Conyza canadensis (Canadian fleabane)
- Summary of Invasiveness
- Taxonomic Tree
- Notes on Taxonomy and Nomenclature
- Plant Type
- Distribution Table
- Risk of Introduction
- Habitat List
- Hosts/Species Affected
- Host Plants and Other Plants Affected
- Biology and Ecology
- Soil Tolerances
- Natural enemies
- Means of Movement and Dispersal
- Pathway Vectors
- Plant Trade
- Wood Packaging
- Impact Summary
- Environmental Impact
- Risk and Impact Factors
- Uses List
- Similarities to Other Species/Conditions
- Prevention and Control
- Distribution Maps
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PicturesTop of page
IdentityTop of page
Preferred Scientific Name
- Conyza canadensis (L.) Cronq. (1943)
Preferred Common Name
- Canadian fleabane
Other Scientific Names
- Erigeron canadensis L.
- Erigeron pusillus Nutt.
- Trimorpha canadensis (L.) Lindm.
International Common Names
- English: horseweed
- Spanish: erigeron del Canada; escoba dura; hierba de caballo; hierba impia; olivarda
- French: erigéron du Canada; vergerette du Canada
- Portuguese: avoadinha
Local Common Names
- Canada: mare's tail
- Colombia: cvenadillo
- Cuba: conyza; zancarana
- France: erigeron de Canada; vergerette de Canada
- Germany: Kanadischer berufkraut; Kanadisher katzenschweif
- India: jarayupriya
- Iraq: thail el-faras
- Italy: impi; saeppola
- Japan: himemukashiyomogi
- Madagascar: sarijamala
- Mauritius: herbe gandi
- Mexico: pegajosa
- Netherlands: fijnstraal, Canadeese
- Norway: canadese fijnstraal; hestehamp
- Poland: przymiotno kasnadyjskie
- Puerto Rico: pascueta; rozuz
- South Africa: armoedskruid; kanadese skraalhans
- Spain: altabaca; canem bord; erigeron de Canada; zamarraga
- Sweden: kanadabinka
- Turkey: sifa out
- USA: butterweed; Canada horseweed; fireweed; hogweed
- Yugoslavia (Serbia and Montenegro): repusnjaca
- ERICA (Erigeron canadensis)
- ERIPS (Erigeron pusillus)
Summary of InvasivenessTop of page C. canadensis is now a common weed in temperate to tropical regions. It is a mainly annual herbaceous weed spreading by producing high numbers of wind-dispersed seeds. It prefers undisturbed sites and is a particular problem in low-tillage systems such as orchards, 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 contaminant of cereals, forage seeds and cotton, 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 as it is an early-successional species, often being replaced by perennial grasses.
Taxonomic TreeTop of page
- Domain: Eukaryota
- Kingdom: Plantae
- Phylum: Spermatophyta
- Subphylum: Angiospermae
- Class: Dicotyledonae
- Order: Asterales
- Family: Asteraceae
- Genus: Conyza
- Species: Conyza canadensis
Notes on Taxonomy and NomenclatureTop of page The species was first described by Linnaeus as Erigeron canadensis in 1753, and transferred to the genus Conyza in 1943 by Cronquist. C. canadensis is a clearly defined species and is not nomenclaturally confused with any other related species, unlike several others within the genus (see datasheets on C. bonariensis and C. sumatrensis). It is, however, still widely referred to by its older name, Erigeron canadensis. Thebauld and Abbott (1995) noted that C. canadensis was the only diploid species of five invasive European species tested, and was more closely related to the genus Erigeron than the other taxa. This supports a hypothesis that C. canadensis is older in evolutionary terms.
DescriptionTop of page C. canadensis is an erect annual with a long taproot and one or more stems arising from a basal rosette, it is usually about 1 m high but may be much taller. Leaves are up to 10 cm long and about 1 cm wide with some shallow teeth, clear green (not greyish as in other common Conyza species), almost glabrous on the surfaces, but with some scattered hairs. Leaf margins ciliate and with longer conspicuous hairs towards the leaf base. Flower heads are very numerous on short pedicels, only 2-3 mm in diameter when fresh (broader in pressed specimens), involucral bracts about 5 mm long, glabrous. Disc florets yellow, contrasting with distinct white ray florets which are 0.5 to 1 mm long, the latter distinguishing C. canadensis from other common weedy Conyza species. Seeds 1.0-1.3 mm long with 10-25 off-white pappus hairs, 2-4 mm long (Holm et al., 1997).
