the 16S ribosomal RNA gene was designed and combined with a reverse bacterial primer located at the end of the gene. The specificity of this PCR system for bacilli and related taxons was confirmed on the basis of tests with diverse strains as well as with soil DNA. Analysis of a soil DNA derived clone library showed that the amplified fragments affiliated exclusively with sequences of gram-positive bacteria, with up to 95% of the sequences originating from putative Bacillus species. In particular, sequences affiliated to those of B. mycoides, B. pumilus, B. megaterium, B. thuringiensis, and B. firmus, as well as to related taxa such as Paenibacillus, were obtained. A minority, i.e., less than 6%, of the clones affiliated with other gram-positive bacteria, such as Arthrobacter spp., Frankia spp., and uncultured gram-positives. The amplified fragments were used as templates for a second PCR using bacterial 16S rDNA primers, yielding PCR products of about 410 bp, which were separated by denaturing gradient gel electrophoresis (DGGE). Amplicons indicating Bacillus spp. were found in the gel between 45% and roughly 60% denaturant, whereas those representing other, high-G+C% bacteria, were localized in gel regions with denaturant concentrations exceeding about 60%, thus allowing the distinction between these two groups of sequences. We applied this system to compare the group-specific diversity in bacterial communities in an agricultural soil under different regimes, i.e., permanent grassland, grassland recently turned to arable land, and arable land under agricultural rotation. Differences in the Bacillus-related community structures between the treatments were clearly detected. Higher diversities, as judged by Shannon-Weaver indices calculated on the basis of the molecular profiles, were consistently observed in the permanent grassland and the grassland turned into arable land, as compared to the arable land.

The re-emergence of fusarium head blight throughout the world and especially in Western Europe prompted a survey of the situation in the Netherlands. To allow for a high throughput screening of large numbers of samples, a diagnostic PCR method was developed to detect the most common species of Fusarium occurring on wheat. Seven primer pairs were tested for their ability to identify isolates of Fusarium avenaceum, F. culmorum, F. graminearum, F. poae, F. proliferatum and Microdochium nivale var. majus and M. nivale var. nivale. Each primer pair only generated a PCR product with the corresponding Fusarium species and all PCR fragments had different molecular sizes. This allowed the generation of these amplicons using a mixture of all seven primer pairs. The robustness of this multiplex PCR encouraged us to screen a large series of isolates collected in 2000 and 2001. In both years 40 fields were sampled leading to a collection of 209 isolates from 2000 and 145 isolates from 2001. The results of the multiplex PCR demonstrated that F. graminearum was the most abundant species in the Fusarium complex on wheat in both years. This is in sharp contrast to reports from the 1980s and early 1990s, which found F. culmorum as the predominant species. Primers derived from the tri7 and tri13 genes, which are implicated in the acetylation and oxygenation of the C-4 atom of the backbone of the trichothecene molecule, were used to discriminate between deoxynivalenol and nivalenol (NIV) producers. The populations of F. culmorum and F. graminearum both showed a slight increase in NIV-producers in 2001

The ability of Bacillus subtilis, strain BB, to colonise cabbage seedlings endophytically was examined following seed inoculation. Strain BB was recovered from different plant parts including leaves (cotyledons), stem (hypocotyl) and roots. While high bacterial populations persisted in the roots and lower stem, they were lower in the upper stem and leaves through time. In addition to cabbage, strain BB colonised endophytically the roots of 5 other vegetable brassicas. Fatty acid methyl ester (FAME) and PCR fingerprinting analysis confirmed the reliability of the detection method. Studies conducted with transmission electron microscope (TEM) showed that BB mainly colonised intercellular spaces of cortical tissues including intercellular spaces close to the conducting elements of roots and stem of cabbage seedlings. Gold labelling was specifically associated with BB and the fibrillar material filling the intercellular spaces where bacterial cells were found.

