Proper landscape-scale deployment of disease resistant genotypes of agricultural crop species could make those crops less vulnerable to invasion by resistance breaking genotypes. Here we develop a multi-scale, spatiotemporal model of the potato late blight pathosystem to investigate spatial strategies for the deployment of host resistance. This model comprises a landscape generator, a potato late blight model, and a suite of aerobiological models, including an atmospheric dispersion model. Within individual growing regions, increasing the number of host genotypes caused the greatest reduction in epidemic extent, followed by reduction of the proportion of potato in the landscape, lowering the clustering of host fields, and reducing the size of host fields. Deployment of host resistance in genotype mixtures had a large effect on disease invasion. The use of space as an isolation barrier was effective in scenarios involving two distinct potato growing regions. It was possible to completely eliminate the risk of epidemic spread from one region to another using inter-regional separation distances ranging from 8 to 32 km. The overall efficacy of this strategy was highly dependent, however, on the degree of spatial mixing of potato genotypes within each region. Deployment of host resistance in genotype mixtures in both regions served to reduce the overall level of incidence in the landscape and the inter-regional separation distance required to eliminate relevant levels of between-region spread of diseas

Kessel, G.J.T.

Rossing, W.A.H.

Skelsey, P.

Werf, W., van der

English

Wageningen Yield 

Eleventh EuroBlight workshop
Hamar (Norway), 2008
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PPO-Special Report no. 13 (2009), 97 - 102
Invasion of a Virulent Phytophthora infestans Genotype at the 
Landscape Level; Does Spatial Heterogeneity Matter?
PETER SKELSEY 1, GEERT J. T. KESSEL2, WALTER A. H. ROSSING3, & 
WOPKE VAN DER WERF4
1Wageningen University, Department of Plant Sciences, Crop and Weed Ecology Group,
P.O. Box 430, 6700 AK Wageningen, The Netherlands
2Plant Research International, P.O. Box 16, 6700 AA Wageningen, The Netherlands
3Wageningen University, Department of Plant Sciences, Biological Farming Systems Group,
P.O. Box 9101, 6700 HB Wageningen, The Netherlands
4Wageningen University, Department of Plant Sciences, Crop and Weed Ecology Group,
P.O. Box 430, 6700 AK Wageningen, The Netherlands
SUMMARY
Proper landscape-scale deployment of disease resistant genotypes of agricultural crop species could 
make those crops less vulnerable to invasion by resistance breaking genotypes. Here we develop 
a multi-scale, spatiotemporal model of the potato late blight pathosystem to investigate spatial 
strategies for the deployment of host resistance. This model comprises a landscape generator, a potato 
late blight model, and a suite of aerobiological models, including an atmospheric dispersion model.
Within individual growing regions, increasing the number of host genotypes caused the greatest 
reduction in epidemic extent, followed by reduction of the proportion of potato in the landscape, 
lowering the clustering of host fields, and reducing the size of host fields. Deployment of host 
resistance in genotype mixtures had a large effect on disease invasion. 
The use of space as an isolation barrier was effective in scenarios involving two distinct potato 
growing regions. It was possible to completely eliminate the risk of epidemic spread from one region 
to another using inter-regional separation distances ranging from 8 to 32 km. The overall efficacy 
of this strategy was highly dependent, however, on the degree of spatial mixing of potato genotypes 
within each region. Deployment of host resistance in genotype mixtures in both regions served 
to reduce the overall level of incidence in the landscape and the inter-regional separation distance 
required to eliminate relevant levels of between-region spread of disease. 
KEYWORDS
Phytophthora infestans, invasion, Gaussian plume model, landscape design
INTRODUCTION
Crop heterogeneity can profoundly affect epidemics caused by the transmission of infectious agents 
(e.g., Zhu et al., 2000). Landscape design and strategic deployment of host resistance therefore 
emerge as a means to spatially separate aggressive resistance-breaking pathogen genotypes from 
other susceptible local host populations. In this paper we raise an important question regarding 
the spatial epidemiology of Phytophthora infestans – does spatial heterogeneity in host populations 
matter, and if so, what scale is relevant?
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We describe a simulation framework to investigate which spatial strategies reduce the development 
of epidemics when a breakthrough of resistance occurs in a proportion of the host population, i.e., 
when a new, aggressive strain of the pathogen evolves. The framework comprises a recently validated 
potato late blight model (Skelsey et al., in press) and a suite of aerobiological models, including 
a long range atmospheric spore dispersion model. Invasion opportunities of virulent pathogen 
genotypes are assessed under a variety of historical weather conditions and theoretical landscape 
configurations. The effects of landscape characteristics are studied using scenarios that vary a potato 
growing region (1 region scenarios) using a limited set of design parameters, and two spatial scales of 
mixing of host genotypes: genotype mixing within a field or between fields. Invasion opportunities 
of virulent pathogen genotypes are also assessed at larger spatial scales using scenarios that vary 
