Taraxacum officinale complex (dandelion)
- Summary of Invasiveness
- Taxonomic Tree
- Notes on Taxonomy and Nomenclature
- Plant Type
- Distribution Table
- History of Introduction and Spread
- Risk of Introduction
- Habitat List
- Hosts/Species Affected
- Host Plants and Other Plants Affected
- Biology and Ecology
- Air Temperature
- Rainfall Regime
- Soil Tolerances
- Natural enemies
- Notes on Natural Enemies
- Means of Movement and Dispersal
- Pathway Vectors
- Plant Trade
- Wood Packaging
- Impact Summary
- Environmental Impact
- Impact: Biodiversity
- Threatened Species
- Social 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
- Taraxacum officinale complex Weber ex Wigg.
Preferred Common Name
Other Scientific Names
- Leontodon taraxacum L.
- Taraxacum dens-leonis Desf.
- Taraxacum officinale Weber ex Wigg.
- Taraxacum vulgare Schrank
International Common Names
- English: bitterwort; blowball; cankerwort; clock; common dandelion; crowparsnip; dandelion; dindle; face clock; fortune-teller; golden milk; grunsel; heart-fever grass; horse-gowan; Irish daisy; milk witch; milk-gowan; monk's head; peasant's cloak; priest's crown; schoolboy clock; stink davie; swine's snout; telltime; time-table; wishes; witch-gowan; yellow-gowan
- Spanish: achicoria silvestre; anagrón; diente de leon; diente-de-lion; taraxacon; taraxacon
- French: coq; dent-de-lion; dent-de-lion commun; florion d'or; laitron; pissenlit; pissenlit; pissenlit officinal
- Portuguese: dente-de-leao
Local Common Names
- Albania: lule qumështore; luleshurdha
- Argentina: chicoria; radicha
- Bolivia: amargon
- Brazil: radice bravio; taraxaco
- Canada/Newfoundland and Labrador: dumble-dor
- Chile: lechugilla
- China: huang-hua-tii-ting
- Czech Republic: smetanka lékaøská
- Denmark: maelkbotte
- Finland: rikkavoikukka
- Germany: Gemeine Kuhblume; Wiesen- Loewenzahn
- Hungary: pongyola pitypang
- Indonesia: jombang
- Italy: capo dè fratre; dente di leone; piscacane; radichiella; soffione; tarassaco
- Japan: seiyotanpopo
- Korea, Republic of: mim-tol-nje; min-deul-rre
- Netherlands: paardebloem
- Norway: lovetann
- Poland: mniszek pospility
- Romania: papadie
- Russian Federation: oduvancik; oduvancik lekarstvennyi
- Serbia: maslacak
- Slovakia: púpava lekárska
- South Africa: perdeblom
- Sweden: maskros
- UK/England and Wales: dant y llew
- USA/Hawaii: lauhele; laulele
- Yugoslavia (Serbia and Montenegro): maslacak
- TAROF (Taraxacum officinale)
Summary of InvasivenessTop of page
Taxonomic TreeTop of page
- Domain: Eukaryota
- Kingdom: Plantae
- Phylum: Spermatophyta
- Subphylum: Angiospermae
- Class: Dicotyledonae
- Order: Asterales
- Family: Asteraceae
- Genus: Taraxacum
- Species: Taraxacum officinale complex
Notes on Taxonomy and NomenclatureTop of page
The origin of the name Taraxacum is uncertain but Holm et al. (1997), Jenniskens (1984) and Mitich (1989) have reviewed possible sources. Taraxacum is thought to originate from the Arabic name for the dandelion 'tarachakum' (meaning wild cherry), 'tarakhshaqun' (meaning wild chicory), 'tharachschakuh', 'talkh chakok' or 'tarashqun' meaning 'bitter herb' (Dwyer, 1977; Jenniskens, 1984; Mitich, 1989). In another explanation, the name was derived from the Greek words 'taraxis', an eye disease, 'tarassen' or 'tarasos' meaning disorder, 'trogimon' meaning edible and 'akeomai' or 'akos' meaning to cure or remedy (Powell, 1972; Jenniskens, 1984; Mitich, 1989). Officinale means medicinal or capable of producing medicine (Schmidt, 1979), or 'of the shops', meaning it was sold as a remedy for man's illnesses (Dwyer, 1977; Holm et al., 1997).
The common name of dandelion is an alteration of 'dent de lion', a phrase thought to be based on the Welsh 'Dant y Llew' of the thirteenth century (Hedrick, 1972), meaning 'tooth of the lion'. This name may have evolved because of the shape of the immature seeds (Lovell and Rowan, 1991), the jagged shape of the leaves (Jackson, 1982), the appearance of the yellow florets of the inflorescence (Angier, 1980), or the strong white taproot (pulling it from a lawn is like trying to extract a lion's tooth) (Dwyer, 1977). The French name, pissenlit, is attributed to the diuretic activity of the plant parts (Lovell and Rowan, 1991).