Plant TypeTop of page Annual
DistributionTop of page C. canadensis is native to North America (USA and Canada) but spread to Europe as early as the 1600s (Michael, 1977) and later to much of Asia and Australia, including tropical regions such as in the Americas. However, in Africa it is so far restricted to north and south subtropical regions (Holm et al., 1997). In Bhutan, it is restricted to higher elevations, over 2000 m (Parker, 1992) but is apparently not so restricted in Central America. The native range elsewhere in the Americas remains obscure, but it is considered as exotic in Central America and the Caribbean in this datasheet. Weaver (2001) reported that the species was native to all provinces of Canada except Newfoundland, whereas USDA-ARS (2004) included Newfoundland in the native range.
Distribution TableTop of page
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/Region||Distribution||Last Reported||Origin||First Reported||Invasive||Reference||Notes|
|China||Present||Introduced||Holm et al., 1979|
|-Hebei||Present||Introduced||Wang et al., 1990|
|-Henan||Present||Introduced||Wang et al., 1990|
|-Jiangxi||Present||Introduced||Wang et al., 1990|
|-Shaanxi||Present||Introduced||Wang et al., 1990|
|-Shandong||Present||Introduced||Wang et al., 1990|
|-Shanxi||Present||Introduced||Wang et al., 1990|
|-Sichuan||Present||Introduced||Wang et al., 1990|
|-Yunnan||Present||Introduced||Missouri Botanical Garden, 2004|
|-Zhejiang||Present||Introduced||Wang et al., 1990|
|India||Widespread||Introduced||Holm et al., 1979|
|-Haryana||Present||Introduced||Dahiya et al., 1988|
|-Jammu and Kashmir||Present||Introduced||Siddiq et al., 1987|
|Indonesia||Present||Introduced||Holm et al., 1997|
|Iran||Present||Introduced||Holm et al., 1979|
|Iraq||Present||Introduced||Holm et al., 1979|
|Israel||Present||Introduced||Holm et al., 1979|
|Japan||Present||Introduced||Holm et al., 1979|
|-Ryukyu Archipelago||Widespread||Introduced||Morita, 1997|
|Jordan||Present||Introduced||Holm et al., 1979|
|Korea, DPR||Widespread||Introduced||Holm et al., 1979|
|Korea, Republic of||Widespread||Introduced||Holm et al., 1979|
|Pakistan||Present||Introduced||Holm et al., 1979|
|Philippines||Widespread||Introduced||Holm et al., 1979|
|Taiwan||Widespread||Introduced||Wang et al., 1990; Holm et al., 1979|
|Thailand||Present||Introduced||Holm et al., 1979|
|Turkey||Present||Introduced||Holm et al., 1979|
|Vietnam||Present||Introduced||Holm et al., 1979|
|Lesotho||Present||Introduced||Wells et al., 1986|
|Madagascar||Present||Introduced||Holm et al., 1997|
|Mauritius||Present||Introduced||Holm et al., 1979|
|Mozambique||Present||Introduced||Holm et al., 1979|
|Nigeria||Present||Introduced||Holm et al., 1979|
|South Africa||Present||Introduced||Holm et al., 1979|
|-Canary Islands||Present||Introduced||Invasive||Cronquist, 1976|
|Sudan||Present||Introduced||Bebawi and Neugebohrn, 1991|
|Swaziland||Present||Introduced||Wells et al., 1986|
|Tunisia||Present||Introduced||Holm et al., 1979|
|Zimbabwe||Present||Introduced||Holm et al., 1979|
|Canada||Widespread||Native||Holm et al., 1979|
|-British Columbia||Present||Native||USDA-ARS, 2004|
|-Newfoundland and Labrador||Present||Native||USDA-ARS, 2004|
|-Northwest Territories||Present||Native||USDA-ARS, 2004|