Studies were conducted in annual crops of strawberry (Fragaria x ananassa) (cv. Elsanta to assess the relative importance of petals as an inoculum source of grey mould, caused by Botrytis cinerea and to identify during which period of flower and fruit development the presence of petals has a significant effect on development of grey mould on fruits. In 1998, the incidence of B. cinerea on flower parts was assessed, and the symptoms of grey mould on fruits were characterised with regards to their starting point. The incidence of B. cinerea on petals was 65-85% of those flowers that harboured B. cinerea. The starting point of symptoms was located underneath the sepals in 65-85% of fruits with grey mould, and petals were present at this site in about 50% of the cases. In 1999, four field experiments were conducted to assess the effect of petal removal at different stages of flower and fruit development on incidence of grey mould. The incidence of grey mould on fruits with petals retained till harvest was circa 55% more than on fruits where petals were removed or had dropped naturally by the end of flowering, regardless of planting date or inoculum level of B. cinerea. The incidence of grey mould was hardly different between treatments where petals were removed at young flower stage, old flower stage, or green fruit stage. It is concluded that petals are an effective and stable source of inoculum for fruit infection by B. cinerea. Considering the high probability that retention of petals during fruit expansion and ripening results in fruit infection, the elimination of petals as an inoculum source appears worthwhile even when the incidence of petal retention on fruits is relatively low

In this paper we discuss the differences and similarities in the growth response of tomato plants to N and P limitation, and to their interaction. Two detailed growth experiments, with varied N or P supply, were conducted in order to unravel the effects of N and P limitation on growth of young tomato plants (Lycopersicon esculentum Mill.). Relative growth rate (RGR) initially increased sharply with increasing plant P concentration but leveled off at higher plant P concentrations. In contrast, RGR increased gradually with increasing plant N concentration before it leveled off at higher plant N concentrations. The relationship of RGR with organic leaf N and P showed the same shape as with total N and P concentrations, respectively. The difference in response is most likely due to the different roles of N and P in the machinery of the plant's energy metabolism (e.g., photosynthesis, respiration). Plant N concentration decreased with increasing P limitation. We show that this decrease cannot be explained by a shift in dry-mass partitioning. Our results suggest that the decrease in N concentration with increasing P limitation may be mediated by a decrease in leaf cytokinin levels and is less likely due to decreased energy availability at low P conditions. Dry-mass partitioning to the roots was closely linearly related to the leaf reduced-N concentration. However, treatments that were severely P limited deviated from this relationship.

Epidemics of onion leaf spot caused by Botrytis spp. depend on necrotic leaf tissue for inoculum build up in the crop. Ulocladium atrum Preuss. is a strong competitor on necrotic above-ground plant tissues. The potential of the antagonist to reduce colonisation of necrotic leaf tissue by Botrytis spp. and subsequent sporulation was studied in two field experiments. U. atrum colonised necrotic tissues and consistently reduced the sporulation of fungal competitors. Although incidence of Botrytis spp. was low, significantly lower spore loads of Botrytis spp. were found on spore traps located within U. atrum treated onion plots as compared to untreated control plots. The number of leaf spots caused by Botrytis spp. was low in both field experiments but was significantly reduced by 60 fter U. atrum applications in the second experiment. Spraying of the fungicide Ronilan resulted in the same control level. Results show that U. atrum has a potential for biological control against diseases caused by Botrytis spp. in onions.

To analyze the dynamics of growth, water and nutrient uptake, the effects of 1, 3 and 7 d of calcium starvation and the recovery capability during 7 d afterwards were investigated in vegetative tomato plants. Results showed that after only 1 d of Ca-starvation, leaf photosynthesis, leaf expansion and leaf transpiration were already reduced and root growth had stopped completely. Leaf Ca concentration was reduced by a Ca-starvation with the strongest effect in the youngest leaves. When 1 d Ca-starvation period was followed by 7 d of recovery with full-strength nutrient solution, leaf photosynthesis and growth completely recovered. During the recovery period, Ca concentrations in the young leaves increased to higher values than these concentrations in the control plants. During the recovery period calcium uptake showed positive linear relationships with dry-matter increase of the plant as well as with water uptake. Most of the measured growth parameters showed a strong correlation with the plant Ca concentration, but the relations were not always identical during starvation and recovery. We discuss the role different forms and functions of Ca may play in regulating growth and developmental processes.

