Papers by Hendrik Pieter Spijkerboer
From lesion to region: epidemiology and management of potato late blight
Aerial dispersal of Phytophthora infestans spores from distant sources to crops is an essential p... more Aerial dispersal of Phytophthora infestans spores from distant sources to crops is an essential part of
the epidemiology of potato late blight. This makes late blight a regional problem. An interdisciplinary
analysis of the regional late blight problem is carried out through model development, experimental
parameterisation and analysis and scenario studies that investigate possibilities for effective control of
the disease at the regional level.
A new equation was derived to estimate the relative exponential growth rate r (d−1) of a plant
disease epidemic from commonly used component parameters for pathogen aggressiveness and host
resistance, such as the latency period, infection efficiency, sporulation intensity and lesion growth rate.
The use of the equation is demonstrated with field measurements of resistance components against late
blight for five potato cultivars. Infection efficiency and lesion growth rate together explained most of
the variation in cultivar resistance.
To describe the dispersal of spores at distances up to 10 km downwind from a source of inoculum,
the Gaussian plume model was used. A field experiment was set up to calibrate the Gaussian plume
model, as applied to the dispersal of spores. A comparison of estimated concentrations with the
measurements confirmed that spore clouds originating from a point source take the form of a Gaussian
plume: the coefficient of correlation between measured spore concentrations and fitted concentrations
was 0.8. The fraction of spores that escaped the canopy and was available for long distance dispersal
amounted to 64% ± 17%.
To model deposition and loss of spores from the spore plume at distances between 50 m and 10 km
from the source, the source depletion method was used. This is a practical method, but it is simplified
in its description of spore loss. The accuracy of the source depletion method was determined by
comparing it with the more realistic surface depletion method in a modelling study. It was found that
under worst case conditions, the source depletion method may lead to an error of at most a factor 4 in
calculated deposition of Phytophthora infestans spores.
The infection pressure on receptor crops caused by inoculum from a distant source was calculated
with a newly developed model. The sensitivity analysis showed that disease level at the source had by
far the greatest impact on infection pressure, followed by distance from the source. Subsequent
scenario studies indicated that eradication of sources with high disease levels and spatial separation of
cropping systems with different disease tolerances are more effective than use of more resistant
cultivars for the receptor crop or a ban on the growing of susceptible cultivars.
The conditions and possibilities for practical implementation of the effective control strategies as
well as their consequences for fungicide requirements are discussed.
Science lacks a unanimous conceptualisation of what is ‘evolution’. There is of course the biolog... more Science lacks a unanimous conceptualisation of what is ‘evolution’. There is of course the biological theory of Darwin. But confusion arises when people use evolution also to speak about successions of forms in relation to ideas, cars, designs, galaxies etc. In relation to existing problems with the concept of evolution, a recent study in Nature asks the question of whether, after Darwin and the Modern Synthesis, the concept of evolution is in need of a ‘rethink’. To contribute to this matter, we explore the possibility of constructing an a priori framework for the conceptualisation of evolution that is valid both inside and outside biology. We expect that such a framework will also offer a context for relating different measures of evolution.
To work towards a conceptual framework, the present study includes the following steps: 1. An introduction to the problem, 2. A review of historical and modern approaches to evolution for identifying major schools of thought, 3. An analysis of “What is evolution actually?”, which leads to the suggestion that the concept of evolution combines both processes and ‘measures’, which combination we identify as a ‘pattern of evolution’. 4. To define individual patterns of evolution, one needs a proper context. As a context we invoke a recently developed systems theory, named the ‘operator hierarchy’. The operator hierarchy allows a stringent typing and hierarchical ranking of entities, which helps us solving questions about ‘objects’ and ‘levels’ that play a role in patterns of evolution. 5. We construct a minimal pattern of evolution and show how this can be extended to a pattern of life history evolution. 6. As a next step we integrate ‘combinatorial’ evolution (Brian Arthur 2009), by making a distinction between evolution processes in which the number of objects increases (which we refer to as diffluent evolution), and processes in which such a number decreases, because particles combine to a single product (which we refer to as confluent evolution). 7. We continue with an exploration of an abstract pattern of evolution allowing the integration of abiotic processes into what we named Big Evolution; the chain of events which connects all the particle types in the operator theory. 8. Based on the preceding steps we suggest a general conceptual framework that includes a ‘family’ of patterns of evolution. If we apply the patterns of evolution in this family, certain classical uses of ‘evolution’ no longer comply with the criteria. In relation to this, and with the aim of reducing confusion and ambiguity, we suggest a renaming of certain current uses of ‘evolution’. 9. We discuss the implications of our results and explore directions for further research.
