Invasive Species Compendium

Detailed coverage of invasive species threatening livelihoods and the environment worldwide


Conyza bonariensis
(hairy fleabane)



Conyza bonariensis (hairy fleabane)


  • Last modified
  • 15 November 2018
  • Datasheet Type(s)
  • Invasive Species
  • Pest
  • Host Plant
  • Preferred Scientific Name
  • Conyza bonariensis
  • Preferred Common Name
  • hairy fleabane
  • Taxonomic Tree
  • Domain: Eukaryota
  •   Kingdom: Plantae
  •     Phylum: Spermatophyta
  •       Subphylum: Angiospermae
  •         Class: Dicotyledonae
  • Summary of Invasiveness
  • C. bonariensis is a mainly annual herbaceous weed overwinters as a rosette, and spreads by producing high numbers of wind-dispersed seeds. It prefers undisturbed sites and is a particular problem in low-tillage systems such as orchards and plantation...

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Mature C. bonariensis.
TitleMature plant
CaptionMature C. bonariensis.
Copyright©Chris Parker/Bristol, UK
Mature C. bonariensis.
Mature plantMature C. bonariensis.©Chris Parker/Bristol, UK
Flowers and leaves of C. bonariensis.
TitleFlowers and leaves
CaptionFlowers and leaves of C. bonariensis.
Copyright©Chris Parker/Bristol, UK
Flowers and leaves of C. bonariensis.
Flowers and leavesFlowers and leaves of C. bonariensis.©Chris Parker/Bristol, UK
Fruiting plants of C. bonariensis (left) C. sumatrensis (right).
TitleFruiting plants
CaptionFruiting plants of C. bonariensis (left) C. sumatrensis (right).
Copyright©Chris Parker/Bristol, UK
Fruiting plants of C. bonariensis (left) C. sumatrensis (right).
Fruiting plantsFruiting plants of C. bonariensis (left) C. sumatrensis (right).©Chris Parker/Bristol, UK
Leaves of C. canadensis (left), C. sumatrensis (middle) and C. bonariensis (right).
TitleLeaf comparison
CaptionLeaves of C. canadensis (left), C. sumatrensis (middle) and C. bonariensis (right).
Copyright©Chris Parker/Bristol, UK
Leaves of C. canadensis (left), C. sumatrensis (middle) and C. bonariensis (right).
Leaf comparisonLeaves of C. canadensis (left), C. sumatrensis (middle) and C. bonariensis (right).©Chris Parker/Bristol, UK


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Preferred Scientific Name

  • Conyza bonariensis (L.) Cronq. (1943)

Preferred Common Name

  • hairy fleabane

Other Scientific Names

  • Conyza ambigua DC.
  • Conyza bonariensis var. leiotheca (S.F. Blake) Cuatrec.
  • Conyza crispus (Pourr.) Rupr.
  • Conyza linifolia (Willd.) Tackh.
  • Erigeron bonariensis L. (1753)
  • Erigeron crispus Pourr.
  • Erigeron crispus subsp. naudinii (Bonnet) Bonnier
  • Erigeron linifolius Willd. (1804)
  • Leptilon bonariense (L.) Small
  • Leptilon linifolium (Willd.) Small

International Common Names

  • English: Argentine fleabane; fleabane
  • Spanish: carnicera (Argentina); rama negra (Argentina); venadillo; yerba de la vida (Argentina); zamarraga
  • French: erigéron de Bonard
  • Portuguese: avoadinha-peluda; buva

Local Common Names

  • Australia: flaxleaf fleabane
  • Brazil: margaridinha-do-campo
  • Germany: Krauser Katzenscweif; Krauses berufkraut
  • Italy: ceppica campestre
  • Japan: arechinogiku
  • Netherlands: fijnstraal, vlasbladige
  • New Zealand: wavy-leaf fleabane
  • Norway: vlasbadige fiijnstraal
  • USA: asthma weed; asthmaweed; hairy fleabane

EPPO code

  • ERIBO (Erigeron bonariensis)

Summary of Invasiveness

Top of page C. bonariensis is a mainly annual herbaceous weed overwinters as a rosette, and spreads by producing high numbers of wind-dispersed seeds. It prefers undisturbed sites and is a particular problem in low-tillage systems such as orchards and plantations but also in some agricultural crops. It may be controlled by tillage at a suitable growth stage, but otherwise, it has developed resistance to many herbicides in a large number of countries. It has been introduced internationally as a seed contaminant and there is a risk of further similar introduction to countries where it is not yet established. It could become a problem invasive in protected areas, though may be controlled naturally by succession.