The prevalence of selected streptomycin (Sm)-resistance genes, i.e. aph (3''), aph (6)-1d, aph (6)-1c, ant (3'') and ant (6), was assessed in a range of pristine as well as polluted European habitats. These habitats included bulk and rhizosphere soils, manure from farm animals, activated sludge from wastewater treatment plants and seawater. The methods employed included assessments of the prevalence of the genes in habitat-extracted DNA by PCR, followed by hybridisation with specific probes, Sm-resistant culturable bacteria and exogenous isolation of plasmids carrying Sm-resistance determinants. The direct DNA-based analysis showed that aph (6)-1d genes were most prevalent in the habitats examined. The presence of the other four Sm-modifying genes was demonstrated in 58% of the tested habitats. A small fraction of the bacterial isolates (8%) did not possess any of the selected Sm-modifying genes. These isolates were primarily obtained from activated sludge and manure. The presence of Sm-modifying genes in the isolates often coincided with the presence of IncP plasmids. Exogenous isolation demonstrated the presence of plasmids of 40-200 kb in size harbouring Sm-resistance genes from all the environments tested. Most plasmids were shown to carry the ant (3'') gene, often in combination with other Sm-resistance genes, such as aph (3'') and aph (6)-1d. The most commonly found Sm-modifying gene on mobile genetic elements was ant (3''). Multiple Sm-resistance genes on the same genetic elements appeared to be the rule rather than the exception. It is concluded that Sm-resistance genes are widespread in the environmental habitats studied and often occur on mobile genetic elements and ant (3'') was most often encountered.

In this study, water and nutrient uptake from the substrate and responses of the crop to fertigation strategies were analysed. In an experiment and simulation study, the amount and frequency of fertigation and electrical conductivity (EC) of the fertigation water were changed for a tomato crop grown on rockwool. In a climate chamber, tomato plants were grown on rockwool while five different EC levels and two fertigation frequencies were maintained. Effects on plant growth, water uptake, and nutrient uptake were measured. The experimental data were compared with simulated results. We used two simulation models: a plant growth model and a model that describes water movement and nutrient transport within the substrate, i.e. rockwool, and root uptake of water and nutrients from the substrate. The key idea behind the simulation of uptake of water and nutrients is to consider transport (mass flow and diffusion) from the bulk substrate surrounding the roots towards the root surface. Water uptake may also be hindered due to osmotic potential differences between substrate solution and root sap. The results confirmed the hypothesis that at lower EC growth is reduced due to limitations in nutrient availability (transport rate towards the root) while at higher EC growth is reduced mainly due to water stress (low osmotic potential). INTRODUCTION The goal of managing water and nutrient supply to greenhouse crops is to control crop growth and product quality as well as to minimise losses of water and nutrients to the environment. Precise supply of water and nutrients is necessary to reach this goal, where supply of water and the individual nutrients is in proportion to the demand of the crop. In order to do so, quantitative information on demand and uptake of water and the individual nutrients is required. Information on crop behaviour when either of them is in surplus or in shortage is also necessary. Knowledge of the transport of water and nutrients from the bulk substrate towards the root surface must be known in order to determine whether the plant demand can be fulfilled or not (shortage). Such information could be obtained from a large number of experiments, but a simulation model in combination with a limited number of experiments is a more powerful tool for such an analysis. The electrical conductivity (EC) of the nutrient solution as well as the amount and frequency of fertigation (irrigation with nutrient solution) are the most important variables used by growers to control the supply of water and nutrients to the crop. The aim of this study was to analyse the water and nutrient uptake from the substrate and the responses of the crop to fertigation strategies. In an experiment and simulation study, the amount and frequency of fertigation and EC were changed to explore the following two hypotheses for a tomato crop grown on rockwool: Crop growth reduction may occur at both low and high EC. At low EC, not enough nutrients may be available to the roots (unless in a well-mixed water culture) resulting in a reduction of nutrient uptake, which may result in reduced crop growth. At high EC, ample nutrients are available. However, reduction of water uptake may Proc. 4 IS on Cropmodels Eds. J.H. Lieth & L.R. Oki Acta Hort. 593, ISHS 2002

