4,451 research outputs found
Numeriek model voor de bepaling van het zandtransport onder invloed van golven
No full textEr zijn door o.a. Vetlinga en Schepers proeven gedaan in een laboratoriumgoot ter bepaling van zandtransport in de voortplantingsrichting van golven onder invloed van die golven. Doel van deze proeven is om neer inzicht te krijgen in de opwoeling van het bcdemmateriaal aan de bcdem en op die manier in het zandtransportmechanisme . In al die proeven is als bcdemmateriaal zand van verschillende dianeter gebruikt. Aangezien het verschijnsel aan de bcdem zeer gecompliceerd is, is uitgegaan van een benadering via een rechtstreekse bepaling van het zandtransport. Bij de proeven van Schepers, die tevens een aantal proeven deed met fijn zand, D50 = 125 um, bleek dat dit zand anders reageerde op de golfbeweging dan het wat grovere zand door Vellinga en Schepers gebruikt. Omdat daar in eerste instantie geen sluitende verklaring voor was te vinden is een aanzet gemaakt tot een cornputerberekening. Aan de hand van een geschematiseerde aanpak van het opwoelingsmechanisme en het neerslaan van dit zand is het effect van de valsnelheid te onderzoeken. In eerste instantie is, het probleem simpel gehouden om duidelijk te herkennen wat een variatie in de valsnelheid voor gevolgen heeft. Omdat ook het faseverschil tussen de top van de lste harmonische en de top van de resulterende 2de hanronische een rol speelt, is dit aan de hand van berekeningen nagegaan. Bij enkele berekeningen is een eenparige stroom toegevoegd. Hoewel dUidelijk was dat het effect hiervan niet zou overeenkomen met de proeven van Schepers op dit gebied, was het als indicatie voor proeven met wat grotere eenparige stroming waardevol.Hydraulic EngineeringCivil Engineering and Geoscience
Experimental control strategies reducing the fungicide input at a practical scale
No full textPhytophthora infestans is the most devastating disease in potato cultivation. Chemical control is necessary to ensure a healthy crop. At the same time Dutch governmental policy asks for a reduction of the environmental impact of potato late blight control by 75% in 2012 as compared to 1996-1998. The aim of the experiments was to compare Decision Support Systems with different approaches to blight risk management for their ability to reduce the fungicide input without compromising control efficacy. Considerable savings, up to 81% when compared to weekly, full dose rate, spray schedules, can be achieved by using information on cultivar resistance, length of the critical period and disease pressure to decide whether or not to spray. The subroutine calculating the potential for viable transport of spores is only effective on resistant varieties as the threshold was exceeded with every critical period on less resistant cultivars. Implications of the experimental control strategies for agricultural practise are discusse
Epidemiology and integrated control of Potato Late Blight in Europe
No full textPhytophthora infestans, the causal agent of late blight, is a major threat to potato production in northwestern Europe. Before 1980, the worldwide population of P. infestans outside Mexico appeared to be asexual and to consist of a single clonal lineage of A1 mating type characterized by a single genotype. It is widely believed that new strains migrated into Europe in 1976 and that this led to subsequent population changes including the introduction of the A2 mating type. The population characteristics of recently collected isolates in NW Europe show a diverse population including both mating types, sexual reproduction and oospores, although differences are observed between regions. Although it is difficult to find direct evidence that new strains are more aggressive, there are several indications from experiments and field epidemics that the aggressiveness of P. infestans has increased in the past 20 years. The relative importance of the different primary inoculum sources and specific measures for reducing their role, such as covering dumps with plastic and preventing seed tubers from becoming infected, is described for the different regions. In NW Europe, varieties with greater resistance tend not to be grown on a large scale. From the grower’s perspective, the savings in fungicide input that can be achieved with these varieties are not compensated by the higher (perceived) risk of blight. Fungicides play a crucial role in the integrated control of late blight. The spray strategies in NW Europe and a table of the specific attributes of the most important fungicides in Europe are presented. The development and use of decision support systems (DSSs) in NW Europe are described. In The Netherlands, it is estimated that almost 40% of potato growers use recommendations based on commercially available DSS. In the Nordic countries, a new DSS concept with a fixed 7-day spray interval and a variable dose rate is being tested. In the UK, commercially available DSSs are used for c. 8% of the area. The validity of Smith Periods for the new population of P. infestans in the UK is currently being evaluated