Taxonomic Tree

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  • Domain: Eukaryota
  •     Kingdom: Plantae
  •         Phylum: Spermatophyta
  •             Subphylum: Angiospermae
  •                 Class: Dicotyledonae
  •                     Order: Asterales
  •                         Family: Asteraceae
  •                             Genus: Conyza
  •                                 Species: Conyza bonariensis

Notes on Taxonomy and Nomenclature

Top of page Originally decribed by Linnaeus as Erigeron bonariensis, the species was transferred to the genus Conyza by Cronquist in the mid 1900s, maintaining its species epithet. Conyza floribunda and C. sumatrensis are sometimes used as synonyms for C. bonariensis (e.g. Holm et al., 1979; Lorenzi, 1982; USDA-NRCS, 2004) but are now generally regarded as distinct species (for C. sumatrensis see the separate datasheet). Taxonomic confusion is continued with USDA-NRCS (2004) incorrectly noting C. bonariensis var. leiotheca and C. floribunda as synonyms of C. bonariensis; synonyms are used here according to the Royal Botanic Garden Edinburgh (2003). A further reason for confusion arises from Erigeron crispus subsp. naudinii being noted as a synonym, whereas C. naudinii is recorded a synonym of C. floribunda (Royal Botanic Garden Edinburgh, 2003). There also continues to be some confusion as to the placement of taxa into genera including Conyza, with, for example, USDA-ARS (2004) noting the existence of only eight species of Conyza, without mention of C. floribunda or C. triloba (Sida, 2003) even as non-preferred names.


Top of page C. bonariensis is an erect annual with one or more stems from a basal rosette, up to 60 cm or occasionally 100 cm in height. All parts of the plant are finely pubescent and greyish in colour. Leaves linear to oblanceolate, mostly about 5 mm wide, entire, but often wavy-edged, with very short or hooked hairs less than 0.5 mm long. The inflorescence has long branches resulting in an almost corymbose effect, with most flowering heads about the same level. Individual flower heads are greyish-green, 4-5 mm diameter when fresh (broader in pressed specimens) with cream-coloured disc florets and no ray florets. The involucral bracts are pubescent with hairs almost as long as the bract width. The pappus is white or pinkish and 4-5 mm long; seeds are about 1 mm long.

Plant Type

Top of page Annual
Seed propagated


Top of page C. bonariensis was first described from Argentina and is probably native to the more temperate parts of South America (Michael, 1977), but it is now widely spread through most warmer regions of Europe, Africa, Asia, the Caribbean and Central America. The situation in North America is somewhat confused, with USDA-NRCS (2004) noting a continuous distribution across the southern USA from California to Florida, north to Oregon, North Carolina and Virginia but not Hawaii or Puerto Rico; whereas USDA-ARS (2004) records presence only California in mainland USA, and Hawaii and Puerto Rico also. This may be explained by the use of C. floribunda as a synonym by USDA-NRCS (2004), thus these additional USA states records may belong to this species and not C. bonariensis in the sense used in this datasheet. Missouri Botanical Garden (2004) do note, however, presence in Lousiana, Mississippi and Florida in addition to California, indicating that the situation is not clear and requires resolution.

Distribution Table

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The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.

Continent/Country/RegionDistributionLast ReportedOriginFirst ReportedInvasiveReferenceNotes


AfghanistanPresentIntroducedHolm et al., 1979
BahrainPresentIntroducedChaudhary et al., 1981
BhutanIndigenous, localizedIntroducedParker, 1992
ChinaPresentIntroducedHolm et al., 1979
-Hong KongPresentIntroducedMichael, 1977
-HunanPresentIntroducedXie and Yao, 1989
-JiangsuPresentNativeWu and Qiang, 2003
-ShanghaiPresentWang et al., 2007
-ShanxiPresentLiu and Kang, 2008
-ZhejiangPresentHong et al., 2008
IndiaWidespreadIntroducedHolm et al., 1979
-Jammu and KashmirPresentAmit and Singh, 2005
-Tamil NaduPresentVictor et al., 2001
-Uttar PradeshPresentSatya and Kanchan, 2004
IndonesiaWidespreadIntroducedHolm et al., 1979
IranPresentIntroducedFatehi et al., 1993
IsraelWidespreadIntroducedHolm et al., 1979
JapanPresentIntroducedMorita, 1997
Korea, Republic ofPresentIntroducedKim et al., 2003
LebanonPresentIntroducedEdgecombe, 1970
OmanPresentIntroducedChaudhary et al., 1981
PakistanPresentIntroducedKhalid, 1995
Saudi ArabiaPresentIntroducedChaudhary et al., 1981
TaiwanPresentIntroducedHolm et al., 1979
TurkeyPresentIntroducedRoyal Botanic Garden Edinburgh, 2003; USDA-ARS, 2004
United Arab EmiratesPresentIntroducedChaudhary et al., 1981
YemenPresentIntroducedChaudhary et al., 1981