In a previous paper, we proposed a fungal growth model (Lamour et al., 2001 IMA J. Math. Appl. Med. Biol., 17, 329-346), describing the colonization and decomposition of substrate, subsequent uptake of nutrients, and incorporation into fungal biomass, and performed an overall-steady-state analysis. In this paper we assume that where nutrient dynamics are much faster than the dynamics of fungal biomass and substrate, the system will reach a quasi-steady-state relatively quickly. We show how the quasi-steady-state approximation is a simplification of the full fungal growth model. We then derive an explicit fungal invasion criterion, which was not possible for the full model, and characterize parameter domains for invasion and extinction. Importantly, the fungal invasion criterion takes two forms: one for systems where carbon is limiting, another for systems where nitrogen is limiting. We focus attention on what happens in the short term immediately following the introduction of a fungus to a fungal-free system by analysing the stability of the trivial steady state, and then check numerically whether the fungus is able to persist. The derived invasion criterion was found to be valid also for the full model. Knowledge of the factors that determine invasion is essential to an understanding of fungal dynamics. The simplified model allows the invasion criterion to be tested with experimental data.

The flood of sequence data resulting from the large number of current genome projects has increased the need for a flexible, open source genome annotation system, which so far has not existed. To account for the individual needs of different projects, such a system should be modular and easily extensible. We present a genome annotation system for prokaryote genomes, which is well tested and readily adaptable to different tasks. The modular system was developed using an object-oriented approach, and it relies on a relational database backend. Using a well defined application programmers interface (API), the system can be linked easily to other systems. GenDB supports manual as well as automatic annotation strategies. The software currently is in use in more than a dozen microbial genome annotation projects. In addition to its use as a production genome annotation system, it can be employed as a flexible framework for the large-scale evaluation of different annotation strategies. The system is open source

The dynamics of competitive colonisation of necrotic cyclamen tissue by the plant pathogenic fungus Botrytis cinerea and the saprophytic fungal antagonist Ulocladium atrum were studied immuno-histologically, while sporulation was studied macroscopically. The effect of different time intervals between inoculation of both fungi on resource capture by each species was explored. Colonisation and sporulation were used as indicators for competitive resource capture and the effectiveness of biological control of B. cinerea, using U. atrum. Mycelial biomass and sporulation showed logistical time courses in both species, in monocultures as well as in mixed cultures. Final colonisation and sporulation levels were lowered by competition, indicating competitive resource capture. Analysis of the extent to which sporulation of either fungus could be reduced by co-inoculation with the other fungus at different times, showed that B. cinerea can be completely excluded by ‘early’ pre-inoculation with U. atrum, but not vice versa, indicating that U. atrum can exploit resources in the leaf that are not accessible to B. cinerea. A model of competitive substrate colonisation and resource capture was developed on the basis of the experimental results. Model results confirm that competition for resources provides a sufficient biological explanation for the dynamic interactions between the fungi. The model provides a tool to optimise dose and timing of U. atrum applications providing effective biological control of B. cinerea.

Antibiotic resistance genes have become highly mobile since the development of antibiotic chemotherapy. A considerable body of evidence exists proving the link between antibiotic use and the significant increase in drug-resistant human bacterial pathogens. The application of molecular detection and tracking techniques in microbial ecological studies has allowed the reservoirs of antibiotic resistance genes to be investigated. It is clear that the transfer of resistance genes has occurred on a global scale and in all natural environments. The considerable diversity of bacteria and mobile elements in soils has meant that the spread of resistance genes has occurred by all currently known mechanisms for bacterial gene transfer. Trans-kingdom transfers from plants to bacteria may occur in soil. Hot spots for gene transfer in the soil/plant environment have been described and colonized niches such as the rhizosphere and other nutrient-enriched sites, for example manured soil, have been identified as reservoirs of resistance genes. Although exposure and selection for tolerance of antibiotics is considerable in clinical environments there is increasing evidence that selection for resistant phenotypes is occurring in natural environments. Antibiotic-producing bacteria are abundant in soil and there is evidence that they are actively producing antibiotics in nutrient-enriched environments in soil. In addition there is clear evidence that the self-resistance genes found within antibiotic gene clusters of the producers have transferred to other non-producing bacteria. Perhaps most important of all is the use of antibiotics in agriculture as growth promotants and for treatment of disease in intensively reared farm animals. These treatments have resulted in gut commensal and pathogenic bacteria acquiring resistance genes under selection and then, due to the way in which farm slurries are disposed of, the spread of these genes to the soil bacterial community. Integrons with multiple resistance gene cassettes have been selected and disseminated in this way; many of these cassettes carry other genes such as those conferring heavy metal and disinfectant resistance which have been co-selected in bacteria surviving in effluents and contaminated soils, further maintaining and spreading the antibiotic resistance genes.