DescriptionTop of page
The basal rosette gives rise to one to numerous, erect to decumbent, simple, glabrous, hollow, cylindrical scapes (peduncles), 5-50 cm tall, decreasing in diameter along their length from base to tip. Each scape bears a terminal capitulum (inflorescence) of 2-5 cm diameter (Gier and Burress, 1942; Gleason, 1963; Holm et al., 1997). Each capitulum is subtended by an oval-cylindrical involucre with lanceolate-obtuse, green to brownish, herbaceous bracts, in two rows of phyllaries, with the outer phyllaries shorter and wider than the inner phyllaries (Holm et al., 1997). The inner phyllaries are of uniform length and one-serrate, while the outer ones are unequal, one-third to one-half as long as the inner bracts and many-serrate. All bracts are reflexed at maturity, with a convex, minutely pitted receptacle, without paleae (Holm et al., 1997).
The capitulum is composed of up to 250 ligulate, perfect, yellow florets (Holm et al., 1997), which usually have coloured stripes below. Each floret has a corolla of five united petals with one side prolonged, strap-shaped, and five-notched at the tip. Each floret contains five stamens fused into a tube with a sagittate base, filiform basal lobes and an obtuse apex (Holm et al., 1997). The warty spherical pollen grains are 30 µm in diameter (Gier and Burress, 1942). In each floret, the inferior ovary contains one basal, inverted ovule with a single integument. A single style branches into two stigmatic arms, which are 1-1.5 mm in length and 0.06 mm in diameter, and covered with fine hairs (Gier and Burress, 1942; Sood and Sood, 1992; Holm et al., 1997). Each ovule gives rise to a pale grey-brown to olive-brown, narrowly obovoid-oblong, rough-surfaced cypsela (seed), 3-4 mm in length and 1 mm width. Each cypsela is 5-8 ribbed on each side with upwardly pointed teeth at the beaked apex and with a white pappus composed of numerous hairs, 3-4 mm in length, mostly white, persistent and fused at the base, for efficient wind dispersal (Gleason, 1963; Holm et al., 1997). Thus, each capitulum resembles a 'puffball' or 'clock' when mature.
All parts of the plant when broken yield a whitish milky latex that dries to a blackish substance. The latex from another Taraxacum species has been used as a rubber substitute (Krotov, 1945).
The genus Taraxacum is diagnosed by the following combination of characters: leaves all basal; scapes simple, hollow; inflorescences with yellow ligulate florets subtended by two rows of involucral bracts. In the remainder of this datasheet, for the sake of simplicity, the following terms will be used: scape to describe a peduncle, inflorescence to describe a capitulum and seed to describe a cypsela.
Phenotypic variability in T. officinale complex increases its ability to colonize a wide range of habitats. In cool or dry weather, or in closely-mown lawns, the leaves usually spread flat against the surface of the ground to form an almost prostrate rosette (Longyear, 1918; Lovell and Rowan, 1991). In warmer weather or in areas where it is crowded by taller vegetation, the leaves stand in more or less erect tufts (Longyear, 1918). The rosette enables it to survive mowing, grazing and competition with grasses (Baker, 1974). The possession of toothed leaves, which resemble those of thistles, and the bitter white latex, are thought to be adaptations to deter grazing animals (Richardson, 1985).
T. officinale complex displays a wide range of leaf shapes, from a smooth rounded (juvenile) form to a deeply incised runcinate (adult) form (Sánchez, 1971). The length:breadth ratio decreases as the leaf number increases (Sánchez, 1971) and the ratio and depth of incisions in the runcinate form are influenced by light, mediated by the phytochrome system (Wassink, 1965; Sánchez, 1967). Therefore, leaf shape can be regulated by light intensity and quality, with rounded blades developing at low light energy values and runcinate blades developing at high light energy values (Sánchez, 1967; Slabnik, 1981). An increase in light intensity increases the degree of lobing and decreases the length:breadth ratio (Slabnik, 1981).
The complex possesses a deep tap root that can extend below the level of competing grass roots (Loomis, 1938), and make it difficult to remove plants manually (Lovell and Rowan, 1991). The root system can be widely-branched and surmounted by a crown, which can divide to form numerous (up to 22) crown branches, depending on the degree of crowding by other plants and plant age (Roberts, 1936). The roots are also highly regenerative, capable of producing shoots and roots within 1-2 weeks from very small segments (Longyear, 1918; Warmke and Warmke, 1950; Mann and Cavers, 1979). When cut off below the crown, the root usually produces several new shoots so that a cluster of new plants is formed (Longyear, 1918). At the end of the growing season, the root shortens and draws the crown slightly into the soil, where it is better protected from adverse conditions (Longyear, 1918). The ease of regeneration is reportedly related to the ability of the parenchymatous cells of the secondary phloem and xylem in the root to readily dedifferentiate and develop into new shoots and roots (Higashimura, 1986).