|-Nova Scotia||Present||Native||USDA-ARS, 2004|
|-Ontario||Present||Native||Whittle et al., 1997|
|-Prince Edward Island||Present||Native||USDA-ARS, 2004|
|-Quebec||Present||Native||Leroux et al., 1996|
|-Yukon Territory||Present||Native||USDA-ARS, 2004|
|Mexico||Present||Introduced||Holm et al., 1979; USDA-ARS, 2004|
|USA||Present||Present based on regional distribution.|
|-Alabama||Widespread||Native||Lorenzi and Jeffery, 1987|
|-Arizona||Widespread||Native||Lorenzi and Jeffery, 1987|
|-Arkansas||Widespread||Native||Lorenzi and Jeffery, 1987|
|-California||Widespread||Native||Lorenzi and Jeffery, 1987|
|-Colorado||Widespread||Native||Lorenzi and Jeffery, 1987|
|-Connecticut||Widespread||Native||Lorenzi and Jeffery, 1987|
|-Delaware||Widespread||Native||Lorenzi and Jeffery, 1987|
|-Florida||Widespread||Native||Lorenzi and Jeffery, 1987|
|-Georgia||Widespread||Native||Lorenzi and Jeffery, 1987|
|-Hawaii||Widespread||Introduced||Holm et al., 1979|
|-Idaho||Widespread||Native||Lorenzi and Jeffery, 1987|
|-Illinois||Widespread||Native||Lorenzi and Jeffery, 1987|
|-Indiana||Widespread||Native||Lorenzi and Jeffery, 1987|
|-Iowa||Widespread||Native||Lorenzi and Jeffery, 1987|
|-Kansas||Widespread||Native||Lorenzi and Jeffery, 1987|
|-Kentucky||Widespread||Native||Lorenzi and Jeffery, 1987|
|-Louisiana||Widespread||Native||Lorenzi and Jeffery, 1987|
|-Maine||Widespread||Native||Lorenzi and Jeffery, 1987|
|-Maryland||Widespread||Native||Lorenzi and Jeffery, 1987|
|-Massachusetts||Widespread||Native||Lorenzi and Jeffery, 1987|
|-Michigan||Widespread||Native||Lorenzi and Jeffery, 1987|
|-Minnesota||Widespread||Native||Lorenzi and Jeffery, 1987|
|-Mississippi||Widespread||Native||Lorenzi and Jeffery, 1987|
|-Missouri||Widespread||Native||Lorenzi and Jeffery, 1987|
|-Montana||Widespread||Native||Lorenzi and Jeffery, 1987|
|-Nebraska||Widespread||Native||Lorenzi and Jeffery, 1987|
|-Nevada||Widespread||Native||Lorenzi and Jeffery, 1987|
|-New Hampshire||Widespread||Native||Lorenzi and Jeffery, 1987|
|-New Jersey||Widespread||Native||Lorenzi and Jeffery, 1987|
|-New Mexico||Widespread||Native||Lorenzi and Jeffery, 1987|
|-New York||Widespread||Native||Lorenzi and Jeffery, 1987|
|-North Carolina||Widespread||Native||Lorenzi and Jeffery, 1987|
|-North Dakota||Widespread||Native||Lorenzi and Jeffery, 1987|
|-Ohio||Widespread||Native||Lorenzi and Jeffery, 1987|
|-Oklahoma||Widespread||Native||Lorenzi and Jeffery, 1987|
|-Oregon||Widespread||Native||Lorenzi and Jeffery, 1987|
|-Pennsylvania||Widespread||Native||Lorenzi and Jeffery, 1987|
|-Rhode Island||Widespread||Native||Lorenzi and Jeffery, 1987|
|-South Carolina||Widespread||Native||Lorenzi and Jeffery, 1987|
|-South Dakota||Widespread||Native||Lorenzi and Jeffery, 1987|
|-Tennessee||Widespread||Native||Lorenzi and Jeffery, 1987|
|-Texas||Widespread||Native||Lorenzi and Jeffery, 1987|
|-Utah||Widespread||Native||Lorenzi and Jeffery, 1987|
|-Vermont||Widespread||Native||Lorenzi and Jeffery, 1987|
|-Virginia||Widespread||Native||Lorenzi and Jeffery, 1987|
|-Washington||Widespread||Native||Lorenzi and Jeffery, 1987|
|-West Virginia||Widespread||Native||Lorenzi and Jeffery, 1987|
|-Wisconsin||Widespread||Native||Lorenzi and Jeffery, 1987|
|-Wyoming||Widespread||Native||Lorenzi and Jeffery, 1987|