the separation distance between two distinct potato growing regions (2 region scenarios). In the 2 
region scenarios, an additional spatial scale of genotype mixing is simulated; between-region mixing, 
whereby each region is homogeneous for a particular potato genotype.
MATERIALS AND METHODS
Simulation framework 
Host and pathogen life cycles, host-pathogen-environment interactions, and fungicide applications 
(protectants and eradicants) are simulated on gridded (raster) landscapes composed of potato and 
non-potato areas. The number, size, aggregation and classification of potato fields can be varied. In 
the 1 region scenarios, each potato growing region measures 6.4 x 6.4 km. In the 2 region scenarios, 
each potato growing region measures 2 x 2 km, with a variable amount of non-potato area between 
them.
An adaptation of the spatiotemporal/integrodifference equation model of the potato late blight 
pathosystem originally developed by Skelsey et al.,(2005) is used to provide field-scale dynamics. 
In this study, two potato phenotypes are represented: susceptible and (partially) resistant. The 
susceptible phenotype represents the genotype with the broken resistance, while the other phenotype 
represents all other (partially) resistant genotypes. Partially resistant varieties can still become 
infected, although the rate of infection is far reduced in comparison to the ‘broken’ or susceptible 
genotype. Host-pathogen interactions are characterized in the model using quantitative components 
of resistance (lesion growth rate, sporulation intensity, and infection efficiency) measured in the 
laboratory on potato leaflets of two cultivars, providing parameter values for the susceptible (broken) 
and the (partially) resistant interaction (Skelsey et al., in press). The field-scale potato late blight 
model was recently validated for these and other cultivar-isolate interactions using data from field 
trials in the Netherlands (Skelsey et al., in press).
Individual fields in the landscape are linked through models describing: spore release from 
sporangiophores; spore escape from the canopy (de Jong et al., 2002); spore dispersion and 
deposition (partial-reflection Gaussian plume model - Overcamp, 1976); and spore survival during 
transportation, according to the dose of global radiation received (Mizubuti et al., 2000). 
Within this modeling framework, the composition, configuration and connectivity of host 
populations are manipulated in order to reveal their influence on epidemic progress. The results of 
these manipulations are used to develop spatial strategies for the deployment of resistance genes. 
Spatial scenario analyses – 1 region scenarios 
Four basic scenario analyses are defined. These analyses address the influence of: (1) the proportion 
of potato in the landscape; (2) the number of different resistant potato genotypes; (3) the size of 
fields; and (4) the degree of spatial clustering of potato fields on the rate and extent of invasion of a 
new, resistance breaking P. infestans genotype. In these scenarios, potato genotypes can be deployed 
at two spatial scales of mixing. Under the first scheme, designated ‘between-field mixing,’ each 
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individual field in the landscape contains a single host genotype; either resistant or susceptible. 
Under the second scheme, designated ‘within-field mixing,’ each field contains a mixture of resistant 
and susceptible varieties, i.e., a genotype mixture. 
2 region scenarios
The use of space as a barrier to pathogen invasion from one potato growing region to another is 
investigated in ‘2-region scenarios.’ This scenario analysis addresses: (5) the influence of the 
separation distance between two growing regions on epidemic extent. One region is the ‘donor’ 
(infected), and the other the ‘receptor’ (disease-free). In addition to simulating both the between-
field and within-field mixing schemes, a third level of genotype mixing is included: ‘between-region 
mixing,’ whereby the donor region is entirely composed of the susceptible potato phenotype, and the 
receptor region is entirely composed of the resistant potato phenotype. The spatial parameter values 
defining all 5 sets of spatial scenarios are given in Table 1. The parameter setting defining landscape 
maps, as in Table 1, in combination with the spatial scale of mixing genotypes, defines a scenario.
RESULTS AND DISCUSSION
The results of this study indicate that spatial heterogeneity in host populations does matter for 
P.  infestans, and that landscapes can be designed that suppress invasions of virulent pathogen 
genotypes. The more effective of the strategies tested for individual growing regions were those 
that increased the number of potato genotypes, and/or increased the degree of spatial mixing of 
genotypes, i.e. within-field genotype mixing (Fig. 1). Landscape designs that focused on spatial 
isolation of aggressive pathogen strains through manipulation of field size and clustering of potato 
fields were found to have limited effect. There was little evidence of thresholds in the response of 
epidemics to manipulation of spatial landscape variables, i.e., in the 1 region scenarios, no situations 
were identified in which spatial design had quantum effects on the prevalence of P. infestans. 
The use of space as an isolation barrier was effective in the 2 region scenarios, but the efficacy of this 
strategy was again dependent on the degree of spatial mixing of potato genotypes (Fig. 2).
The results of the 2 region scenarios act to confirm the scant information in the literature on the 
large capacity of P. infestans for long distance dispersal (e.g., Zwankhuizen et al., 1998; Mizubuti 