During the development of the inflorescence, the growth rate and georesponse of the scape varies (Clifford and Oxlade, 1989). The scape elongates to bloom, then bends down close to the ground while the seeds mature, where it can escape injury from lawnmowers or grazers (Longyear, 1918; Richardson, 1985). When seeds are nearly mature, the scape elongates again up to 75 cm, maximizing its height for effective dispersal of the wind-blown seeds (Longyear, 1918; Jackson, 1982; Richardson, 1985). Therefore, the scape grows upright (negatively orthgeotropic) when extension growth is rapid, as it is prior to flowering and during formation of the capitulum (Oxlade and Clifford, 1981). However, between these stages, when extension growth is minimal and the inflorescence is closed, the scape can grow parallel to the ground (diageotropic) for some or most of its length (Oxlade and Clifford, 1981). The outer tissue layers of T. officinale complex scapes are held in a state of longitudinal tension by internal stem tissues, which are held in a reciprocal state of compression (Niklas and Paolillo, 1998).
Fasciation has been recorded in T. officinale complex in two forms, confined to the reproductive tissues of the plant (Dekker and Dekker, 1987). In plants with multiple scapes, the central scapes can be fused together to form one broad (1-2 cm) scape; or inflorescences (2-4) can be fused at their base to form a longitudinal floral structure (Dekker and Dekker, 1987).
Plant TypeTop of page
DistributionTop of page
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.Last updated: 10 Jan 2020
|Continent/Country/Region||Distribution||Last Reported||Origin||First Reported||Invasive||Reference||Notes|
|-Jammu and Kashmir||Present|
|Uzbekistan||Present||Original citation: Turaev & Khurramov, 1981|
|Czechia||Present||Introduced||Original citation: Klimes et al. (2003)|
|Federal Republic of Yugoslavia||Present||Introduced|
|Denmark||Present||Introduced||Original citation: Molgaard (1977)|
|-British Columbia||Present, Widespread||Introduced|
|-New Brunswick||Present, Widespread||Introduced|
|-Newfoundland and Labrador||Present, Widespread||Introduced|
|-Northwest Territories||Present, Widespread||Introduced|
|-Nova Scotia||Present, Widespread||Introduced|
|-Prince Edward Island||Present, Widespread||Introduced|
|United States||Present, Widespread||Introduced|
|-New Hampshire||Present, Widespread||Introduced|
|-New Jersey||Present, Widespread||Introduced|
|-New Mexico||Present, Widespread||Introduced|
|-New York||Present, Widespread||Introduced|
|-North Carolina||Present, Widespread||Introduced|
|-North Dakota||Present, Widespread||Introduced|
|-Rhode Island||Present, Widespread||Introduced|
|-South Carolina||Present, Widespread||Introduced|
|-South Dakota||Present, Widespread||Introduced|
|-West Virginia||Present, Widespread||Introduced|
|-New South Wales||Present, Widespread||Introduced|
|-Northern Territory||Present, Widespread||Introduced|
|-South Australia||Present, Widespread||Introduced|
|-Western Australia||Present, Widespread||Introduced|
History of Introduction and SpreadTop of page
Risk of IntroductionTop of page
HabitatTop of page
It prefers disturbed ground (Witty and Bing, 1985; Lovell and Rowan, 1991; Hamill, 1997; Holm et al., 1997) and, in fact, in undisturbed communities, a plant may not flower until its fourth season (Gorchakovskii and Abramchuk, 1996). T. officinale complex responds positively to repeated cutting (Dahmen and Kuhbauch, 1990; Teyssonneyre et al., 2002) or heavy grazing (Rogalski et al., 1997; Harker et al., 2000). Establishment and performance of T. officinale complex in grass vegetation is dependent on the height and cutting frequency of the grass (Molgaard, 1977). Increasing grass height led to a decrease in the density of T. officinale complex, at least partly due to shading (Molgaard, 1977). Also, the reproductive morphology of T. officinale complex in alfalfa fields was different, facilitating colonization of open areas, compared to the reproductive morphology on undisturbed sites with a high density of grass (Welham and Setter, 1998).
