Californian sudden oak death epidemic'unstoppable,' new epidemics must bemanaged earlier2 May 2016

Large-scale tree mortality in northern Sonoma County,California. Credit: David Rizzo

New research shows the sudden oak deathepidemic in California cannot now be stopped, butthat its tremendous ecological and economicimpacts could have been greatly reduced if controlhad been started earlier. The research alsoidentifies new strategies to enhance control offuture epidemics, including identifying where andhow to fell trees, as "there will be a next time".

Sudden oak death - caused by Phytophthoraramorum, a fungus-like pathogen related to potatoblight - has killed millions of trees over hundreds ofsquare kilometres of forest in California. Firstdetected near San Francisco in 1995, it spreadnorth through coastal California, devastating theregion's iconic oak and tanoak forests. In 2002 astrain of the pathogen appeared in the south westof England, affecting shrubs but not oaks, sinceEnglish species of oak are not susceptible. In 2009the UK strain started killing larch - an important

tree crop - and has since spread widely across theUK.

In a study published today in PNAS, researchersfrom the University of Cambridge have usedmathematical modelling to show that stopping oreven slowing the spread of Phytophthora ramorumin California is now not possible, and indeed hasbeen impossible for a number of years.

Treating trees with chemicals is not practical or cost-effective on the scales that would be necessary foran established forest epidemic. Currently the onlyoption for controlling the disease is to cut downinfected trees, together with neighbouring trees thatare likely to be infected but may not yet showsymptoms. "By comparing the performance of alarge number of potential strategies, modelling cantell us where and how to start chopping down treesto manage the disease over very large areas,"explains Nik Cunniffe, lead author fromCambridge's Department of Plant Sciences.

The authors say that preventing the disease fromspreading to large parts of California could havebeen possible if management had been started in2002. Before 2002 not enough was known aboutthe pathogen to begin managing the disease. Theirmodelling also offers new strategies for moreeffectively controlling inevitable future epidemics.

Models developed in Cambridge are already anintegral part of the management programme for thePhytophthora ramorum epidemic in the UK. Themodels are used to predict where the disease islikely to spread, how it can be effectively detectedand how control strategies can be optimised, inclose liaison with colleagues from DEFRA and theForestry Commission.

Sudden oak death is known to affect over one

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hundred species of tree and shrub, presenting asignificant risk to the biodiversity of manyecosystems. The death of large numbers of treesalso exacerbates the fire risk in California whenfallen trees are left to dry out. There is now concernthat the disease may spread to the AppalachianMountains, putting an even larger area of trees atrisk.

Map shows risk of infection in 2030 under no control onleft; control on and ahead of wave-front on right. Credit:Nik Cunniffe

"Our study is the first major retrospective analysisof how the sudden oak death epidemic in Californiacould have been managed, and also the first toshow how to deal with a forest epidemic of thismagnitude," explains Cunniffe.

"Even if huge amounts of money were to beinvested to stop the epidemic starting today, theresults of our model show this cannot lead tosuccessful control for any plausible managementbudget. We therefore wanted to know whether itcould have been contained if a carefully-optimisedstrategy had been introduced sooner. Our modelshowed that, with a very high level of investmentstarting in 2002, the disease could not have beeneradicated, but its spread could have been slowedand the area affected greatly reduced."

The model also indicates how policymakers mightbetter plan and deploy control when futureepidemics emerge.

"It is a tool by which we can make a better job nexttime, because it is inevitable that there will be anext time," says Chris Gilligan, senior author alsofrom the Department of Plant Sciences. "With thissort of epidemic there will always be more sites totreat than can be afforded. Our model shows whenand where control is most effective at differentstages throughout a developing epidemic so thatresources can be better targeted."

"It can be tempting for authorities to start cuttingdown trees at the core of the infected area, but forthis epidemic our research shows that this could bethe worst thing to do, because susceptiblevegetation will simply grow back and becomeinfected again," explains Cunniffe.

Cunniffe, Gilligan and colleagues found that insteadtreating the 'wave-front' - on and ahead of theepidemic in the direction that disease is spreading -is a more effective method of control. They alsofound that 'front-loading' the budget to treat veryheavily early on in the epidemic would greatlyimprove the likelihood of success.

"Unlike other epidemic models, ours takes accountof the uncertainty in how ecological systems willrespond and how the available budget may change,allowing us to investigate the likelihood of successand risks of failure of different strategies at differentpoints after an epidemic emerges," says Gilligan.

"Whenever a new epidemic emerges, controlling itbecomes a question of how long it takes for us tohave enough information to recognise that there isa problem and then to make decisions about how todeal with it. In the past we have been starting fromscratch with each new pathogen, but the insightgenerated by this modelling puts us in a betterposition for dealing with future epidemics," he adds.

The researchers say that the next step in dealingwith well-established epidemics such as suddenoak death is to investigate how to protectparticularly valuable areas within an epidemic that -as they have demonstrated - is already too big tobe stopped.

The methodology is already being applied to createrelated models for diseases that threaten food

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security in Africa, such as pathogens that attackwheat and cassava.

More information: Modeling when, where, andhow to manage a forest epidemic, motivated bysudden oak death in California, PNAS, www.pnas.org/cgi/doi/10.1073/pnas.1602153113

Provided by University of CambridgeAPA citation: Californian sudden oak death epidemic 'unstoppable,' new epidemics must be managedearlier (2016, May 2) retrieved 23 March 2022 from https://phys.org/news/2016-05-californian-sudden-oak-death-epidemic.html

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