et al., 2000; Sunseri et al., 2002). Nevertheless, it was possible to completely eliminate the risk of 
epidemic spread from one region to another. Geographic separation of growing regions according 
to potato phenotype (between-region mixing) was particularly successful in this respect, with a 
Table 1.  Spatial parameter values defining the five sets of spatial scenarios
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Figure 1.  Influence of spatial host population characteristics on the spatial extent of simulated potato late blight 
epidemics within individual growing regions. Panels A to D correspond to spatial scenario analyses 1 
to 4 (Table 1). Incidence is defined as the number of potato hectares infected (disease severity ≥ 1%) 
relative to the number of potato hectares in the landscape. Circular data markers show predictions 
for the between-field genotype mixing scheme, and triangular data markers show predictions for the 
within-field genotype mixing scheme.
Figure 2.  Influence of inter-regional separation distance and spatial scale of mixing of potato genotypes on the 
spread of potato late blight disease from a ‘ donor’ (inoculated) to a ‘receptor’ (disease-free) potato 
growing region (Table 1; scenario analysis 5): (A) between-field genotype mixing; (B) within-field 
genotype mixing; and (C) between-region genotype mixing. Incidence is calculated in two ways: 
open data markers show the number of potato hectares infected (disease severity ≥ 1%) relative to 
the number of potato hectares in the landscape, and solid data markers show the number of potato 
hectares infected in the receptor region relative to the number of potato hectares in the receptor region.
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complete elimination of between-region spread at a separation distance of 8 km. As landscape 
planners, however, we must concern ourselves with the total amount of disease in the landscape 
(both donor and receptor regions). Landscape incidence levels were markedly reduced when the 
degree of spatial mixing of potato genotypes within regions was increased.
CONCLUSIONS
In combining this knowledge, we arrive at a unified spatial strategy for deployment of host 
resistance, with a view to suppressing invasions of a virulent strain of P. infestans: landscape-wide, 
homogenous (non-aggregated) deployment of diverse genotype mixtures in small fields. Simulation 
results suggest that this strategy would be effective in reducing spatial increase in disease within and 
between growing regions, and thus in minimizing the consequences of a breakthrough in resistance. 
Secondary effects could include an increase in the performance and durability of resistance, and a 
reduction in the need for plant protection products.
ACKNOWLEDGEMENTS
Funding for this study was provided by the Dutch Ministry of Agriculture, Nature Management and 
Fisheries through the DuRPh project - theme: “Resistance management,” project no. 3340043100, 
and through the Umbrella Plan Phytophthora (DWK 427). Thanks to Joost de Groot for his help 
with the computer cluster. 
REFERENCES
de Jong, M., Bourdôt, G.W., Powell, J., and Goudriaan, J. 2002. A model of the escape of Sclerotinia 
sclerotiorum ascospores from pasture. Ecological Modelling 150, 83-105.
Mizubuti, E.S.G., Aylor, D.E. and Fry, W.E. 2000. Survival of Phytophthora infestans sporangia 
exposed to solar radiation. Phytopathology 90, 78-84. 
Overcamp, T.J. 1976. A general Gaussian diffusion-deposition model for elevated point sources. 
Journal of Applied Meteorology 15, 1167-1171.
Skelsey, P., Kessel, G.J.T., Rossing, W.A.H., and van der Werf, W. Parameterization and evaluation 
of a spatiotemporal model of the late blight pathosystem. Phytopathology. In press.
Skelsey, P., Rossing, W.A.H., Kessel, G.J.T., Powell, J., and van der Werf, W. 2005. Influence of 
host diversity on development of epidemics: an evaluation and elaboration of mixture theory. 
Phytopathology 95, 328-338. 
Sunseri, M.A., Johnson, D.A., and Dasgupta, N. 2002. Survival of detached sporangia of 
Phytophthora infestans exposed to ambient, relatively dry atmospheric conditions. American 
Journal of Potato Research 79, 443-450. 
Zhu, Y., Chen, H., Fan, J., Wang, Y., Li, Y., Chen, J., Fan, J., Yang, S., Hu, L., Leung, H., Mew, 
T.W., Teng, P.S., Wang, Z., and Mundt, C.C. 2000. Genetic diversity and disease control in 
rice. Nature 406, 718-722.
Zwankhuizen, M.J. 1998. Development of potato late blight epidemics: disease foci, disease 
gradients, and infection sources. Phytopathology 88, 754-763.
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A general Gaussian diffusion-deposition model for elevated point sources.

A model of the escape of Sclerotinia sclerotiorum ascospores from pasture.

Development of potato late blight epidemics: disease foci, disease gradients, and infection sources.

Genetic diversity and disease control in rice.

Influence of host diversity on development of epidemics: an evaluation and elaboration of mixture theory.

Parameterization and evaluation of a spatiotemporal model of the late blight pathosystem.

Survival of detached sporangia of Phytophthora infestans exposed to ambient, relatively dry atmospheric conditions.

Survival of Phytophthora infestans sporangia exposed to solar radiation.

Invasion of a Virulent Phytophthora infestans Genotype at the Landscape Level; Does Spatial Heterogeneity Matter?

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Invasion of a Virulent Phytophthora infestans Genotype at the Landscape Level; Does Spatial Heterogeneity Matter?

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