Grey mould is an important disease of strawberries resulting from flower and fruit infection by Botrytis cinerea Pers. The saprophytic fungus Ulocladium atrum Preuss is a promising biological control agent for control of B. cinerea in strawberry and other crops. The objective of this research was to determine the efficacy of U. atrum to control grey mould by a single application of a spore suspension (2 x 106 conidia/ml) at different flower and fruit development stages. Four experiments were carried out in 1999, two under natural and two under enhanced inoculum levels of B. cinerea. In each experiment, flowers and young fruits in six distinct stages of development were sprayed with either water or U. atrum suspension. U. atrum suppressed B. cinerea sporulation on petals by 15 to 54ÐOne to four days after spraying, U. atrum was present on less than 30␘f stamens and did not affect the incidence of B. cinerea on these flower parts. The efficacy of the U. atrum sprays in controlling grey mould was low to moderate, and resulted on average in a reduction of 21␒n disease incidence on ripe fruits. Low control efficacy was probably due to poor coverage with or colonisation of stamens by U. atrum spores, and a relatively low level of suppression of the colonisation of flower parts by B. cinerea. Significant reductions of grey mould in comparison to the control (p ≤ 0.10; on average 41␛eduction) were found most frequently when the antagonist was introduced at late flowering or early fruit stages. Therefore, these are the most suitable stages to apply U. atrum. Further studies are needed to improve the spray coverage and persistence of U. atrum inoculum.

EDU (ethylenediurea) and non-EDU-treated bean plants (Phaseolus vulgaris) L. cv. Lit) were exposed to ambient air at four rural sites in the Netherlands during the growing seasonsof 1994 through 1996 to investigate the responses to ambient ozone. Ozone-induced foliar injury was observed each year anddifferences in injury between sites depended on year. On average,injury amounted to 27% in 1994, to 8% in 1995 and to 1% in 1996. Injury increased with increasing ozone exposure (AOT40) and the estimated AOT40 value corresponding with 5% injury wascirca 3650 nl l-1 h ozone. The highest ozone levels accumulated at each site for five consecutive days before injuryexceeded the proposed short-term critical level for injury development. EDU reduced injury and its protective effect was positively related to the injury intensity in non-EDU-treatedplants. Yield of green marketable pods (intermediate harvest) andmature pods (final harvest) was generally reduced in non-EDU-treated plants compared to EDU-treated plants and differences inyield reduction between harvests varied between years. The yield of mature pods was reduced in 1994 and 1996 while the yield ofgreen pods was reduced in 1995 by ozone only. Since yield reduction was not correlated with AOT40, the EDU method was notvalid to determine an ozone exposure-yield reponse relationshipfor bean.

