Modelling offers hope for future invaders

Nik Cunniffe, Chris Gilligan and colleagues have published a paper in PNAS modelling potential management of sudden oak death in California. Sudden oak death – caused by Phytophthora ramorum, an oomycete related to potato blight – has killed millions of trees and has affected hundreds of square kilometres of forest in California since the early 1990s. However, the work predicts that the area affected by the disease will increase approximately ten-fold by 2030. The model also shows that stopping or even slowing the spread of the epidemic at state wide scales is now effectively impossible.

However, the study offers insight into how to better manage future epidemics. The work suggests that stopping the spread of disease to large parts of California could have been possible, had management been started in 2002, around the time enough was known about the disease for control to be practicable. By allowing millions of potential control strategies to be tested and compared, mathematical models offer a rational methodology to optimise treatment within a limited budget. For sudden oak death in California, the work shows that a wave-front strategy, repeatedly treating on and ahead of the epidemic in the direction that disease is spreading, would have been the most effective method of control, had it been started early enough.

For sudden oak death in California, the next step must now be localised site-specific management by forest managers and state agencies to protect particular areas that still can be saved. However, for current and future invaders – in the UK as well as the US – modelling provides a transferrable tool to develop strategies for how where, when and how epidemics should be managed. A variant of the model is already being used to inform the management of Phytophthora ramorum here in the UK. In the light of recent arrivals including ash dieback, and with a number of other pests and diseases – including the emerald ash borer and Xylella fastidiosa – very firmly on the horizon, mathematical modelling is an essential part of the toolbox for protecting our forests.

• Modeling when, where, and how to manage a forest epidemic, motivated by sudden oak death in California. NJ Cunniffe, RC Cobb, RK Meentemeyer, DM Rizzo, CA Gilligan. PNAS. DOI 10.1073/pnas.1602153113