Development and persistence of resistance to fungicides in Sphaerotheca fuliginea in cucumbers in the Netherlands
No full textErgosterol biosynthesis inhibitors (EBIs) have a remarkably broad spectrum of antifungal activity. They belong to the commercial fungicides which exhibit the highest activity known to date. Resistance to EBIs was found in vitro, but the level of resistance and the decreased fitness of resistant strains led to the hypothesis that resistance to EBIs was rather unlikely to develop in practice. Despite the fact that EBIs were introduced as early as 1972, up to 1980 only data on EBI-resistant laboratory strains were available. However, gradually there appeared reports on decreased sensitivity to EBIs in practice. This made it of interest to investigate the potential development of resistance to EBIs in vivo.In this study the pathogen used to gather the desired information was Sphaerotheca fuliginea, as this fungus is widespread in cucumber glasshouses in the Netherlands, intensively controlled with fungicides and resistance-prone. This thesis contains five papers describing several aspects of resistance to fungicides in S.fuliginea.In the first paper it has been shown that S.fuliginea is present on cucumber plants all year round. A gradual increase in the number of infected crops, in the form of S-shaped curves, was observed from planting until May. In the districts with the highest crop density (Pijnacker), mildew was generally observed early in the growing season, while the apparent rate of increase of infected crops was higher than in other districts. In the district with the lowest crop density (Northern Netherlands), mildew was observed late in the growing season and the apparent rate of increase of infected crops was low. Early in the growing season the pathogen is probably dispersed by transportation of infected planting stock and by man. When the disease pressure increases, inoculum is probably dispersed by wind. This implies that inoculum is highly mobile and that, in the case of development of resistance to fungicides, the resistant strains will disperse to crops where the spray regime did not cause resistance to develop.As S. fuliginea developed resistance to dimethirimol, benzimidazoles and pyrazophos in 1971, 1972 and 1979, respectively, it was thought that presence of strains resistant to these fungicides might provide information on the long-term fitness of these strains. The results of an investigation are presented in the second paper. Strains resistant to these fungicides still persisted in the pathogen population. Although this finding may indicate that the long-term fitness of these strains is equal to that of wild-type strains, several other factors might be involved in the persistence of resistance.From 1981 to 1984, S.fuliginea in the Netherlands was primarily controlled with EBIs. The short-term fitness of isolates of S. fuliginea resistant to EBIs and their behaviour under practical conditions are described in the third, fourth and fifth paper.Several fitness parameters of EBI-resistant isolates collected from commercial glasshouses were compared to those of isolates with a wild-type sensitivity to EBIs. Fitness parameters studied were germination of conidia, growth of germ tubes and mycelium, penetration, sporulation and competitive ability. The results are described in the third paper. One or more values of fitness parameters for EBI-resistant isolates were significantly lower than those for the wild-type isolates. However, in general it was concluded that the fitness of EBI-resistant S. fuliginea isolates, collected in commercial glasshouses, was hardly reduced. Within the group of EBI-resistant isolates tested no significant relation was observed between the degree of resistance to EBIs and the degree of fitness.In the fourth and fifth paper the resistance to EBIs of S.fuliginea over a three-year period has been described. In 1981 the sensitivity of glasshouse isolates to EBIs was lower than that of the wild-type isolates. In 1982 and 1983 the sensitivity decreased further. In 1984, no significant differences in sensitivity with the 1983 level were apparent. Per district, the degree of resistance appeared to be positively correlated to the frequency of application of EBIs. Isolates collected in the district of Limburg, where EBIs were applied less frequently than in the district of