Central America and Caribbean
|Belize||Present||Introduced||Missouri Botanical Garden, 2004; USDA-ARS, 2004|
|Costa Rica||Present||Introduced||Missouri Botanical Garden, 2004; USDA-ARS, 2004|
|Cuba||Present||Introduced||Invasive||Oviedo Prieto et al., 2012|
|El Salvador||Present||Introduced||Missouri Botanical Garden, 2004; USDA-ARS, 2004|
|Guatemala||Present||Introduced||Missouri Botanical Garden, 2004; USDA-ARS, 2004|
|Honduras||Present||Introduced||Holm et al., 1979|
|Jamaica||Present||Introduced||Holm et al., 1979|
|Nicaragua||Present||Introduced||Missouri Botanical Garden, 2004; USDA-ARS, 2004|
|Panama||Present||Introduced||Missouri Botanical Garden, 2004; USDA-ARS, 2004|
|Puerto Rico||Present||Introduced||Holm et al., 1979|
|Trinidad and Tobago||Present||Introduced||Holm et al., 1979|
|Turks and Caicos Islands||Present||Introduced||Missouri Botanical Garden, 2004|
|Brazil||Present||Introduced||Holm et al., 1997|
|Chile||Present||Introduced||Holm et al., 1979|
|Colombia||Present||Introduced||Holm et al., 1979|
|Ecuador||Present||Introduced||Holm et al., 1979|
|Guyana||Present||Introduced||Holm et al., 1997|
|Peru||Present||Introduced||Holm et al., 1997|
|Suriname||Present||Introduced||Holm et al., 1979|
|Venezuela||Present||Introduced||Holm et al., 1997|
|Czechoslovakia (former)||Present||Introduced||Invasive||Cronquist, 1976|
|Greece||Present||Introduced||Invasive||Cronquist, 1976; Baliousis, 2014|
|Russian Federation||Present||Present based on regional distribution.|
|-Central Russia||Present||Introduced||Invasive||Royal Botanic Garden Edinburgh, 2003|
|-Southern Russia||Present||Introduced||Invasive||Royal Botanic Garden Edinburgh, 2003|
|San Marino||Present||Introduced||Invasive||Cronquist, 1976|
|Yugoslavia (former)||Present||Introduced||Invasive||Cronquist, 1976|
|Yugoslavia (Serbia and Montenegro)||Present||Introduced||Invasive||Cronquist, 1976|
|Australia||Present||Introduced||Holm et al., 1979|
|-Queensland||Present||Introduced||Holm et al., 1997|
|-South Australia||Present||Introduced||Holm et al., 1997|
|-Tasmania||Present||Introduced||Holm et al., 1997|
|-Western Australia||Present||Introduced||Holm et al., 1997|
|New Zealand||Present||Introduced||Holm et al., 1979|
|Papua New Guinea||Widespread||Introduced||Henty and Pritchard, 1975|
Risk of IntroductionTop of page Although there is a risk of accidental introduction of C. canadensis, the fact that it is already so widespread means that the phytosanitary risk is relatively low. It is present in many regions where it is able to survive and grow and exclusion from other areas where it is not yet present may prove 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 mainly undisturbed ground, the potential risks to such sites may be considerable.
HabitatTop of page C. canadensis is a weed of agriculture and forestry in temperate and subtropical climates and at higher elevations in some parts of the tropics. It is associated with perennial crops, fallows and field borders and, in annual crops, is favoured by reduced tillage. It is sometimes associated with sandy soils and irrigation, but is not limited to these conditions. It is also a weed of roadsides, old fields and is also associated with recently disturbed land, as an early successional species (Thebaud et al., 1996).