Ralstonia solanacearum biovar 2, the causative agent of brown rot in potato, has been responsible for large crop losses in Northwest Europe during the last decade. Knowledge on the ecological behaviour of R. solanacearum and its antagonists is required to develop sound procedures for its control and eradication in infested fields. A polyphasic approach was used to study the invasion of plants by a selected R. solanacearum biovar 2 strain, denoted 1609, either or not in combination with the antagonistic strains Pseudomonas corrugata IDV1 and P. fluorescens UA5-40. Thus, this study combined plating (spread and drop plate methods), reporter gene technology (gfp mutants) and serological (imunofluorescence colony staining [IFC]) and molecular techniques (fluorescent in situ hybridization [FISH], PCR with R. solanacearum specific primers and PCR–DGGE on plant DNA extracts). The behaviour of R. solanacearum 1609 and the two control strains was studied in bulk and (tomato) rhizosphere soil and the rhizoplane and stems of tomato plants. The results showed that an interaction between the pathogen and the control strains at the root surface was likely. In particular, R. solanacearum 1609 CFU numbers were significantly reduced on tomato roots treated with P. corrugata IDV1(chr::gfp1) cells as compared to those on untreated roots. Concomitant with the presence of P. corrugata IDV1(chr::gfp1), plant invasion by the pathogen was hampered, but not abolished. PCR–DGGE analyses of the tomato rhizoplane supported the evidence for antagonistic activity against the pathogen; as only weak R. solanacearum 1609 specific bands were detected in profiles derived from mixed systems versus strong bands in profiles from systems containing only the pathogen. Using FISH, a difference in root colonization was demonstrated between the pathogen and one of the two antagonists, i.e. P. corrugata IDV1(chr::gfp1); R. solanacearum strain 1609 was clearly detected in the vascular cylinder of tomato plants, whereas strain IDV1 was absent. R. solanacearum 1609 cells were also detected in stems of plants that had developed in soils treated with this strain, even in cases in which disease symptoms were absent, indicating the occurrence of symptomless infection. In contrast, strain 1609 cells were not found in stems of several plants treated with either one of the two antagonists. The polyphasic analysis is valuable for testing antagonistic strains for approval as biocontrol agents in agricultural practice

Lathrolestes ensator (Brauns) was the only parasitoid species reared from larvae of apple sawfly Hoplocampa testudinea (Klug) collected in Dutch orchards. The life history parameters of apple sawfly and its parasitoid L. ensator were investigated in field and semi-field experiments. Usually, the adult parasitoids emerge in synchrony with the preferred host stage, the second instar sawfly larvae. Parasitoid females carried 120-175 eggs, but never laid more than half this number. The mortality of sawfly eggs and larvae varied from 75 and 99&Eth;The eggs and mining larvae are most vulnerable, as the older larvae survive for 90 n average. Three factors at least determine how many of the descending larvae become adult sawfly or parasitoid next spring. The failure of descending larvae to construct a cocoon varied from 7 to 31 nd was highest in clay soil. Probably a similar mechanical hindrance prohibited more emerging adults, of both sawfly and L. ensator, from attaining the surface in heavier soils next spring. Relatively more sawfly prepupae than parasitoids died in the cocoon stage, from fungus disease or other causes, but more parasitoids than hosts stayed in prolonged diapause. Mainly due to this last factor, the overall result was a decrease of the parasitoid : host ratio during the first season underground. A life table based on survival rates during various life stages yields a net reproductive rate of up to 2.4 daughters per female sawfly. It indicates that 60 f the sawfly larvae need to be parasitized to stop population growth, or correspondingly less when more than 75 f the host eggs and young larvae are killed by predators or other causes.

In a previous paper, we proposed a fungal growth model (Lamour et al., 2001 IMA J. Math. Appl. Med. Biol., 17, 329-346), describing the colonization and decomposition of substrate, subsequent uptake of nutrients, and incorporation into fungal biomass, and performed an overall-steady-state analysis. In this paper we assume that where nutrient dynamics are much faster than the dynamics of fungal biomass and substrate, the system will reach a quasi-steady-state relatively quickly. We show how the quasi-steady-state approximation is a simplification of the full fungal growth model. We then derive an explicit fungal invasion criterion, which was not possible for the full model, and characterize parameter domains for invasion and extinction. Importantly, the fungal invasion criterion takes two forms: one for systems where carbon is limiting, another for systems where nitrogen is limiting. We focus attention on what happens in the short term immediately following the introduction of a fungus to a fungal-free system by analysing the stability of the trivial steady state, and then check numerically whether the fungus is able to persist. The derived invasion criterion was found to be valid also for the full model. Knowledge of the factors that determine invasion is essential to an understanding of fungal dynamics. The simplified model allows the invasion criterion to be tested with experimental data.

Tijdens een inventarisatie met enkele honderden bodemvallen in het natuurgebied de Blauwe Kamer bij Wageningen werd een enkel exemplaar aangetroffen van Elaphropus hoemorrhoidales, een nieuwe loopkever voor de Nederlandse fauna. Het betreft een mediterrane soort, die in heel Midden-Europa zeer zeldzaam is. Uit de weinige ecologische gegevens blijkt dat de soort vooral gebonden is aan moerassige gebieden. Het is de vraag of zich in de Blauwe Kamer een populatie bevindt, of dat er sprake is van een incidentele waarneming van een verdwaald exemplaar.