BotswanaPresentIntroducedWells et al., 1986
Côte d'IvoirePresentIntroducedAdams, 1963
EgyptPresentIntroducedHolm et al., 1979
EthiopiaPresentIntroducedStroud & Parker, 1979
GhanaPresentIntroducedAdams, 1963
KenyaWidespreadIntroducedHolm et al., 1979
LesothoPresentIntroducedWells et al., 1986
MoroccoPresentIntroducedHolm et al., 1979
NamibiaPresentIntroducedWells et al., 1986
Sao Tome and PrincipePresentIntroducedAdams, 1963
South AfricaPresentIntroducedHolm et al., 1979
-Canary IslandsPresentSiverio et al., 2011
SwazilandPresentIntroducedWells et al., 1986
TanzaniaPresentIntroducedHolm et al., 1979
UgandaPresentIntroducedLind and Tallantire, 1971
ZimbabwePresentIntroducedHolm et al., 1979

North America

MexicoPresentIntroduced Invasive Morgado-Arroya & Urzua-Soria, 1995
USAPresentPresent based on regional distribution.
-CaliforniaPresentIntroduced Invasive Kempen and Graf, 1981
-HawaiiPresentIntroduced Invasive Holm et al., 1979

Central America and Caribbean

GuatemalaPresentIntroducedUSDA-ARS, 2004
JamaicaPresentIntroduced Invasive Holm et al., 1979
Lesser AntillesPresentIntroduced Not invasive USDA-ARS, 2004
PanamaPresentIntroducedUSDA-ARS, 2004
Puerto RicoPresentIntroducedUSDA-ARS, 2004
Trinidad and TobagoPresentIntroduced Invasive Holm et al., 1979

South America

ArgentinaWidespreadNativeHolm et al., 1979
BrazilWidespreadNativeHolm et al., 1979
-AlagoasPresentNativeLorenzi, 1982
-AmazonasPresentNativeLorenzi, 1982
-BahiaPresentNativeLorenzi, 1982
-CearaPresentNativeLorenzi, 1982
-Espirito SantoPresentNativeLorenzi, 1982
-GoiasPresentNativeLorenzi, 1982
-MaranhaoPresentNativeLorenzi, 1982
-Mato GrossoPresentNativeLorenzi, 1982
-Mato Grosso do SulPresentNativeLorenzi, 1982
-Minas GeraisPresentNativeLorenzi, 1982
-ParaPresentNativeLorenzi, 1982
-ParaibaPresentNativeLorenzi, 1982
-ParanaPresentNativeLorenzi, 1982
-PernambucoPresentNativeLorenzi, 1982
-PiauiPresentNativeLorenzi, 1982
-Rio de JaneiroPresentNativeLorenzi, 1982
-Rio Grande do NortePresentNativeLorenzi, 1982
-Rio Grande do SulPresentNativeLorenzi, 1982
-RondoniaPresentNativeLorenzi, 1982
-Santa CatarinaPresentNativeLorenzi, 1982
-Sao PauloPresentNativeLorenzi, 1982
-SergipePresentNativeLorenzi, 1982
ChilePresentNativeFinot et al., 1996
ColombiaPresentNativeHolm et al., 1979
EcuadorPresentNativeUSDA-ARS, 2004
PeruPresentNativeHolm et al., 1979
UruguayPresentNativeHolm et al., 1979
VenezuelaPresentNativeHolm et al., 1979


AlbaniaPresentIntroduced Invasive Cronquist, 1976
CroatiaPresentIntroduced Invasive Silic and Solic, 1999
Czech RepublicPresentIntroduced Invasive Sida, 2003
FrancePresentIntroduced Invasive Cronquist, 1976
GreecePresentIntroduced Invasive Cronquist, 1976
ItalyPresentIntroduced Invasive Cronquist, 1976
-SicilyPresentBlando and Mineo, 2005
MaltaPresentIntroduced Invasive Lanfranco, 1976
NetherlandsPresentIntroduced Invasive Reutelingsperger, 2000
PortugalPresentIntroduced Invasive Cronquist, 1976
-AzoresPresentIntroduced Invasive Cronquist, 1976
RomaniaPresentNegrean and Ioana, 2012
SpainPresentIntroduced Invasive Cronquist, 1976
Yugoslavia (former)PresentIntroduced Invasive Cronquist, 1976


