Rustfungushasa complexlife-cycle
andlaunchesa two-prongedattackonits
host.It targetstheleavesthroughoutthe
growingseason,weakeningtheplantand
reducingitsabilitytosetseed.Then,after
overwinteringinthesoil,it awakensinthe
springtoattackthestemsofanyseedlings,
renderingthemweakandtwisted,before
hittingtheleavesonceagaintocontinue
thecycle.Thatoverwinteringstageis
thereforea keypartoftheprocess.
“Littlepustulesontheundersideof
leavesaretheeasiestwaytotellif a plantis
infected,”explainedthelateCarolEllison,
whowasa rustfungusspecialistatCABI.
“Wipeyourfingeracrossthem,andyou’ll
seea spreadofspores.”
Despitetheearlyoptimismfromthe
lab,resultsinthefieldfollowingthe
2015 releasewerepuzzlinglyslow,with
infectionratesmuchlowerthanexpected
ata numberofsites.
Thescientistsdelveddeeper,carrying
outmolecularanalysison100-year-old
samplesofHimalayanbalsamatKew.
TheynowthinkthattheplanthasmadeTheoptionsforcontrollingthisinvader
aren’tpromising,particularlyatthe
riverside.Physicallyshiftingbalsamonany
scalerequiresvastnumbersofpeople,and
it oftengrowsinsuchdifficulttoaccess
placesthatremovalis simplynotfeasible.
Chemical‘enemies’wemightwanttouse
areseverelyrestrictedthroughoutEurope.
Thereis onlyoneherbicide,glyphosate,
thatcanbeusednearwater,andthefateof
thatis underreview.
Thereis,however,a thirdmethod–
biocontrol.Thismeansintroducingthe
naturalenemiesofaninvasivespecies,
whichco-evolvedwithit initshome
range,intothenewenvironmentalready
colonised.Theperfectagentwillhave
evolvedsocloselywiththeinvaderthatit
posesminimalthreattonativespecies.
Theaimis noteradication,butcontrol.
Theapproachhasitscritics.Science
journalistFredPearcearguesinhisbook
TheNewWildthatweshouldlearnto
acceptinvasivespeciesasa consequenceof
themodernworldwelivein.A newplant
andanimalcommunitywillbecreatedin
time– weshouldstoptryingtowagea war
againstthealiens.
AccordingtoRichardShaw,
countrydirectorattheCentre
ofAgricultureandBioscience
International(CABI),somethingmayindeed
evolvetokeepthenumbersof
theseinvadersincheck,butit
maytakethousandsofyears–
wejustdon’tknow.Whatabout
thedamagetheyaredoingin
themeantime,notleasttoour
nativebiodiversity?Focussingonfungi
Since2006,CABIscientists
havebeenconductingsurveys
throughoutthebalsam’snative
rangetoidentifypotential
biocontrolagents.Manyhavebeen
discounted,butone,a rustfungusinthe
IndianHimalayas,wasobservedtohave
significantimpactsonitsbalsamhost.In
2010,it wasbroughttoCABI’sUKlab
forquarantinedtests.Fieldtrialsbeganin
2014,andinspring2015,thefunguswas
releasedat 25 sitesinEnglandandWales.
Initially,thefunguswasspreadby
introducinginfectedplantsamongexisting
standsofbalsam,butthesprayingof
fungalspores,suspendedinliquid,directly
ontothegrowingleavesprovedfarmore
efficient.A patchofabout10m would
bezonedoff,andplantsinabouthalfof
theareatreatedthreetimesduringthe
summer,optimisingtherust’schanceto
establishitself.Rustfungico-evolvedso
closelywithbalsamthat
theypreferentiallyattack
plantsfromtheirregion.
Left: when
left to its
own devices,
Himalayan
balsam can
soon grow
and spread.
Below: ‘balsam
bashing’ is
one way of
controlling
this invasive
species. Bottom
left: wasps
benefit from
the balsam’s
abundance
of nectar.NEWS FEATURE
August 2020