De schimmel Ulocladium atrum is geselecteerd als een antagonist van Botrytis cinerea. De ecologische eigenschappen van deze antagonist en de toepassingen op bovengrondse plantendelen worden in dit artikel beschreven. Gegevens bij de bijgaande figuren: 1) Effect van Ucladium atrum op de grauwe schimmel in druif; 2) Schade aan Botrytis cinerea in tomaat en de bestrijding hiervan door U. atrum; 3) Schade van Botrytis cinerea in cyclaam en de bestrijding hiervan door U. atrum

Willow is often used in bio-energy plantations for its potential to function as a renewable energy source, but knowledge about its effect on soil carbon dynamics is limited. Therefore, we investigated the temporal variation in carbon dynamics in willow, focusing on below-ground allocation and sequestration to soil carbon pools. Basket willow plants (Salix viminalis L.) in their second year of growth were grown in pots in a greenhouse. At five times during the plants growth, namely 0, 1, 2, 3 and 4 months after breaking winter dormancy, a subset of the plants were continuously labelled with 14CO2 in an ESPAS growth chamber for 28 days. After the labelling, the plants were harvested and separated into leaves, first and second year stems and roots. The soil was analysed for total C and 14C content as well as soil microbial biomass. Immediately after breaking dormancy, carbon stored in the first year stems was relocated to developing roots and leaves. Almost half the newly assimilated C was used for leaf development the first month of growth, dropping to below 15 n the older plants. Within the second month of growth, secondary growth of the stem became the largest carbon sink in the system, and remained so for the older age classes. Between 31 and 41 f the recovered 14C was allocated to below-ground pools. While the fraction of assimilated 14C in roots and root soil respiration did not vary with plant age, the amount allocated to soil and soil microbial biomass increased in the older plants, indicating an increasing rhizodeposition. The total amount of soil microbial biomass was 30 arger in the oldest age class than in an unplanted control soil. The results demonstrate a close linkage between photosynthesis and below-ground carbon dynamics. Up to 13 f the microbial biomass consisted of carbon assimilated by the willows within the past 4 weeks, up to 11 f the recovered 14C was found as soil organic matter

Antibiotic resistance genes have become highly mobile since the development of antibiotic chemotherapy. A considerable body of evidence exists proving the link between antibiotic use and the significant increase in drug-resistant human bacterial pathogens. The application of molecular detection and tracking techniques in microbial ecological studies has allowed the reservoirs of antibiotic resistance genes to be investigated. It is clear that the transfer of resistance genes has occurred on a global scale and in all natural environments. The considerable diversity of bacteria and mobile elements in soils has meant that the spread of resistance genes has occurred by all currently known mechanisms for bacterial gene transfer. Trans-kingdom transfers from plants to bacteria may occur in soil. Hot spots for gene transfer in the soil/plant environment have been described and colonized niches such as the rhizosphere and other nutrient-enriched sites, for example manured soil, have been identified as reservoirs of resistance genes. Although exposure and selection for tolerance of antibiotics is considerable in clinical environments there is increasing evidence that selection for resistant phenotypes is occurring in natural environments. Antibiotic-producing bacteria are abundant in soil and there is evidence that they are actively producing antibiotics in nutrient-enriched environments in soil. In addition there is clear evidence that the self-resistance genes found within antibiotic gene clusters of the producers have transferred to other non-producing bacteria. Perhaps most important of all is the use of antibiotics in agriculture as growth promotants and for treatment of disease in intensively reared farm animals. These treatments have resulted in gut commensal and pathogenic bacteria acquiring resistance genes under selection and then, due to the way in which farm slurries are disposed of, the spread of these genes to the soil bacterial community. Integrons with multiple resistance gene cassettes have been selected and disseminated in this way; many of these cassettes carry other genes such as those conferring heavy metal and disinfectant resistance which have been co-selected in bacteria surviving in effluents and contaminated soils, further maintaining and spreading the antibiotic resistance genes.