AustraliaWidespreadIntroduced Invasive Michael, 1977
-New South WalesPresentIntroduced Invasive Clements, 1983
-QueenslandPresentIntroduced Invasive Michael, 1977
New ZealandPresentIntroduced Invasive Michael, 1977
TokelauPresentIntroduced Invasive Cronquist, 1976

History of Introduction and Spread

Top of page Michael (1977) records that it was already present in Europe in the early 1700s and was recorded in Australia in the 1840s. Little else is known about exact dates of introduction, although possible means were discussed by Sida (2003), who concludes that seed may have been introduced with cotton imports, with early infestations being recorded around textile factories.

Risk of Introduction

Top of page Although the risk of accidental introduction of C. bonariensis remains high, the fact that it is already so widespread means that the phytosanitary risk is relatively low. It is present in most regions where it is able to survive and grow, and exclusion from other areas where it is not yet present may be impossible. However, certain quarantine measures may ensure that it does not spread into certain specified areas within a country or region, such as protected areas. Being a weed of undisturbed ground, the potential risks to such sites may be considerable.


Top of page C. bonariensis occurs typically on waste land, around field edges, roadsides, in fallows and in orchards, in both tropical and subtropical regions, and to some extent in temperate zones. It generally prefers undisturbed habitats, and thus when occurring as a weed, is a problem in perennial crops.

Habitat List

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Terrestrial – ManagedCultivated / agricultural land Present, no further details Harmful (pest or invasive)
Managed forests, plantations and orchards Present, no further details
Managed forests, plantations and orchards Present, no further details Harmful (pest or invasive)
Managed grasslands (grazing systems) Present, no further details Harmful (pest or invasive)
Disturbed areas Present, no further details Harmful (pest or invasive)
Rail / roadsides Present, no further details Harmful (pest or invasive)
Terrestrial ‑ Natural / Semi-naturalNatural forests Present, no further details Harmful (pest or invasive)

Hosts/Species Affected

Top of page C. bonariensis is usually a weed of perennial crops, especially in orchards of both temperate and tropical fruit trees such as apple, olive, mango, etc., vineyards, plantation crops (date palm, tea), and also in pastures and in fallows (Perez and Duarte, 1991). It may, however, also occur in annual crops such as barley, wheat, maize, soyabean, sugar beet and oilseed rape, but in these cases is more prevalent in low-tillage or no-till farming practices.

Host Plants and Other Plants Affected

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Biology and Ecology

Top of page Genetics

C. bonariensis is a hexaploid, with a chromosome number (2n) of 54 (Thebaud and Abbott, 1995), the same as recorded for C. sumatrensis, with which it has been previously confused. The other common weedy species, C. canadensis, is by comparison, a diploid (2n=18). The role of ploidy in its evolution and spread is as yet unknown.

Reproductive Biology

C. bonariensis is mainly an annual plant, germinating in autumn and persisting as a rosette of leaves over winter before shooting and flowering in the following spring. However, it may often behave as a biennial in temperate climates. It rarely, if ever, persists for a second season after flowering (unlike C. sumatrensis). Seeds need a temperature of 10-25°C, and require light for germination (Zinzolker et al., 1985). Establishment occurs mainly in relatively undisturbed situations, and cultivation in annual crops apparently buries most of the seed and greatly reduces emergence. After establishment as a rosette, elongation of the stem is inhibited by short days but occurs rapidly under longer day conditions (Zinzolker et al., 1985). Seed production can be as great as 226,000 seeds per plant in the USA (Kempen and Graf, 1981), and seed dispersal by wind is made highly efficient by the pappus.

Environmental Requirements

Although occurring mainly in warm temperate climates, it has wide adaptation to hotter climates and can be found in many sub-tropical regions, and even tropical zones especially at higher altitudes. In Europe, it is found mainly in the Mediterranean region, though has recently been recorded further north. There is little evidence for preference regarding soil type. In Bhutan, it occurs at a wide range of altitudes, whereas C. floribunda is found mainly in lowlands, below 2000 m and C. canadensis tends to be restricted to higher altitudes, over 2000 m (Parker, 1992). C. bonariensis has been recorded at altitudes up to 3900 m in Bolivia (Missouri Botanical Garden, 2004).


