GRDC GreenBridge FS 6pp

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    PAGE 1

    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 thes