Habitat ListTop of page
|Terrestrial||Managed||Cultivated / agricultural land||Present, no further details||Harmful (pest or invasive)|
|Terrestrial||Managed||Protected agriculture (e.g. glasshouse production)||Present, no further details||Harmful (pest or invasive)|
|Terrestrial||Managed||Managed forests, plantations and orchards||Present, no further details||Harmful (pest or invasive)|
|Terrestrial||Managed||Managed grasslands (grazing systems)||Present, no further details||Harmful (pest or invasive)|
|Terrestrial||Managed||Disturbed areas||Present, no further details||Harmful (pest or invasive)|
|Terrestrial||Managed||Rail / roadsides||Present, no further details||Harmful (pest or invasive)|
|Terrestrial||Managed||Urban / peri-urban areas||Present, no further details||Harmful (pest or invasive)|
|Terrestrial||Natural / Semi-natural||Natural forests||Present, no further details||Harmful (pest or invasive)|
|Terrestrial||Natural / Semi-natural||Natural grasslands||Present, no further details||Harmful (pest or invasive)|
|Terrestrial||Natural / Semi-natural||Riverbanks||Present, no further details||Harmful (pest or invasive)|
|Terrestrial||Natural / Semi-natural||Wetlands||Present, no further details|
|Terrestrial||Natural / Semi-natural||Cold lands / tundra||Present, no further details||Harmful (pest or invasive)|
|Terrestrial||Natural / Semi-natural||Deserts||Present, no further details|
|Littoral||Coastal areas||Present, no further details||Harmful (pest or invasive)|
Hosts/Species AffectedTop of page
Host Plants and Other Plants AffectedTop of page
|Aronia melanocarpa (black chokeberry)||Rosaceae||Other|
|Beta vulgaris (beetroot)||Chenopodiaceae||Main|
|Brassica napus var. napus (rape)||Brassicaceae||Main|
|Daucus carota (carrot)||Apiaceae||Other|
|Fragaria ananassa (strawberry)||Rosaceae||Other|
|Glycine max (soyabean)||Fabaceae||Main|
|Hordeum vulgare (barley)||Poaceae||Main|
|Lactuca sativa (lettuce)||Asteraceae||Main|
|Lolium perenne (perennial ryegrass)||Poaceae||Main|
|Malus domestica (apple)||Rosaceae||Other|
|Medicago sativa (lucerne)||Fabaceae||Main|
|Mentha piperita (Peppermint)||Lamiaceae||Other|
|Nicotiana tabacum (tobacco)||Solanaceae||Other|
|Poa pratensis (smooth meadow-grass)||Poaceae||Main|
|Pyrus communis (European pear)||Rosaceae||Other|
|Secale cereale (rye)||Poaceae||Other|
|Solanum tuberosum (potato)||Solanaceae||Other|
|Trifolium pratense (red clover)||Fabaceae||Other|
|Trifolium repens (white clover)||Fabaceae||Main|
|Triticum aestivum (wheat)||Poaceae||Main|
|Vitis vinifera (grapevine)||Vitaceae||Other|
|Zea mays (maize)||Poaceae||Main|
Biology and EcologyTop of page
T. officinale complex ranges in ploidy level from diploid to hexaploid (x=8), possessing 16 to 48 individual chromosomes (Richards, 1973). North American individuals of this complex are generally triploid (x=8, 3x=24), and sexual reproduction is rare or may even be absent (Solbrig and Simpson, 1974; Lyman and Ellstrand, 1984). With more than 2000 reported micro-species of this genus in Europe, approximately 90% are polyploids that reproduce asexually by obligate agamospermy (apomixis). The majority of the remaining 10% are diploid species that reproduce sexually and are obligate outcrossers. However, a small number of more primitive forms are capable of self-fertilization (Hughes and Richards, 1985). In T. officinale apomicts, almost every floret automatically produces a seed which is genetically identical to the mother so that mother-daughter lines form 'seed clones' (Richards, 1973; Hughes and Richards, 1988; Richards, 1996), also known as 'biotypes', which are usually given specific status (for example, Dudman and Richards, 1997).
As Taraxacum species exhibit extremely variable biology and morphology, the genus is treated as many micro-species in Europe, as previously mentioned; however, it is treated as one species exhibiting considerable phenotypic plasticity in North America (Richards, 1973). This extensive variation may be somewhat unexpected since North American populations are generally considered to be apomictic and so do not exchange genes (Solbrig and Simpson, 1974; Taylor, 1987). There are differences of opinion regarding the extent to which the observed variation is due to phenotypic plasticity versus genotypic differentiation arising from multiple introductions of European microspecies (Taylor, 1987). Janzen (1977) suggested that there is very little genetic variation among populations, while Abbott (1979) argued that this assumption is premature. Taylor (1987) stated that intra-populational morphological variation was as great as or greater than inter-populational variation and, therefore, morphological variation was largely due to phenotypic plasticity. However, Kennison (1978) found that in populations from Washington State, variation among populations was consistently greater than variation within populations.
Ford (1981a) observed that agamospecies growing in particular habitats differed from site to site in characters such as population flux, survivorship and fecundity. Furthermore, agamospecies represent ecologically highly specialized population units, relevant to a fine scale of heterogeneity of habitat, and two or more agamospecies can coexist in a broad habitat (Ford 1981b, 1985). Small and Catling (1999) provide an excellent summary of variation in this complex in Canada. King (1993) used restriction enzyme analysis of ribosomal DNA and chloroplast DNA to assess the relative contribution of hybridization and mutation as sources of genotypic variation in dandelions of North America. She found that multiple hybridization events in populations (prior to their introduction to North America) were a more important source of genotypic variation than mutation in populations.
Although sexual reproduction is rare (Solbrig and Simpson, 1974), natural hybrids have been reported to occur between T. officinale complex and T. platycarpum in Japan (Watanabe et al., 1997). More than 90% of the plants classified as T. officinale complex had alleles introduced from T. platycarpum and were morphologically intermediate between the two species with respect to the number of marginal hairs in the outer involucral bract, the length of corniculate appendages on the outer involucral bract, and the size of the seed (Watanabe et al., 1997).
