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2009

GREEN BRIDGEFACT SHEET

EMMALEONARD

Level 1, Tourism House | 40 Blackall Street, Barton ACT 2600 | PO Box 5367, Kingston ACT 2604 | t. +61 2 6166 4500 | f. +61 2 6166 4599 | e. [email protected] | w. www.grdc.com.au

The essential crop management tool green bridge control is integral topest and disease management

KEY POINTS

The green bridge provides abetween season host for insects

and diseases (particularly rusts)

that pose a serious threat to future

crops and can be expensive to

control later in the season.

Outright killof the weeds andvolunteers is the only certainwayto avoid them hosting diseases

and insects.

Diseases and insects can quicklyspread from the green bridge,

jeopardising crops and current

control methods including the

effectiveness of chemicals and

genetic breeding for resistance.

Effective control of the pest anddisease risks requires neighbours

to work together to simultaneously

eradicate weeds and crop

volunteers.

Weed growth during summer andautumn also depletes soil moisture

and nutrients that would otherwise

be available to following crops and

can have an allelopathic effect.

Control of the green bridge gives crops a better chance to reach theirpotential by reducing the risk of pests and diseases surviving betweenseasons, while preserving valuable soil moisture and nutrients.

Action is needed on a communitylevel to remove crop volunteersand summer weeds to effectivelymanage the risk of pests anddisease that find harbour in thegreen bridge between crops.


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The term green bridge describes therole of weeds and crop volunteers inhelping prevent pests and diseasesthat threaten crops cross from onecropping season into the next.

This mass of vegetation grows onpaddocks, headlands, roadsides andnon-crop land after summer rain although it can also refer to growthduring winter, between summer crops.

A green bridge consists of cropvolunteers usually from last yearscrop, and sometimes from crops

grown one or two years before, plusweeds emerging from seeds set overmany years, or from new windborneweed seeds.

A shared risk

Control of the green bridge is acommunity issue, not just an individualone, as many pests and diseases caneasily spread from adjacent propertiesand commonly-owned land.

Rust spores, aphids and mites alltravel on the wind, and easily crossfarm boundaries. Aphids can also flyupwind. Where crops are sown close

to a green bridge containing plantsinfected with fungal or viral diseases,and their vectors, the infection willquickly spread to the newly establishedcrop. The impact of a disease will bemore widespread and severe the earlierinfection occurs after germination.

Widespread infection places pressureon current chemical controls. Highlevels of rust carryover on volunteers,for instance, can make control withfungicides more difficult and alsoplaces pressure on genetic resistancein cereal varieties and increases thelikelihood of pathogen mutation.

By providing a host for pests anddisease the green bridge can triggerepidemics of insects and diseaseslater in the year, which cannot beeasily or economically controlled. Insome instances there are no effectivechemical controls for the insects ordiseases transmitted from the greenbridge, such as wheat streak mosaicvirus transmitted by the wheat curl mite.

While individual farmers will benefitfrom efforts to eradicate the green

bridge on their own properties,

effective control requires neighboursto work together to remove volunteersand weeds simultaneously.

Competition for soil andnutrients

While not all weeds act as a greenbridge for pests and diseases, anyweeds that persist during summer,together with volunteers, will consumevaluable soil moisture and nitrogenfrom the soil. Research in WesternAustralia and New South Waleshas identified that removing weedsshortly after they begin to emerge can

Rusts are the most important foliarcereal diseases in Australia andinfection is much more widespreadand damaging in years where summer/autumn rainfall has resulted in asignificant green bridge. Spores arewindbourne and are easily spread; as

little as one infected leaf per 12 hectaresof regrowth surviving through summerand early autumn can produce a rustepidemic in the following cereal crop.

Rusts cause red, orange or yellowpowdery pustules on leaves, stems orheads of plants, and volunteer cerealsare the primary reservoir of infectionfrom one cropping season to the next.If high levels of rust are present in agreen bridge when crops are sown,even crops selected for their rustresistance are likely to be severelyaffected. Rust will infect crops during

the most susceptible establishmentphase, before resistance traits developin the adult plants.

Stripe rust can be a serious problemon wheat in regions where cool

temperatures prevail through thegrowing season. Barley and someother grasses and cereals are alsoaffected. It survives in the green bridgeon volunteer wheat, barley and triticale,as well as on barley grass and othercereal grasses, brome grass and

phalaris.

Stem rust most commonly affectswheat, but barley, oats and rye canalso be affected. It is less commonthan stripe rust, but can cause totalcrop losses. The most important hostsare susceptible wheat volunteers, butit can also survive on barley, triticale,and some grasses. Where conditionsfavour the spread of stem rust it candestroy an entire crop. Epidemics aremore frequent in the rust-prone, veryhigh rainfall districts in northern andsouthern agricultural areas.