Habitat ListTop of page
|Terrestrial – Managed||Cultivated / agricultural land||Present, no further details||Harmful (pest or invasive)|
|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-natural||Natural forests||Present, no further details||Harmful (pest or invasive)|
Hosts/Species AffectedTop of page C. canadensis occurs in a wide range of crops, annual and perennial, especially where tillage is less intensive. It has been reported in many orchard and other tree crops, including forestry, in grassland and forage crops, and also in annual crops being managed under minimal or reduced tillage regimes. It is also found as a weed in fields producing flowering bulbs and perennials, in forest nurseries, and in ornamental situations in turfgrass.
Host Plants and Other Plants AffectedTop of page
|Ananas comosus (pineapple)||Bromeliaceae||Other|
|Arachis hypogaea (groundnut)||Fabaceae||Main|
|Avena sativa (oats)||Poaceae||Other|
|Beta vulgaris (beetroot)||Chenopodiaceae||Other|
|Brassica napus var. napus (rape)||Brassicaceae||Other|
|Camellia sinensis (tea)||Theaceae||Main|
|Coffea arabica (arabica coffee)||Rubiaceae||Main|
|Elaeis guineensis (African oil palm)||Arecaceae||Main|
|Glycine max (soyabean)||Fabaceae||Main|
|Helianthus annuus (sunflower)||Asteraceae||Other|
|Hordeum vulgare (barley)||Poaceae||Main|
|Ipomoea batatas (sweet potato)||Convolvulaceae||Other|
|Linum usitatissimum (flax)||Other|
|Malus domestica (apple)||Rosaceae||Other|
|Manihot esculenta (cassava)||Euphorbiaceae||Main|
|Medicago sativa (lucerne)||Fabaceae||Other|
|Musa x paradisiaca (plantain)||Musaceae||Main|
|Nicotiana tabacum (tobacco)||Solanaceae||Other|
|Oryza sativa (rice)||Poaceae||Main|
|Pinus banksiana (jack pine)||Pinaceae||Other|
|Pinus densiflora (Japanese umbrella pine)||Pinaceae||Other|
|Pyrus communis (European pear)||Rosaceae||Other|
|Saccharum officinarum (sugarcane)||Poaceae||Main|
|Secale cereale (rye)||Poaceae||Other|
|Solanum lycopersicum (tomato)||Solanaceae||Other|
|Solanum tuberosum (potato)||Solanaceae||Main|
|Triticum aestivum (wheat)||Poaceae||Main|
|Vitis vinifera (grapevine)||Vitaceae||Main|
|Zea mays (maize)||Poaceae||Main|
|Zoysia japonica (zoysiagrass)||Poaceae||Other|
Biology and EcologyTop of page Genetics
C. canadensis is a diploid species, with a chromosome number of 2n=18. This is in contrast with several other species of Conyza, which were ascertained as allopolyploids (Thebaud and Abbot, 1995). As there is a tendency in plants to increasing ploidy levels, it may be assumed that C. canadensis is an ancestral member of the genus, and other species may have arisen via hybridization events. Chromosome size has been studied by Gosteva (1998).
Physiology and Phenology
C. canadensis is predominantly an annual plant, germinating in autumn and persisting as a rosette of leaves over the winter before bolting and flowering the following spring. It may, however, behave as a biennial in temperate climates, but rarely, if ever, persists for a second season after flowering. Seeds need a temperature of 10-25°C and require light for germination (Zinzolker et al., 1985). Establishment occurs mainly in occasionally disturbed situations. Intensive cultivations for annual crops apparently bury most of the seed and greatly reduce emergence, while in completely uncultivated situations other vegetation tends to interfere with its establishment (Escarré et al., 1998; Németh et al., 1998). 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). For further information on the biology of C. canadensis, refer to Weaver (2001).
Seed production can be immense, up to 250,000 seeds per plant (Holm et al., 1997; Weaver, 2001), and seed dispersal by wind is made highly efficient by the pappus (Weaver, 2001). Seed size is small, but also variable, with Fenner (1983) describing the rather complex relation between seed size, seedling establishment and vigour.
C. canadensis is native to an area with a broad climatic amplitude, though is most common in temperate and mediterannean zones, it is found from tundra and taiga to the sub-tropics. Where introduced, it can even be found in tropical regions, though generally at higher altitudes.
There is little evidence for preference regarding soil type, with C. canadensis apparently able to grow in a wide range of soil types.