Physiology and Phenology
T. officinale complex leaves are rich in fibre, potassium, iron, calcium, magnesium, phosphorus, vitamins A and C, the B vitamins thiamine and riboflavin, and protein (Schmidt, 1979; Jackson, 1982; Gail, 1994). Gail (1994) reported that they are also nature's richest vegetable source of beta-carotene at 0.84 mg/g tissue compared to carrots (Daucus carota) at 0.61 mg/g tissue. They rank above broccoli (Brassica oleracea) and spinach (Spinacia oleracea) in overall nutritional value (Haytowitz and Matthews, 1984), and Minnich (1983) ranked them out of all vegetables (including grains, seeds and greens) as tied for ninth best, higher than lettuce (Lactuca sativa). Also, the roots of T. officinale complex are rich in iron, copper and other trace elements (Dwyer, 1977). The most prominent therapeutic property of T. officinale complex is the diuretic activity, which is based on the high potassium content of the plant (Hook et al., 1993). It is superior to other diuretics because it reduces the likelihood of hypokalaemia, a common side-effect of many diuretics (Houghton, 1995).
The major and trace element content of T. officinale complex alters with growth stage (Müller and Kirchgessner, 1972). In a Finnish study, the vitamin C content was lowest, while dry matter, soluble solids and mineral content were highest in late summer (Kuusi et al., 1982). Dandelion mineral content was investigated by van der Kley (1956) to assess the suitability of this species as feed for livestock. The high amounts of protein and beta-carotene, favourable mineral composition, and low nitrate content throughout the growing season in Poland provided a high value feed (Falkowski et al., 1990). In UK studies, the availability of these elements was equivalent to that in perennial ryegrass (Lolium perenne), a popular forage species (Wilman and Derrick, 1994). T. officinale complex also had a lower proportion of cell walls in dry matter than perennial ryegrass (Derrick et al., 1993). The true dry matter digestibility was as high as that of ryegrass, but the in vivo digestibility was lower (Derrick et al., 1993).
T. officinale complex is a C3 species (Kemp et al., 1977). In US studies, plants from different altitudes showed no significant differences in enzyme activity, net photosynthesis, dark respiration, photorespiration, transpiration rates or temperature responses of gas exchange (Kemp et al., 1977; Oulton et al., 1979). This is in disagreement with earlier US reports, which indicated that there were differences in the photosynthetic Hill reaction and enzyme activity among the same altitudinally diverse populations (May and Villarreal, 1974; May, 1976). Activity of the enzyme invertase, which was only present in the petiole and central midrib of the developing leaf, was correlated with leaf growth rate and its level was controlled by light (Slabnik, 1981). A US study showed that the leaf size of T. officinale complex plants decreased linearly with increasing elevation and a corresponding decline in nocturnal infrared irradiation from the sky (Jordan and Smith, 1995). Differences in plant leaf structure and physiology traditionally associated with daytime sun exposure may also be influenced by night-time sky exposure and susceptibility to frost (Jordan and Smith, 1995). Red light and far-red light influenced the water exchange of epidermal cells of T. officinale complex and phytochrome appeared to be involved (Carceller and Sánchez, 1972).
Various chemicals and cellular structures play roles in the geotropic behaviour of T. officinale complex. UK studies showed that elevated levels of endogenous ethylene were associated with the diageotropic behaviour and reduced extension growth after flowering (Clifford and Oxlade, 1989). There were significant differences in indol-3yl-acetic acid (IAA) levels across the scape after geostimulation, indicating a role for auxin in geotropism (Clifford et al., 1985). In an earlier British study, Clifford and Barclay (1980) showed that amyloplasts (colourless plastids that form starch granules) in the cells of scapes sediment much faster than previously reported and were involved in the initiation of geotropism in T. officinale complex.
Plants exposed to elevated levels of carbon dioxide grew faster, exhibited more deeply incised leaf margins and had relatively more slender leaf laminae than those exposed to ambient levels (Thomas and Bazzaz, 1996; Staddon et al., 1999). These effects were most pronounced when T. officinale complex plants were grown individually, but detectable differences were also found in plants grown at high density (Thomas and Bazzaz, 1996). This supports the hypothesis that leaf carbohydrate levels play an important role in regulating heteroblastic leaf development, although elevated carbon dioxide may also affect leaf development through direct hormonal interactions or increased leaf water potential (Thomas and Bazzaz, 1996). However, Teyssonneyre et al. (2002) found that the population of T. officinale complex in a grassland community did not increase in response to elevated CO2.