Leaf rust most commonly affectssusceptible wheat varieties. It is morewidespread, but less severe thanstem or stripe rust, reducing yieldsaround 20 per cent and reducing

grain quality. The most common hostsare volunteers of susceptible cerealvarieties.

Wheat Streak Mosaic Virus(WSMV)is transmitted by the wheat curl mite(WCM) and can also be spread by

infected seed. It causes severe leafsymptoms and reduced yield, andif there is a severe infection in anemerging crop, WSMV can result intotal crop loss. The virus affects wheat,oats, rye and a range of grasses.

The virus and its vector require a greenbridge to survive between growingseasons. Hosts for WSMV and WCMinclude volunteer wheat, barley grass,annual ryegrass, small burr grass, stinkgrass and witch grass.

Wheat curl mites are less than 0.3mmlong and can only be seen with the aid

of a microscope. Currently there is noeffective miticide for WCM in Australia,so removal of the green bridge isan essential strategy to prevent thespread of WSMV.

WHAT IS THE GREEN BRIDGE?

COMMON GREEN BRIDGE PESTS AND DISEASES

Crop volunteers and weedsgerminating in summer and autumnhost diseases and insects, which canquickly spread into newly established

winter crops across propertyboundaries.


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preserve 50 to 75 millimetres of soilmoisture.

Researchers have also identified yieldlosses of 0.5-1.0 tonnes per hectarein cereals where the green bridgeremained uncontrolled up to sowing;farmer experience parallels this.

Nutrients used by the green bridgeare no longer available to crops atseeding even if the green bridge is laterkilled by herbicide. Summer grassesare known to forage for nitrogen inparticular, depleting nutrients availableto following crops.

Late summer and early autumn rainfallis the key trigger for the establishmentof the green bridge in those partsof Australia where winter croppingdominates.

Seasonal conditions provide anindication of the green bridge riskfrom year to year, with some stategovernment agencies providingawareness campaigns in years of highrisk. These campaigns identify wherethere has been enough summer/autumn rain to generate problematicweed and volunteer growth.

The more weed vegetation there isduring the summer/autumn period,the longer it is in place and thecloser it survives to the start of thenew cropping season, the higher thedisease and pest risk associated withthe green bridge. Information andawareness campaigns generally beginfrom February onwards.

Cereal crops make up an increasingpercentage of the landscape, astraditional grazing regions diversifyinto cropping, such as Victoria and

Tasmanias high rainfall zones. This

increases the potential to generatecereal volunteers between crops,raising the green bridge risk. Morecrops also mean more area at risk.

Weed risks

Some regions have experienced achange in traditional rainfall patternsduring the past decade, with highersummer rainfall. This has resulted inmore widespread establishment of asummer green bridge and increasedlevels of disease, particularly rusts, infollowing cereal crops.

New weeds are appearing withchanges in traditional summer rainpatterns, and their capacity to hostdisease and viruses are still beingevaluated.

Some weeds, such as mintweed inWA, are also allelopathic they puttoxins into the soil that prevent plantsfrom growing, further reducing theproductive potential of crops.

In many areas there are now far feweranimals within farming systems (thenational sheep flock has fallen from

ASSESSING THE GREEN BRIDGE RISK

pulses. Hosts for these viruses andtheir vectors include broadleaf weedsand pasture legumes such as lucerne,medics and clovers. These virusescause stunted plants and reducedyields and may kill heavily infectedplants depending on the combinationof crop and virus. Infection with BLRVcauses severe symptoms in pulses.

Hosts for these viruses and theirvectors include broadleaf weeds andpasture legumes such as lucerne,medics and clovers. These viruses canresult in 30 to 50 per cent yield losses.

Diamond Back Moth (DBM), orcabbage moth, can devastate canola.DBM multiplies over summer oncanola volunteers and other weedgrowth including wild raddish. In wintercrop growing areas where conditionshave remained mild, DBM caterpillarshave damaged emerging canola. DBMare usually most destructive later in theseason when canola is at the mid tolate flowering stage, as high caterpillarnumbers can cause yield loss. Onewell-timed spray can control thecaterpillars but DBM caterpillars arealready resistant to some insecticides,

Stem rust is less common than stripe rust but can cause total crop losses.

BRADCOLLIS

EMMA LEONARD

Barley Yellow Dwarf Virus(BYDV)infects all cereals, having significantimpacts on wheat, oats and barley.It is transmitted mainly by the oatand maize aphids, and even lowpopulations of these aphids cancause significant spread of BYDV.

