1,685 research outputs found
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
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
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
Virtueller Sex und die uneindeutigen Folgen – ein systemtheoretisch inspirierter Ausblick
No full textLewandowski S. Virtueller Sex und die uneindeutigen Folgen – ein systemtheoretisch inspirierter Ausblick. In: Lemmen K, Schepers J, Sweers H, Tillmann K, eds. Sexualität wo•hin ? Hinblicke. Einblicke, Ausblicke. Aids-Forum DAH . Vol 49. Berlin: Deutsche Aids-Hilfe; 2005: 127–131
Exploring cause and effect relationships of soil fertility on corn yield variability.
No full textIn an experimenl conducted 10 asses lhe potencial causes of corn yield variabilily, the variation of phosphorus (P) and manganese (Mn), accounted for 79% of the variability in grain yield. Based on yield map the experimental field was divided in two general areas of management. One area (a) with grain yield below the average (11.30 Mg ha-1) and another one (b) with grain yield above average. Three treatments were applied 10 the corn: (1) 92 kg of P2O5 ha.-1 banded at 10 cm beside of planls and incorporated inlo lhe soil; (2) Mn applied on the com foliage at rate of 1.1 kg ha.-1 at the four and eight- leaf growth stages and, (3) combination of trealment 1 plus 2. Also, with lhe use of CERES-Maize model we eslimated the corn grain yield in area (a) by simulating two situations: (1) appliction of manure at 25 Mg ha-1 (dry matter basis), plus 100 kg ha-1 de ammonium phosphale al sowing time and 50 kg of N ha-1 applied side-dress as ammonium nitrate; (2) no manure, 100 kg ha-1 de ammonium phosphate at sowing time and 100 kg of N ha-1 applied side-dress as ammonium nitrate. The application of P and Mn fertilizers did not improve significantly the corn grain yield. Compared to the control, extra fertilizer application increase the grain yield by 1.25,0,86, and 1.27 Mg ha-1 due to applicalion of Mn, P, and combination of both fertilizers, respectively. These findings indicate that either yield was limited by constraints other than P and Mn, or application of P or Mn fertilizers did not adequately alleviate the deficiency of these nutrients in eroded soils. Corn grain yields measured in area (a) in 1997, ranged from 8.5 to 11.0 Mg ha-1 and averaged 10.5 Mg ha-1. The grain yield of 10.9 Mg ha-1simulated by CERES-Maize model withoul use of manure, was similar 10 the yield (10.5 Mg ha-1) measured for area (a). With the use of manure, the CERES-Maize model simulated a grain yield of 14.1 Mg ha-1, similar to the high grain yield (13.8 Mg ha-1) measured on the besl area of the field (area b)
Die Entwicklung von Moralität im Kindes- und Jugendalter
No full textPöge A, Schepers D. Die Entwicklung von Moralität im Kindes- und Jugendalter. In: Wallner S, Weiss M, Reinecke J, Stemmler M, eds. Devianz und Delinquenz in Kindheit und Jugend. Neue Ansätze der kriminologischen Forschung. Wiesbaden: Springer VS; 2019: 235-258
Erratum to: High rates of clinically relevant incidental findings by total-body CT scanning in trauma patients: Results of the REACT-2 trial
No full textA technical error led to incorrect rendering of the author group in this article. The correct authorship is as follows: K. Treskes1, S.A. Bos1, L.F.M. Beenen2, J.C. Sierink1, M.J.R. Edwards3, B.J.A. Beuker4 , G.S.R. Muradin5, J. Hohmann6, J.S.K. Luitse1, M.W. Hollmann7, M.G.W. Dijkgraaf8, J.C. Goslings1, on behalf of the REACT-2 study group The collaborators are: T.P. Saltzherr, T. Schepers, V.M. de Jong, R. van Vugt, M. Brink, J. Peters, M. El Moumni, J.S. Harbers, K.W. Wendt, E.M.M. van Lieshout, M.J. Elzinga, E.H. Jansen, C. Zähringer, N. Bless, R. Bingisser The original publication has been corrected
Lokus van beheer en transformasionele leierskap
No full textLocus of control and transformational leadership. The aim of this study was to determine the relationship between locus of control and transformational leadership. These constructs were selected by virtue of the lack of empirical research regarding the assumed relationship between them. The Locus of Control Inventory (LCI) of Schepers (1998) and the Multifactor Leadership Questionnaire (MLQ), Form 5R of Bass and Avolio (1993) were administered to 102 respondents. The results indicate statistically significant relationships between: internal locus of control and transformational leadership; external locus of control and transactional leadership; and autonomy and transformational leadership.
Opsomming
Die doel van die studie was om die verband tussen lokus van beheer en transformasionele leierskap te bepaal. Hierdie konstrukte is gekies op grond van die gebrek aan empiriese navorsing rakende die veronderstelde verband tussen die genoemde konstrukte. Die Lokus van Beheer-vraelys van Schepers (1998) en die Multifaktor Leierskapsvraelys (MLV),Vorm 5Rvan Bass en Avolio (1993) is op 102 respondente toegepas. Die resultate toon statisties beduidende verbande aan tussen: interne lokus van beheer en transformasionele leierskap; eksteme lokus van beheer en transaksionele leierskap; en outonomie en transformasionele leierskap
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