All the topics that are discussed in this study are linked by a common conceptual approach. For this reason we have chosen to publish the contents as a single manuscript, instead of distributing the ideas over a range of strongly interdependent publications. While this choice has resulted in a long text, the reader can now access all the information from a single source.
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Papers by Hendrik Pieter Spijkerboer
the epidemiology of potato late blight. This makes late blight a regional problem. An interdisciplinary
analysis of the regional late blight problem is carried out through model development, experimental
parameterisation and analysis and scenario studies that investigate possibilities for effective control of
the disease at the regional level.
A new equation was derived to estimate the relative exponential growth rate r (d−1) of a plant
disease epidemic from commonly used component parameters for pathogen aggressiveness and host
resistance, such as the latency period, infection efficiency, sporulation intensity and lesion growth rate.
The use of the equation is demonstrated with field measurements of resistance components against late
blight for five potato cultivars. Infection efficiency and lesion growth rate together explained most of
the variation in cultivar resistance.
To describe the dispersal of spores at distances up to 10 km downwind from a source of inoculum,
the Gaussian plume model was used. A field experiment was set up to calibrate the Gaussian plume
model, as applied to the dispersal of spores. A comparison of estimated concentrations with the
measurements confirmed that spore clouds originating from a point source take the form of a Gaussian
plume: the coefficient of correlation between measured spore concentrations and fitted concentrations
was 0.8. The fraction of spores that escaped the canopy and was available for long distance dispersal
amounted to 64% ± 17%.
To model deposition and loss of spores from the spore plume at distances between 50 m and 10 km
from the source, the source depletion method was used. This is a practical method, but it is simplified
in its description of spore loss. The accuracy of the source depletion method was determined by
comparing it with the more realistic surface depletion method in a modelling study. It was found that
under worst case conditions, the source depletion method may lead to an error of at most a factor 4 in
calculated deposition of Phytophthora infestans spores.
The infection pressure on receptor crops caused by inoculum from a distant source was calculated
with a newly developed model. The sensitivity analysis showed that disease level at the source had by
far the greatest impact on infection pressure, followed by distance from the source. Subsequent
scenario studies indicated that eradication of sources with high disease levels and spatial separation of
cropping systems with different disease tolerances are more effective than use of more resistant
cultivars for the receptor crop or a ban on the growing of susceptible cultivars.
The conditions and possibilities for practical implementation of the effective control strategies as
well as their consequences for fungicide requirements are discussed.
To work towards a conceptual framework, the present study includes the following steps: 1. An introduction to the problem, 2. A review of historical and modern approaches to evolution for identifying major schools of thought, 3. An analysis of “What is evolution actually?”, which leads to the suggestion that the concept of evolution combines both processes and ‘measures’, which combination we identify as a ‘pattern of evolution’. 4. To define individual patterns of evolution, one needs a proper context. As a context we invoke a recently developed systems theory, named the ‘operator hierarchy’. The operator hierarchy allows a stringent typing and hierarchical ranking of entities, which helps us solving questions about ‘objects’ and ‘levels’ that play a role in patterns of evolution. 5. We construct a minimal pattern of evolution and show how this can be extended to a pattern of life history evolution. 6. As a next step we integrate ‘combinatorial’ evolution (Brian Arthur 2009), by making a distinction between evolution processes in which the number of objects increases (which we refer to as diffluent evolution), and processes in which such a number decreases, because particles combine to a single product (which we refer to as confluent evolution). 7. We continue with an exploration of an abstract pattern of evolution allowing the integration of abiotic processes into what we named Big Evolution; the chain of events which connects all the particle types in the operator theory. 8. Based on the preceding steps we suggest a general conceptual framework that includes a ‘family’ of patterns of evolution. If we apply the patterns of evolution in this family, certain classical uses of ‘evolution’ no longer comply with the criteria. In relation to this, and with the aim of reducing confusion and ambiguity, we suggest a renaming of certain current uses of ‘evolution’. 9. We discuss the implications of our results and explore directions for further research.
All the topics that are discussed in this study are linked by a common conceptual approach. For this reason we have chosen to publish the contents as a single manuscript, instead of distributing the ideas over a range of strongly interdependent publications. While this choice has resulted in a long text, the reader can now access all the information from a single source.