Environmental factors, such as temperature, photoperiod and rainfall, were studied in Finland for their effect on the bitterness of T. officinale complex (Kuusi and Autio, 1985). It was found that increasing temperature and photoperiod and decreasing rainfall were correlated with an increase in bitterness. However, morphological characters such as leaf shape, main nerve breadth and colour of petiole base were not correlated with bitterness (Kuusi and Autio, 1985). Growth phase and season had a strong influence on bitterness, with plants being less bitter in spring before flowering than in late summer (Kuusi and Autio, 1985). Bitterness in leaves was caused by sesquiterpene lactones, such as taraxinic acid and its glucoside, as well as triterpenoids, such as cycloartenol (Houghton, 1995). Kuusi et al. (1985) identified four bitter compounds: p-hydroxyphenylacetic acid, beta-sitosterol, 11,13-dihydrotaraxine acid 1'-O-beta-D-glucopyranoside and taraxine acid 1'-O-beta-D-glucopyranoside (also identified as an allergen (von Hausen, 1982)). The following triterpene alcohols have been isolated from tissues of T. officinale complex: taraxol, taraxerol, lupeol, taraxasterol, beta-amyrin, stigmasterol and beta-sitosterol. Phenolic acids related to caffeic acid, caffeoyltartaric acids, cinnamic acids, coumarins, flavonoids, beta-fructofuranosidases and the serine proteinase taraxalisin have also been isolated from T. officinale complex tissues (Burrowes and Simpson, 1938; Rutherford and Deacon, 1972; Akashi et al., 1994; Houghton, 1995; Williams et al., 1996; Budzianowski, 1997; Rudenskaya et al., 1998). T. officinale complex is also a valuable source of the essential linolenic acid, apigenin-7-glucoside, lecithin, and cholin (Houghton, 1995; Letchamo and Gosselin, 1995). Unsaturated hydroxy fatty acids such as linolenic acid are important in the chilling-resistant properties of T. officinale complex (Imai et al., 1995).
T. officinale complex possesses allelopathic properties that can reduce germination and growth of other plant species (Dwyer, 1977; Jackson, 1982; Falkowski et al., 1990). In addition, phenolic compounds produced by T. officinale complex are considered responsible for biological control of Fusarium oxysporum f.sp. radicis-lycopersici in greenhouse tomato plantings in Canadian experiments (Kasenberg and Traquair, 1988). Satisfactory control of this pathogen was achieved when residues of T. officinale complex were incorporated into sterilized greenhouse soil. The mode of action is unknown but it may act directly by secretion of allelochemicals or promotion of antagonistic microflora (Jarvis, 1989). T. officinale complex leaves are capable of producing an anti-fungal toxin called lettucenin A (Mizutani, 1989). It is a stress-induced antifungal sesquiterpenoid that is present in sufficient quantity in dandelion to suppress invasion of a pathogen in vivo (Hanawa et al., 1994). It was found that lettucenin A production started at an early stage of fungal infection, before the appearance of symptoms, and ended soon after the death of the pathogen (Hanawa et al., 1994).
Carbohydrate and nitrogen reserves in the roots of T. officinale complex fluctuate with the season (Loomis, 1938). The roots remained viable during the winter and acted as a source of nutrients to facilitate the resumption of growth in early spring (Cyr et al., 1990). Fructans, storage carbohydrates such as inulin and inulo-n-ose, were synthesized in roots by the enzyme fructan-fructan fructosyl transferase (Lüscher et al., 1993; Ernst et al., 1996). This synthesis of inulin was practically unaffected by the height of competing grass vegetation (Molgaard, 1977). High inulin content in roots resulted in high nitrogenase activity (Vlassek and Jain, 1976), which could enrich the soil with nitrogen through asymbiotic nitrogen fixers such as Azotobacter and Clostridium species (Vlassek and Jain, 1978). Fructan hydrolysis occurred during late autumn and provided simple sugars as a readily accessible carbon pool (Cyr et al., 1990). Nitrates, free amino acids and soluble proteins were important vehicles for nitrogen storage (Cyr et al., 1990). Storage reserves remained at peak levels throughout winter and declined prior to the resumption of growth in spring, when inulin was metabolized to provide a high content of mobile fructose and sucrose to enable extensive vegetative growth and flowering (Molgaard, 1977; Cyr et al., 1990). At the time of fruiting, nitrogen reserves were at their lowest concentration (Loomis, 1938). Toward the end of summer, these reserves were restored and the cycle began again (Rutherford and Deacon, 1974).
In other studies, it was found that amino acids accumulated in the roots as autumn senescence progressed in the aerial parts of the plant, and declined in spring when regrowth occurred, with large fluctuations in the amides asparagine and glutamine (Cyr and Bewley, 1990a). An 18-kDa protein increased in T. officinale complex roots during the autumn months, suggesting that it has a role as a storage compound, possibly in cold protection (Cyr and Bewley, 1990b; Xu et al., 2000). The same protein was also found in inflorescences, the vestigial stem and seeds (Cyr and Bewley, 1990b). Interestingly, the protein also possesses properties similar to allergen- and pathogenesis-related proteins (Xu et al., 2000). A UK study showed that leaf compounds of T. officinale complex fluctuated with season, probably due to temperature (Westerman and Roddick, 1981). Free 4-demethyl sterols were maximal during the winter months and levels were correlated negatively with sunshine and temperature (Westerman and Roddick, 1981). Sitosterol ester and cycloartenol ester showed the opposite response, with levels correlating positively with sunshine and temperature (Westerman and Roddick, 1981). The scapes contained predominantly beta-sitosterol and beta-amyrin (Aexel et al., 1967).
