Deploying clean seed to slow pathogen invasions

The Guide at this link provides more detail about the points below.

Invasive pathogens and seed degeneration. When a crop pathogen is invading a country, it is particularly important to use disease-free seed to keep the pathogen from establishing in more locations. There are two considerations for invasive pathogens: each farmer’s short-term economic considerations and the benefit that all farmers in a location gain if the pathogen can be kept out longer.

Clean seed to slow invasions. National programs that produce pathogen-free seed in response to a pathogen invasion will often not be able to produce enough seed for every farmer, especially at the beginning of an invasion. When the program wants to use pathogen-free seed to ‘fight’ an invasive pathogen, it can consider both individual farmer benefits and general benefits from slowing invasions. Any farmer could benefit from pathogen-free seed. Pathogen-free seed for farmers in particular locations may also have a bigger effect for slowing the invasion.

Decisions about where to distribute clean seed. There may be two ‘extremes’ where distributing pathogen-free seed is less useful. In one extreme, if there is so much disease in the area that a plant grown from pathogen-free seed will immediately get infected from outside sources, there is only a momentary benefit. In the other extreme, if there is no disease in the area and everyone uses seed without the disease, anyway, bringing pathogen-free seed may be redundant. In between these two extremes, there are many potential situations where pathogen-free seed will be most useful to individual farmers and have the biggest impact for slowing the pathogen invasion.

Potential strategies for distribution. The most useful locations for pathogen-free seed to slow a pathogen invasion will often be near the ‘front wave’ of the epidemic, where the disease is recently present, and the pathogen may not be widely established. Deciding where exactly to target for pathogen-free seed will generally require data to characterize the epidemic.

Developing decision support for a particular epidemic. Policy makers who are deciding how to distribute pathogen-free seed can benefit from input about the likely outcomes from potential choices. Given a scenario for pathogen-free seed distribution, what are the likely epidemic outcomes?

Data needs for decision support. The first type of data is characterization of where disease is currently present. Second, characteristics of epidemics that will influence the value of a given seed distribution choice include the following. (1) The speed at which the epidemic is moving. A slower epidemic may be better managed by pathogen-free seed near the front line, while a fast-moving epidemic may be better managed by pathogen-free seed further ahead of the front line. (2) The role of each location in spread of the pathogen, through trade or vector/spore movement. (3) The importance of long-distance movement, such as through trade, where long-distance trade makes planning more challenging.

Other considerations. Policy makers need to balance a number of considerations, in addition to prioritizing slowing the spread of invasive pathogens. Choices of where to distribute disease-free seed may be driven by farmer need, as well. If disease does not spread quickly from one field to another, pathogen-free seed may be particularly important and effective for individual farmers in locations where disease is now widespread. But if farmer need is equal, and disease can spread quickly from field to neighboring field, distributing pathogen-free seed following a strategy to slow an epidemic will benefit all farmers where the disease is not yet present.

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