Disease is devastating California’s iconic oak and tanoak forests.
Sudden oak death, which is caused by the fungus-like pathogen called Phytophthora ramorum, has been spreading through California’s forests for years—at least since it was first detected in San Francisco in 1995.
Researchers from the University of Cambridge have now shown that the epidemic cannot be stopped. Their mathematical models—similar to those used to predict how diseases will spread in humans—reveal that earlier intervention could have been effective, but it is too late. The study appears in the latest issue of Proceedings of the National Academy of Sciences.
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Currently, the only practical way to deal with these outbreaks is to cut down infected trees, along with neighboring trees that have potentially been infected. This is where mathematical models can come in useful. “By comparing the performance of a large number of potential strategies, modelling can tell us where and how to start chopping down trees to manage the disease over very large areas,” explained Nik Cunniffe, lead author of the study, in a press release.
According to the authors, preventing the disease from spreading to large parts of California could have been possible if management had been started in 2002. Before 2002, not enough was known about the pathogen to begin managing the disease.
But the pathogen has since become too widespread in California forests, and the rate at which it is spreading has accelerated. “Even if huge amounts of money were to be invested to stop the epidemic starting today, the results of our model show this cannot lead to successful control for any plausible management budget,” said Cunniffe.
Over one hundred tree and shrub species are susceptible to sudden oak death, not just oak trees. Infected trees crack open and dry out, exacerbating the risk of forest fires. The disease also reduces the capacity of forests to sequester carbon.
The models developed by the study’s authors offer strategies to enhance control of future epidemics. For example, the models describe when and where control efforts should be targeted in order to optimize limited financial resources.
As Cunniffe explains, authorities often start cutting down trees at the core of an infected area, but the new research shows that doing this will allow susceptible vegetation to simply grow back and become infected again. The authors found that treating the ‘wave-front’ in the direction that the disease is spreading is more effective.
These types of models are now also being created to control plant diseases in Africa, where food security is being threatened by pathogens that attack key staples such as cassava.
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