Generally, during the first season of growth, T. officinale complex seedlings produce only leaves, usually in rosettes (Longyear, 1918). In the spring of the second season, and each season thereafter, inflorescences are produced (Longyear, 1918). However, under favourable conditions, some seedlings can bloom in their first year (von Hofsten, 1954; Listowski and Jackowska, 1965). The first bud may appear at various times and cannot be correlated to leaf index, although the plant has to have formed at least 20 leaves and enlarged its tap root to store the required energy (Listowski and Jackowska, 1965; Solbrig, 1971). The time of first flowering is partly dependent on the surrounding plant community and, in undisturbed communities, a plant may not flower until its fourth season (Gorchakovskii and Abramchuk, 1996).
In T. officinale complex, flowering occurs over a wide range of photoperiods and light intensities. Studies on seasonal variation in flowering of T. officinale complex showed that plants flowered throughout the year, with most plants flowering in spring when the average daily air temperature was around 16°C and day length was 10-13 h (Gray et al., 1973; AJ Richards, University of Newcastle, UK, personal observation). In the northern hemisphere, this equates to March-May in the UK, April-May in North America, June in Norway and Siberia. A secondary peak occurred in the autumn (September-October), with an average of 21°C and 12-13 h day length (Gray et al., 1973). Therefore, T. officinale complex can be classified as a day-neutral plant (Listowski and Jackowska 1965; Gray et al., 1973), although Solbrig (1971) classified it as a short-day plant due to limited flowering during long summer days. Individual plants that bloom in spring may also bloom again in autumn (Listowski and Jackowska, 1965). Bud formation in these plants occurs during a period of decreasing daylight, when the differentiation of new leaves is limited and existing leaves show symptoms of premature aging (Listowski and Jackowska, 1965). The time course of the main spring flowering period may vary in different years, partly due to differences in microclimate, such as the amount of sunshine and soil temperature (von Hofsten, 1954; Sterk and Luteijn, 1984). The number of times the inflorescences open and close, the length of time that the inflorescences remain open each day, and the length of time that the inflorescences remain closed before opening into mature heads, vary with time of year (Gray et al., 1973).
The development of buds requires approximately 1 week (Solbrig, 1971). A scape is formed between the base of the bud and the tip of the shoot in about 48 h (Solbrig, 1971). On average, inflorescences open during two or three successive days, after which they remain closed until the seeds mature (Longyear, 1918; Gray et al., 1973). The scape and the inflorescence flatten to the ground and, after a couple of days, the scape straightens and the involucral bracts surrounding the closed inflorescence open to reveal seeds (Solbrig, 1971). The time required from the first day of blooming until the seeds ripen and the bracts open to release them, is about 9-12 d (Longyear, 1918; Beach, 1939; Gray et al., 1973).
A study in Japan showed that at low temperatures, inflorescences opened in response to increasing temperature (thermonasty), while at higher temperatures, they opened in response to light (photonasty) (Tanaka et al., 1988). The minimum temperature for photonastic opening was 13°C and inflorescences remained open for 13-14 h (Tanaka et al., 1988). At temperatures of 13-18°C, plants were in full bloom and this was most favourable for nectar secretion, pollen production and bee activity (Kremer, 1950). In Michigan, USA, inflorescences were reported to close when the temperature was over 21°C or during adverse weather, and could remain closed for several days and then re-open when climatic conditions were favourable (Kremer, 1950). Once closed, however, they did not open again on the same day (Kremer, 1950).
Seeds produced in the spring during the peak flowering period mostly emerged that same spring or did not emerge at all (Collins, 2000). However, seeds produced at other times during the year produced seedlings throughout the year. Seedlings produced in the autumn produced seeds in the spring of the following year. Chepil (1946) and Roberts and Neilson (1981) found that seedlings emerged in most months of the year in Canada and for up to four years after sowing. Collins (2000) collected ripe, viable seeds from a single population on the University of Western Ontario campus, London, Canada on the following dates in 1999: May 1, 17, 27; June 10; August 20; September 14, 21; October 4, 20; November 5, 22; and December 13. At least 58% of the seeds collected on each date germinated. P Cavers (University of Western Ontario, London, Canada, personal observation) has collected ripe seeds in every month of the year, but not every month in a single year. He concluded that if there is a January or February thaw that lasts for at least a week, then flowering and seed production can occur.
The survival and regeneration of root fragments vary seasonally (Mann and Cavers, 1979). Minimum survival of fragments occurred at the time of maximum flowering of the source plants and maximum survival of fragments occurred in the second growing season (Mann and Cavers, 1979).
As T. officinale complex is an apomict (the embryo develops without fertilization) and a triploid of hybrid origin, most pollen grains are abortive and sterile, and cannot form pollen tubes (Solbrig, 1971; Jenniskens, 1984). Generally, in the Asteraceae, ligulate or ray-florets are sterile, and tubular or disc-florets are fertile. However, in T. officinale complex, there is no distinction between ray- and disc-florets, either in appearance or function, with all florets being ligulate and equally fertile (Roberts, 1936). The ligulate florets are surrounded by inner and outer involucral bracts that close at night, in overcast weather, when the relative humidity is above 97% or when the temperature is less than 9.4°C. Opening of the inflorescences is inhibited by rain and accelerated by high light intensity (Percival, 1955; Jenniskens et al., 1984).