Pijnacker, showed a significantly higher sensitivity to EBIs than isolates collected in Pijnacker.Triforine showed a very low efficacy in controlling glasshouse isolates. This is in accordance with the experience of growers who, therefore, only use triforine when the disease pressure is low. It is concluded that the decreased sensitivity to triforine can be regarded as resistance. In the case of bitertanol, fenarimol and imazalil, a change to shorter spray intervals has up till now been sufficient to achieve proper control.Contrary to earlier presumptions that development of resistance to EBIs seemed rather unlikely, it gradually developed in normal commercial practice. The slowly decreasing sensitivity to EBIs of cereal powdery mildew is in line with the results described in this thesis. Strategies to delay resistance to EBIs are necessary. Alternation with other site-specific and multi-site fungicides must be advised and can be applied immediately. Non-EBI fungicides are registered for control of cucumber powdery mildew in the Netherlands. Appropriate information and guidance by extension officers and fungicide manufacturers may lower the reluctance of growers to use these strategies.In the long run, control of S. fuliginea might only be maintained by a form of integrated control, which accepts less than perfect disease control. Biological and chemical control, induced resistance of cucumber plants, epidermal coating and mildew-tolerant cucumber cultivars are elements that may fit into a strategy for integrated control of cucumber powdery mildew
Special issue dedicated to Professor Johann M. Schepers
No full textIt is a great pleasure for me to have been associated with this special issue of the SA Journal of Industrial Psychology dedicated to Professor Johann M. Schepers. The purpose of the special issue is to honour Professor Schepers for his contributions to the development of Psychology and Industrial Psychology as empirical fields of study in South Africa. The contributors have worked with Professor Schepers as students or colleagues and share his academic interests. The articles reflect his areas of interest and employ analytic techniques taught and championed by him. We are grateful to Professor Schepers for his cooperation throughout this project. Thanks are due to all the contributors and referees
A Simple Roller-Mill Grinding Procedure for Plant and Soil
No full textABSTRACT Obtaining finely ground, homogeneous plant and soil samples for chemical analysis without cross contamination is a major concern when using direct combustion procedures for determination of total nitrogen (N), total carbon (C), and isotope-ratio analysis. A rollermilling device, using square glass bottles instead of round grinding bottles, was evaluated. Rotation of the square bottles on the roller mechanism increased the milling action by the stainless steel rods inside the bottles. The roller-milling device with square grinding bottles resulted in a sample with smaller particles (99.2% <100 mesh) than with a ball-milling device (83% <100 mesh). The roller-mil
Posibilidad y contingencia. Historia de la terminología filosófica anterior a Leibniz
No full textFil: Schepers, Heinrich. Münster
Minutes of the Classis of Holland
No full textAlbertus C. Van Raalte informed the Classis of Holland on April 29, 1852, that he had received a letter from the Rev. J. J. Schepers written on February 18 which requests that the Classis install Schepers as pastor of the congregation. However, the congregation does not promise to unite with the Albany denomination. But Van Raalte and the Classis feel that this congregation had no right to leave the congregation in Graafschap and without authorization appointed Schepers as pastor.https://digitalcommons.hope.edu/vrp_1850s/1223/thumbnail.jp
Appreciating Johann M. Schepers
No full textAs an expert leader in psychometrics, eminent scholar, gatekeeper, study leader and mentor, Johann M. Schepers has had a profound effect on the development of Psychology and Industrial Psychology in South Africa. By means of an appreciative inquiry the outstanding ability of this man has been highlighted in stories which resulted in a rich profile and a legacy that needs to be protected and nurtured
Appreciating Johann Schepers.
No full textAs an expert leader in psychometrics, eminent scholar, gatekeeper, study leader and mentor, Johann M. Schepers has had a profound effect on the development of Psychology and Industrial Psychology in South Africa. By means of an appreciative inquiry the outstanding ability of this man has been highlighted in stories which resulted in a rich profile and a legacy that needs to be protected and nurtured
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