A detailed study was undertaken by Prieur-Richard et al. (2002) of the invasion by C. bonariensis on Mediterranean annual plant communities. In agreement with earlier results suggesting that high nutrient availability can favour invasions, an abundant legume biomass in communities increased the final biomass and net fecundity of C. bonariensis, due to positive effects on soil nitrate concentration. Survival and establishment of C. bonariensis were mainly favoured by a high biomass of Asteraceae, and additional results from measurements of herbivory suggested that C. bonariensis survival was not related to abiotic conditions but may be due to protection against herbivores in plots with abundant Asteraceae. Establishment was on the other hand likely to be hindered by the effects of abundant grass and legume foliage on light quality, and therefore easier within an Asteraceae canopy. Prieur-Richard et al. (2002) concluded that invasion of Mediterranean old fields by species similar to C. bonariensis could be limited by favouring communities dominated by annual grasses.

Other studies looking at the invasion by C. bonariensis of fallows have observed that early high densities of the species are gradually replaced over several years by other plants, suggesting that C. bonariensis is a species of the early successional stage, having pioneering characteristics.

Soil Tolerances

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Soil drainage

  • free
  • impeded

Soil reaction

  • acid
  • neutral

Soil texture

  • heavy
  • light
  • medium

Special soil tolerances

  • infertile
  • saline
  • shallow
  • sodic

Natural enemies

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Natural enemyTypeLife stagesSpecificityReferencesBiological control inBiological control on
Septoria erigerontis Pathogen Leaves

Means of Movement and Dispersal

Top of page Natural Dispersal (Non-Biotic)

C. bonariensis is principally a wind-dispersed species, facilitated by light seed accompanied by a pappus which aids flight (e.g. Andersen, 1992).

Vector Transmission.

No information is available on the possibility of spread by animals, but if it occurs, it is likely to be only of minor significance in comparison to wind-dispersal.

Agricultural Practices

Mowing along roadsides, especially during seed production, is likely to increase spread. Also, late tillage or other practices at such inappropriate times will also facilitate seed dispersal.

Accidental Introduction

Seed of several Conyza species now widely present as weeds outside of their native ranges were probably introduced to most of their introduced ranges accidentally as contaminants in cotton, cereals or forage grains/seed. The first appearance of C. bonariensis around textile mills in Europe and elsewhere where exotic lead Sida (2003) to conclude that it may have been widely introduced from the New World as a contaminant of cotton.

Also a weed in nurseries, Conyza spp. may be spread as seed present in the soil in pots or other planting containers that accompany nursery stock, either as ornamentals or for establishing plantations. The spread of C. canadensis, along with numerous other weeds in central European forests, was thought to have been assisted by this method (see the datasheet on C. canadensis), and thus, presence in soil must be considered as a potential pathway.

Pathway Vectors

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VectorNotesLong DistanceLocalReferences
Soil, sand and gravel Yes

Plant Trade

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Plant parts liable to carry the pest in trade/transportPest stagesBorne internallyBorne externallyVisibility of pest or symptoms
True seeds (inc. grain) seeds Yes Pest or symptoms usually visible to the naked eye
Plant parts not known to carry the pest in trade/transport
Fruits (inc. pods)
Growing medium accompanying plants
Seedlings/Micropropagated plants
Stems (above ground)/Shoots/Trunks/Branches

Impact Summary

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Animal/plant collections None
Animal/plant products None
Biodiversity (generally) None
Crop production None
Environment (generally) None
Fisheries / aquaculture None
Forestry production None
Human health None
Livestock production None
Native fauna None
Native flora None
Rare/protected species None
Tourism None
Trade/international relations None
Transport/travel None


Top of page C. bonariensis, though only recorded as a major weed in two countries, Argentina and Brazil (Holm et al., 1979), is frequently noted as a dominant weed, especially in orchards (for example, in olives in Spain and in apple in Pakistan) and pastures. Nevertheless, no single species competition studies have been conducted, and any crop loss data are inevitably confounded by the presence of other weed species. The widespread development of resistance to herbicides means, however, that it is tending to increase in importance. Economic impacts may also arise from the effects of C. bonariensis as a host for crop pests, as is common with other Conyzo species. C. bonariensis has been noted as an important host for various ant species, reported to be serious crop pests in China (Xie and Yao, 1989).

Environmental Impact

Top of page That C. bonariensis is principally a weed in undisturbed land, poses the risk that it may become invasive in protected areas.