Infected plants have less above groundbiomass and also a reduced rootsystem, resulting in smaller grain sizeand lower grain yield. Yield losses of 10to 20 per cent have been consistentlyrecorded in trials in long growing

season areas, with up to 60 per centlosses in some instances.

Beet Western Yellow Virus (BWYV)

and Bean Leaf Roll Virus (BLRV)are two viruses that commonly affectoilseed and pulse crops. They rely

on the existence of a green bridgefor their survival, and the survival oftheir vector aphids between growingseasons. BWYV is endemic in canolacrops in southwest Western Australia.BLRV causes severe symptoms in


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and ongoing infestations increasepressure on the effectiveness ofremaining products. Insecticidescontrol only the grubs, and not theadults.

Mites Green bridges can breedhigh populations of mite pests,especially if susceptible crops suchas canola are planted into paddocksthat have a green bridge. Clover miteand balaustium mites can maintainpopulations all through the year,as long as there is a green bridge.

However, redlegged earth mites andblue oat mites require five consecutivedays of temperatures below 20C tohatch from over-summering eggs, anda green bridge greatly helps to increasetheir populations.

Rhizoctoniais a root disease that,while not readily transferable to otherareas, continues to survive in the green

bridge in infected areas. Rhizoctoniaattacks cereals, crop legumes,pastures and weeds and has becomewidespread in the cereal belt, causingyield losses of up to 16 per cent incereal crops. It survives as fine fungal

185 million to 75 million) to control thesummer weeds burden.

Chemical management has becomemore important, and growersshould rotate herbicide groups toprevent resistance developing inweed populations. Chemicals mostcommonly used for summer weedcontrol with growers are Group B(sulphonylureas), Group I (phenoxys),and Group M (glyphosate) products.

Weeds such as melons also impacton the efficiency of sowing, as vines

become tangled in equipment.

Insect risks

Insect pests provide the greatest riskearly in the season, when crops firstemerge.

Insecticides may be required tocounter the direct feeding damage ofthe insects or to prevent the spread ofviruses within the crop, for example, byaphids.

Where insects are in epidemicnumbers, effective control is usually

threads in organic matter and is likelyto be more severe where the fungushas infected the roots of grass weeds

before sowing, which enables thefungus to both multiply and toleratecultivation. There is no chemicalsolution for Rhizoctonia and weedcontrol to remove host plants is one ofthe most effective cultural practices.

achieved by using the higher registeredrates of insecticides and increasing thefrequency of applications. However, the

greater the insect population is, there

is an increased likelihood of mutationsdeveloping and this could lead to morefecund or virulent pest species.

Mintweed puts toxins into thesoil which reduces the growth ofcrops planted in that soil.

Redlegged earth mites

RALPH BURNETT

CESARCONSULTANTS

100

90

70

50

30

10

80

60

40

20

0Feb Mar Apr May June July Aug Sept Oct Nov

Month

Stripe rust (%)

Green bridge Green bridge + APR

No green bridge No green bridge + APR

FIGURE 1 ILLUSTRATION OF THE EFFECTS OF A GREEN BRIDGE AND ADULTPLANT RESISTANCE (APR) ON THE DEVELOPMENT OF STRIPE RUST INWHEAT COMPARED WITH A DRIER SUMMER


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In terms of hosting disease andinsects, crop volunteers provide themost serious risk, particularly if theyare the same crop, and the samevariety as the one about to be planted.

Weeds can be most effectivelycontrolled when they first emergefollowing summer or autumn rain.Smaller weeds are easier to control,and they take less water and nutrientsthey will take from fallow paddocks.

However, in the case of foliar pests anddiseases, where one grower eliminates

the green bridge early in the season,any subsequent weed growth may beinfected by disease or insects fromneighbouring properties. An early-sown crop could also be infectedfrom an adjacent property where theneighbouring grower has a later weedcontrol and sowing program.

Weeds and volunteer cereals alongfencelines, around sheds androadsides should all be targeted aspotential disease hosts, particularlyfor rusts.

Neighbours working togetherto eliminate the green bridgesimultaneously will control the pestand disease risks more effectively thangrowers working independently.

Timing

If the green bridge has not beeneliminated early in the season it isessential to create a break of at leasttwo weeks during which there is nogreen plant matter capable of hostingdisease or insect vectors.

To achieve this, the green bridgeshould be sprayed four to six weeksbefore sowing, to ensure weeds arecompletely dead.

For instance, WCM can survive in the

core of dead-looking plants and canquickly spread into new crops andinfecting them with WSMV. Virusescan be most damaging if weeds arenot quite dead when the new cropemerges.

Spraying weeds within two weeks ofsowing will not reduce the diseaserisk, although it will reduce competitionfrom weeds for water, nutrients andlight as the crop emerges.

