Reducing disease risks to British forests: an exploration of costs and benefits of nursery best practices

Trade-disseminated Phytophthora species are implicated in a recent upsurge of Phytophthora diseases in the UK and the establishment and spread of these pathogens in the wider UK environment via diseased propagation material. The costs and benefits of introducing best practices in the nursery sector and for society at large were explored in relation to avoiding potential impacts of introductions of new pathogens in the wider woodland environment. The analysis formed part of a wider project on the drivers of introduction and spread of Phytophthora species and opportunities for mitigation by plant nurseries. The effect in reducing the risk of introductions of new pathogens is not easily quantified, but the shorter the interval between avoided outbreaks, the greater the benefit of introducing best practices. The maximum interval between avoided outbreaks for the benefits to exceed the costs from a nursery perspective can be estimated. The results indicate that the benefits of introducing nursery best practices are unlikely to outweigh the costs from a nursery perspective where Phytophthora alone is considered. The analysis is extended by also considering benefits in reducing risks of Xylella. Scenario analysis based upon modelling harvested wood and carbon impacts is then used to investigate potential costs that would be expected to be borne by society in the event of the introduction and spread of a new Phytophthora species affecting either oak or Sitka spruce stands in Britain. The results indicate the magnitude of the externality involved, with the cost of an outbreak affecting oak estimated at around 500 pound million and one affecting Sitka spruce at 11 pound 300 million. While approaches taken in the paper are exploratory, there seems a strong case for some social incentives or control in the nursery sector, to reduce the probability of new diseases infecting British forests.