Prieur-Richard et al. (2002) studied in detail the invasion of C. bonariensis in Mediterranean annual plant communities (see the datasheet on C. bonariensis), though comparisons with C. canadensis may be invalid, as Conyza species are observed to have significantly different characteristics in terms of their position in plant succession. Nonetheless, Conyza spp. including C. canadensis are generally early successional species (e.g. Escarré et al., 1998), though Thebaud et al. (1996) noted that C. canadensis was restricted to recently disturbed areas, whereas C. sumatrensis colonized early- to mid-successional old fields. In Japan, they were noted as dominant in two-year-old fields, being able to grow as shade-tolerant rosettes under the canopy of other annuals, and being gradually succeeded by perennial grasses in later years (Ohtsuka, 1998).
Soil TolerancesTop of page
Special soil tolerances
Natural enemiesTop of page
|Natural enemy||Type||Life stages||Specificity||References||Biological control in||Biological control on|
Means of Movement and DispersalTop of page Natural Dispersal (Non-Biotic)
C. canadensis is principally a wind-dispersed species (e.g. Ohtsuka, 1998), facilitated by light seed accompanied by a pappus which aids flight.
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.
C. canadensis is encouraged by irrigation, perhaps partly because of the distribution of seed by irrigation water (Holm et al., 1997). Mowing along roadsides, especially during seed production, is also likely to increase spread. Also, late tillage or other practices at such inappropriate times will also facilitate seed dispersal.
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 means it may have been a contaminant of cotton, and seeds of other Conyza species may have been introduced via this pathway also. C. canadensis was considered by Park et al. (2001) to have been introduced into Korea Republic with imported cereals for concentrate feed or within seeds for forage production. This is a likely pathway, considering how the species is a weed in forage seed production stands, e.g. in lucerne (Dimitrova and Milanova, 2003).
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 (Gallitano and Skroch, 1993) or for establishing forest plantations (Prach et al., 1995). The spread of C. canadensis, along with numerous other weeds in central European forests, was thought to have been assisted by seeds in tree containers (Prach et al., 1995), and thus, presence in soil must be considered as a potential pathway.
Pathway VectorsTop of page
|Soil, sand and gravel||Seeds in potting compost.||Yes|
Plant TradeTop of page
|Plant parts liable to carry the pest in trade/transport||Pest stages||Borne internally||Borne externally||Visibility of pest or symptoms|
|Growing medium accompanying plants||seeds||Yes||Pest or symptoms usually visible to the naked eye|
|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)|
|Stems (above ground)/Shoots/Trunks/Branches|
Wood PackagingTop of page
|Wood Packaging not known to carry the pest in trade/transport|
|Loose wood packing material|
|Processed or treated wood|
|Solid wood packing material with bark|
|Solid wood packing material without bark|
Impact SummaryTop of page
|Fisheries / aquaculture||None|
ImpactTop of page Holm et al. (1997) record that C. canadensis occurs as a weed in 70 countries, in more than 40 crops. It is most common in the less intensively cultivated perennial crops, such as fruit orchards, vines, forestry, tea, grassland, sugarcane, pineapple and ornamental nurseries. It has also become increasingly important in zero-tilled annual crops, such as cotton and soyabean, where the seeds are left on the soil surface and are exposed to the light necessary for germination, and are not controlled by normal pre-planting tillage (Buhler and Owen, 1997). Having germinated in the autumn it is often well grown by the time of planting and thus quite difficult to control. Its importance is increased by its widespread resistance to paraquat and triazines (see Chemical Control). The are few estimates of its competitive effect, but in Romania yield reductions of 64% have been recorded in sugar beet (Sarpe and Torge, 1980) and Holm et al. (1997) note that vegetative growth of vines can be reduced by 28%. There are some reports of damaging allelopathic effects, for example, to Trifolium repens and lettuce (Souto et al., 1990) It may cause additional economic damage as a result of its aromatic oils contaminating those harvested from Mentha spp. (Ogg et al., 1975).
