Abstract
Seed systems are critical for deployment of improved varieties but also can serve as major conduits for the spread of seedborne pathogens. As in many other epidemic systems, epidemic risk in seed systems often depends on the structure of networks of trade, social interactions, and landscape connectivity. In a case study, we evaluated the structure of an informal sweet potato seed system in the Gulu region of northern Uganda for its vulnerability to the spread of emerging epidemics and its utility for disseminating improved varieties. Seed transaction data were collected by surveying vine sellers weekly during the 2014 growing season. We combined data from these observed seed transactions with estimated dispersal risk based on village-to-village proximity to create a multilayer network or “supranetwork.” Both the inverse power law function and negative exponential function, common models for dispersal kernels, were evaluated in a sensitivity analysis/ uncertainty quantification across a range of parameters chosen to represent spread based on proximity in the landscape. In a set of simulation experiments, we modeled the introduction of a novel pathogen and evaluated the influence of spread parameters on the selection of villages for surveillance and management. We found that the starting position in the network was critical for epidemic progress and final epidemic outcomes, largely driven by node out-degree. The efficacy of node centrality measures was evaluated for utility in identifying villages in the network to manage and limit disease spread. Node degree often performed as well as other, more complicated centrality measures for the networks where village-to-village spread was modeled by the inverse power law, whereas betweenness centrality was often more effective for negative exponential dispersal. This analysis framework can be applied to provide recommendations for a wide variety of seed systems.
Highlights
The identification of key locations for surveillance and management is an important problem in plant disease epidemiology, and it remains an open question whether the same locations are best for both in any given system
We evaluated the structure of an informal sweet potato seed system in the Gulu region of northern Uganda for its vulnerability to the spread of emerging epidemics and its utility for disseminating improved varieties
We considered a range of values of the spread parameters of both the inverse power law and negative exponential models selected based on the assumption that there is a greater tendency for farmers to exchange planting material with neighboring villages than with those that are distant (Perales et al 2005; Pusadee et al 2009), and the probability of vector spread decreases with increasing distance
Summary
The identification of key locations for surveillance and management is an important problem in plant disease epidemiology, and it remains an open question whether the same locations are best for both in any given system. There are often multiple mechanisms for pathogen dispersal that must be integrated in models of dispersal risk to identify key locations. Formal and informal seed trade networks and the dispersal of pathogens by vectors may both be important risk components. Seed systems, are a critical component of global food security, but often they serve as Current address of C. E. Buddenhagen: AgResearch, Ruakura, Hamilton 3214, New Zealand
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