Threatened Species

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Threatened SpeciesConservation StatusWhere ThreatenedMechanismReferencesNotes
Panicum fauriei (Carter's panicgrass)NatureServe NatureServe; USA ESA listing as endangered species USA ESA listing as endangered speciesHawaiiCompetition (unspecified)US Fish and Wildlife Service, 2011
Scaevola coriacea (dwarf naupaka)NatureServe NatureServe; USA ESA listing as endangered species USA ESA listing as endangered speciesHawaiiCompetition (unspecified)US Fish and Wildlife Service, 2010a
Schiedea apokremnos (Kauai schiedea)CR (IUCN red list: Critically endangered) CR (IUCN red list: Critically endangered); USA ESA listing as endangered species USA ESA listing as endangered speciesHawaiiCompetition (unspecified)US Fish and Wildlife Service, 2010b
Tetramolopium lepidotum (Waianae Range tetramolopium)USA ESA listing as endangered species USA ESA listing as endangered speciesHawaiiCompetition - stranglingUS Fish and Wildlife Service, 2009

Social Impact

Top of page Although social impacts are not recorded, the use of the common name 'asthmaweed' in the USA indicates the potential allergenic effects of C. bonariensis pollen on people.

Risk and Impact Factors

Top of page Invasiveness
  • Proved invasive outside its native range
  • Highly adaptable to different environments
  • Highly mobile locally
  • Has high reproductive potential
Impact outcomes
  • Negatively impacts agriculture
Impact mechanisms
  • Competition - monopolizing resources
  • Competition - strangling
  • Competition
  • Pest and disease transmission
Likelihood of entry/control
  • Highly likely to be transported internationally accidentally
  • Difficult to identify/detect as a commodity contaminant
  • Difficult/costly to control


Top of page C. bonariensis has only limited uses, though it is cultivated as a medicinal plant in some parts of the world, possibly for the noted antimicrobial effects (USDA-ARS, 2004). Plant extracts have been noted to have antifungal properties with possible uses against crop pathogens (e.g. Arora et al., 2003, Guatam et al., 2003).

Similarities to Other Species/Conditions

Top of page C. bonariensis is superficially similar to several other species occurring as weeds, of which the most widespread is C. canadensis, distinguished by finer, glabrous, yellow-green flower heads, 2-3 mm in diameter, with distinct white ray florets 0.5 to 1 mm long, and broader, greener, shallowly toothed leaves with distinct hairs on the margins (see the separate datasheet on C. canadensis). Two other species confused with C. bonariensis nomenclaturally are C. floribunda and C. sumatrensis. Both of these are somewhat more robust, about 1 m high or more, with seed heads slightly narrower than those of C. bonariensis, brownish grey pappus and much broader shallowly toothed leaves, often over 1 cm wide, without marginal hairs (see the datasheet on C. sumatrensis). The above differences are illustrated by Parker (1992), but for 'C. floribunda' read C. sumatrensis. For diagnostic keys differentiating Conyza bonariensis, C. canadensis and C. sumatrensis, see Reutelingsperger (2000), also Michael (1977) for the former two.

Prevention and Control

Top of page Cultural Control

C. bonariensis establishes from a small seed and the initial rosettes are readily destroyed by tillage. Once established, however, the plant becomes more difficult to control mechanically. Soil solarization is surprisingly ineffective (Silveira et al., 1988).

Chemical Control

C. bonariensis is normally susceptible to a wide range of the standard herbicides for control of broad-leaved weeds, including 2,4-D, dicamba, atrazine, simazine, paraquat and glyphosate. However, it has developed resistance to paraquat and to triazines, and sometimes shows dual resistance to both. The mechanisms of resistance have been much studied and it has been shown that the resistance to paraquat is associated with a single dominant gene. The level of resistance is about 30-fold in the early vegetative stage before bolting but rises to 200- to 300-fold at flowering. The higher level of resistance is associated with increased levels of superoxide dismutase, ascorbate peroxidase and glutathione reductase at the flowering stage but not at the earlier stage (Amsellem et al., 1993; Ye and Gressel, 1994). There is still a lack of agreement on the exact mechanism(s) of resistance (Preston, 1994). The resistance to paraquat automatically confers increased tolerance of atrazine and acifluorfen as well as to sulfur dioxide (Shaaltiel et al., 1988), but not to the related herbicide morfamquat and only partially to diquat (Vaughn et al., 1989). Full resistance to triazines depends on a separate mechanism, the same as that in other weeds (Prado et al., 1989).


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