If the first weeds of the summer/autumn season appear shortly beforesowing, the previous weed-free period

is likely to provide a sufficient break inthe disease cycle.

Chemical control

Spraying weeds currently provides

GREEN BRIDGE CONTROL

the most effective control of the greenbridge. Growers are advised to followthe label directions on herbicides,as prevailing weather conditions,particularly hot weather, can influencethe efficacy of herbicides. Theobjective is to achieve an outright kill ofweeds and volunteers.

Glyphosate and other translocatedherbicides are best as they are usuallythe most effective contact herbicidesare less reliable.

Grazing

Heavy grazing can be an effectivetechnique for reducing the summervegetation load. However, grazingpatterns are often irregular andbecause host plants are not killed, theycan continue to harbour viruses androot diseases in particular.

Cultivation

Tillage can be effective in eliminatingweeds for disease control. It shouldbe carried out two to four weeks priorto planting to remove the green bridgeeffectively.

However, in areas with light soils thatdry quickly during summer monthscultivation can cause loss of soilmoisture and lead to erosion.

Cultivation can be effective in reducingthe severity of Rhizoctonia.

Eliminate weeds along fencelines,buildings and roadsides in your

green bridge control strategy as theycan also harbour pests and disease.

DAFWA EMMA LEONARD


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Acknowledgements: Ralph Burnett, Crop Protection Consultants; Grant Hollaway, Vic DPI; Hugh Wallwork,

SARDI; Geoff Thomas, DAFWA; Svetlana Micic, DAFWA; Peter Newman, DAFWA; Jenny Davidson, SARDI.

DISCLAIMER

Any recommendations, suggestions or opinions conta ined in this publica tion

do not necessarily represent the policy or views of the Grains Research and

Development Corporation. No person should act on the basis of the contents of

this publication without first obtaining specific, independent professional advice.

The Corporation and contributors to this Fact Sheet may identify products by

proprietary or trade names to help readers identify particular types of products.

We do not endorse or recommend the products of any manufacturer referred to.

Other products may perform as well as or better than those specifically referred

to. The GRDC will not be liable for any loss, damage, cost or expense incurred

Useful resources:

For local information about managing the green bridge risk and seasonal assessments of the risk, contact

your local agronomist or:

South Australia Hugh Wallwork SARDI 08 8303 9382 Email: [email protected]

Victoria and Tasmania Grant Hollaway VIC DPI 03 5362 2111 Email: [email protected]

Western Australia Geoff Thomas DAFWA 08 9368 3333 Email: [email protected]

Western Australia Svetlana Micic DAFWA 08 9892 8444 Email: [email protected]

New South Wales Andrew Milgate NSW DPI 02 6938 1990 Email: [email protected]

GRDC website, Rustlinks www.grdc.com.au/rustlinks

GRDC website, Pestlinks www.grdc.com.au/pestlinks

GRDC website, Weedlinks www.grdc.com.au/weedlinks

Crop disease forecasts provided by each state NSW: www.dpi.nsw.gov.au; Victoria: www.dpi.vic.gov.au;

WA: www.agric.wa.gov.au; Queensland: www.dpi.qld.gov.au;

Tasmania: www.dpiw.tas.gov.au SA: www.sardi.sa.gov.au

or arising by reason of any person using or relying on the information in this

publication.

CAUTION: RESEARCH ON UNREGISTERED PESTICIDE USE

Any research with unregistered pesticides or of unregistered products repor ted

in this document does not constitute a recommendation for that particular use by

the authors or the authors organisations.

All pestic ide applications must accord with the currently regis tered label for t hat

particular pesticide, crop, pest and region.

In Western Australia, modelling isused to help identify the green bridgerisk. Daily temperature, rainfall andevaporation data are used to determinethe greenness level or build up andbreak down of green bridge plantmaterial in the southern cropping belt.

Temperature determines the rate atwhich greenness increases. Rainfalland evaporation are used to determinethe fraction of plant-available water,which is then used to calculate themaximum capacity for greenness. Themodel outputs have been calibratedusing satellite Normalised DifferenceVegetation Index (NDVI) imagery andcan be viewed at the Department ofAgriculture and Food Western Australiawebsite, www.agric.wa.gov.au

PRODUCED

BY

www.coretext.com.au

MODEL HELPS TO ASSESS RISK

Green bridge, south-west WA April 1, 2000 Green bridge, south-west WA April 1, 2005

Green bridge, south-west WA April 1, 2006 Green bridge, south-west WA April 3, 2009

IMAGESCOURTESYOFDAFWA
http://www.grdc.com.au/rustlinkshttp://www.agric.wa.gov.au/http://www.agric.wa.gov.au/http://www.grdc.com.au/rustlinks