Abstract
We use a spatially explicit, stochastic model to analyse the effectiveness of different scales of local control strategies in containing the long-term, multi-seasonal spread of a crop disease through a dynamically changing population of susceptible crops in which there is cryptic infection. The model distinguishes between susceptible, infested and symptomatic fields. It is motivated by rhizomania disease on sugar beet in the UK as an exemplar of a spatially structured and partially asymptomatic epidemic. Our results show the importance of matching the scales of local control strategies to prevent intensification and regional spread of disease with the inherent temporal and spatial scales of an epidemic. A simple field-scale containment strategy, whereby the susceptible crop is no longer grown on fields showing symptoms, fails for this system with cryptic infection because the locally applied control lags behind the epidemic. A farm-scale strategy, whereby growers respond to the disease status of neighbouring farms by transferring their quota for sugar beet to farmers in regions of reduced risk, succeeds. We conclude that a soil-borne pathogen such as rhizomania could be managed by movement of susceptible crops in the landscape using a strategy that matches the temporal and spatial scales of the epidemic and which take account of risk aversion among growers. We show some parallels and differences in effectiveness between a ‘culling’ strategy involving crop removal around emerging foci and the local deployment of partially resistant varieties that reduce amplification and transmission of inoculum. Some relationships between the control of plant and livestock diseases are briefly discussed.
Highlights
The invasion of diseases at national and international scales is a frequent and recurring problem in crop and livestock production, in natural and semi-natural ecosystems and human populations
The model shows proliferation of disease around the three sites of initial infestation. This is supported by empirical observation of the continued spread of rhizomania in the UK during the period in which the containment policy was in place (Stacey et al 2004)
Our results show the importance of matching the scales of local control strategies to prevent intensification and regional spread of disease with the inherent temporal and spatial scales of an epidemic, using rhizomania disease of sugar beet as a motivation for the analyses
Summary
The invasion of diseases at national and international scales is a frequent and recurring problem in crop and livestock production, in natural and semi-natural ecosystems and human populations. Et al 2001), cassava mosaic disease in Africa (Legg 1999) and Asian soybean rust in Brazil Global control strategies, such as the deployment of resistance genes and chemical control in crops or prophylactic vaccination of livestock, are seldom feasible for many of these emerging epidemics. Selection of an appropriate scale, to prevent invasion is complicated by the cryptic spread of infection ahead of the appearance of symptomatic individuals (Dybiec et al 2004; Tildesley et al 2006) It is complicated by spatial and temporal heterogeneities in the spread of disease. These reflect the spatial arrangement of susceptible sites (fields or herds) in the landscape, associated with crop or animal husbandry, as well as seasonal disturbances, such as harvesting or culling It follows that many emerging diseases appear to Received 21 February 2007 Accepted 30 May 2007
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