It is recorded as an alternative host of Tomato bushy stunt virus (Grbelja et al., 1988), and as a host for numerous other serious plant viruses including Tomato spotted wilt virus and Cucumber mosaic virus, nematodes such as Meloidogyne javanica (Dahiya et al., 1998) and Rotylenchulus reniformis (Wang et al., 2003), fungal pathogens such as Sclerotinia minor, also aster yellows phytoplasma, and a range of insect pests (e.g. Weaver, 2001).
Environmental ImpactTop of page The presence of C. canadensis is seen to have significant effects on the soil carbon:nitrogen ratio in a subtropical orchard (Chen et al., 2004), and as such can be expected to have similar effects on natural ecosystems if it become invasive there, though the actual importance of any changes to soil have yet to be ascertained in such situations.
Risk and Impact FactorsTop of page Invasiveness
- Proved invasive outside its native range
- Highly adaptable to different environments
- Highly mobile locally
- Has high reproductive potential
- Negatively impacts agriculture
- Competition - monopolizing resources
- Pest and disease transmission
- Highly likely to be transported internationally accidentally
- Difficult to identify/detect as a commodity contaminant
- Difficult/costly to control
UsesTop of page In Japan and China, the essential oils from C. canadensis are used in the treatment of jaundice (Miyazawa et al., 1992), and the oils have been observed to have antifungal effects. Also, plant extracts have some antifeedant effects on insect species, and as such have potential use as repellents in stored grains. There are also some reports of suppression of other weeds, which could perhaps be regarded as beneficial (Varadi et al., 1987).
Uses ListTop of page
- Essential oils
Similarities to Other Species/ConditionsTop of page Several other Conyza spp. are common weeds, overlapping with C. canadensis in distribution, but these are generally distinguished by having more finely hairy, greyish-green foliage, without the scattered long marginal hairs, and flower heads without ray florets (see datasheets on C. bonariensis and C. sumatrensis). See also Reutelingsperger (2000) for a key differentiating these species.
Prevention and ControlTop of page Cultural Control
In traditional farming, C. canadensis is controlled satisfactorily by tillage, hand-weeding and also suitable crop rotation (e.g. Leroux et al., 1996). In more developed systems, non-chemical methods include the use of living mulches of, for example, Trifolium subterraneum (Enache and Ilnicki, 1988). Other mulches, living or inert, are observed to increase control of C. canadensis, which may be expected as seeds require light for germination. The use of cover crops may also have a similar effect, due to direct competition for light and possible water and plant nutrients, and were shown to be effective in controlling C. canadensis in apple orchards in Korea (Jung et al., 1998). It is, however, surprisingly resistant to destruction by soil solarization (Horowitz et al., 1983).
C. canadensis is normally susceptible to most of the herbicides used to control annual broad-leaved weeds, including 2,4-D and dicamba. However, it has developed widespread resistance to paraquat and the triazines in Europe and the USA. The mechanisms of resistance have been studied in detail, including the dual resistance to both herbicide groups in Hungary (Polos et al., 1988; Lehoczki et al., 1992; Darko et al., 1996). Paraquat resistance has been shown to be controlled by a single dominant gene (Yamasue et al., 1992). This resistance automatically confers moderate resistance to diquat, and triazine resistance results in less susceptibility to bromacil and lenacil. Linuron resistance is also reported from France (Beuret, 1988) and terbacil resistance in Hungary (Molnar et al., 1988). One previously isolated report of resistance to glyphosate (Talbert et al., 1975) is now supported by others, e.g. Solymosi (2001) in Hungary and Van Gessel (2001) in the USA, confirming the increased resistance of this species to such herbicides normally effective along with a wide range of other alternatives depending on the particular cropping regime. These include sulfonylurea herbicides (Németh et al., 1998), cyanazine, sulfallate, glufosinate, oxyfluorfen, hexazinone, tebuthiuron, amitrole, asulam, oryzalin, clopyralid and imazapyr. Inconsistent results are reported with diuron, metribuzin, bentazon and acifluorfen, while poor results have been reported with oxadiazon and imazethapyr.
There has been consideration of biological control possibilities in Italy (Pecora, 1977), the insects of interest including the tephritid Procecidochares australis from USA and the coleopteran Agrilus pulchellus [Engyaulus pulchellus] which is known to attack other Conyza/Erigeron species; but there are no reports of any practical progress.
ReferencesTop of page
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