Economic inefficiencies in private management of epidemics spreading between farms

Most plant disease epidemics spread both within and between farms. However, in the absence of collective action, each farmer generally takes disease control decisions based on personal costs and benefits. It is important to identify under which conditions the combination of such private control decisions can have synergistic or antagonistic effects, and can lead to collective economic inefficiencies. We used the game theory framework to investigate these questions, considering a simplified two-period game where two farmers decide whether or not to control an epidemic on their farm. Taking the example of sharka epidemics, caused by plum pox virus in Prunus orchards, we characterized the game and its outcomes according to initial epidemic conditions and focused on those likely to produce economic inefficiencies. Our results show that depending on the initial infection levels, a broad range of games may arise, some of which involving synergistic or antagonistic control decisions. This means that the nature of strategic interactions between famers may change depending on the state of the epidemic. After a thorough characterization of the epidemic conditions for which private management produces collective economic inefficiencies, we investigated the expected effect of different public policy incentives aiming to reduce such inefficiencies.