In the Peace River region of Alberta, Canada, Szabo (1984) found an average daily production of 59.2 inflorescences/m², which is equivalent to 592,700 inflorescences ha/d and, over a 25-d blooming period, represented a potential production of 14,792,500 inflorescences/ha. On a sunny day, inflorescences opened between 0800 and 0900 h, reached a peak at 1100 to 1200 h and closed gradually from 1500 to 2100 h; but the whole period was shorter on dull days since high light intensity accelerates inflorescence opening (Percival, 1955, Szabo, 1984). A UK study found that T. officinale complex presented its pollen from 0900 to 1500 h, with the peak period from 1000 to 1100 h (Percival, 1955). Most inflorescences (89%) opened for two consecutive days, some (7%) for 1 d and some (4%) for 3 d (Szabo, 1984). Quantity and concentration of nectar were significantly higher in inflorescences 2 d old than in those 1 d old (Szabo, 1984). Larger inflorescences produced more nectar, and the nectar-sugar concentration and sugar value increased with increasing temperature (Szabo, 1984). High nectar-foraging activity by honeybees coincided with peak nectar-sugar production (Szabo, 1984), and anthers dehisced over a period of 1-7 d (Percival, 1955).
Although stamens and pistils are present and pollen is produced regularly, the seed of T. officinale complex develops without fertilization (Roberts, 1936). Originally, it was thought that seeds were primarily produced by allogamy, and insects such as honeybees and flies were pollinators (Longyear, 1918). However, it has been suggested by UK investigators that insect visitors, attracted by the bright yellow inflorescences, may be needed to trigger seed set (Williams et al., 1996). In a heavily infested area in Canada, the average number of seeds produced by T. officinale complex was 60,000/m², equivalent to about 600,000,000 seeds/ha (Roberts, 1936). Under near optimal conditions, the number of inflorescences/plant ranged from 48 to 146, with an average of 93, while the number of seeds/inflorescence ranged from 130 to 412, with an average of 252 (Roberts, 1936). This provides an average of 23,436 seeds/plant (Roberts, 1936).
In UK studies on the reproductive success of T. officinale complex, Bostock and Benton (1979) found that of 185.5 ovules produced/inflorescence, 181.7 seeds were produced, and 178.1 were dispersed. As well, an average of 12.2 inflorescences/plant were observed, resulting in approximately 2,170 seeds/plant being produced during the growing season. Ford (1981b) found that the number of inflorescences/plant, number of seeds/inflorescence and, therefore, the number of seeds/plant varied with the habitat in which Taraxacum agamospecies were found. The number of inflorescences/plant was 7.7 for plants on denuded roadsides, 1.7 for plants on non-denuded roadsides, and only 0.6 for plants in upland sites. Similarly, the number of seeds/inflorescence was 151 for plants growing in denuded roadside sites, as compared to 119 in non-denuded roadside sites, and only 62 in upland sites. Similarly, a US study found that the number of inflorescences/plant and florets/inflorescence depended on the size and vigour of the plant (Longyear, 1918). Small stunted plants growing in dry soil may produce only a single inflorescence with 30 florets/inflorescence; while large clumps of plants along roadsides may produce 50 or more inflorescences at once with more than 200 florets/inflorescence (Longyear, 1918).
Collins (2000) found that the mean mass (mg) per seed in the University of Western Ontario, Canada, collection ranged from 0.33 mg in an August collection to 0.68 mg in a late October collection. At the peak of flowering in May, it ranged between 0.43 and 0.49 mg. The seeds produced in late October and early November were significantly heavier than those produced at other times of the year. UK reports of seed weights vary, with Salisbury (1961) stating an average weight of 0.8 mg, Bostock (1978) reporting an average seed weight of 0.583 mg, while Sheldon (1974) reported the average seed weighed 0.0549 mg; one tenth as much as in any other report. Also, the average seed was reported as 10.25 mm in length, with a pappus diameter is 6.94 mm (Sheldon, 1974).
At least 7 d must elapse after the opening of the inflorescence before most seeds of T. officinale complex are mature enough to germinate (Longyear, 1918). Therefore, if all inflorescences, including those that have closed to ripen the seeds, are removed from the plant (e.g. by mowing), the germination of seeds from the dried inflorescences that have ripened after cutting is only 13% (Longyear, 1918; Roberts, 1936). Fully mature seeds of T. officinale complex lack primary dormancy and are able to germinate almost as soon as they leave the plant (Longyear, 1918; Martinková and Honek, 1997). However, the proximity of the seeds to each other influences germination. In a US study, seeds placed singly or in groups of 5, 10 or 25 had germination percentages of 68, 64, 54 and 41%, respectively. This significant decrease in germination with increasing density may be a population-regulating mechanism (Linhart, 1976).
Very large populations of T. officinale complex seeds can be found in soil. For example, in the UK, Champness and Morris (1948) found 1,575,000 seeds/ ha in the top 13 cm of soil in a grassland area, and 2,350,000 seeds/ha in the top 18 cm of an arable field, although not all will germinate. The germination capacity of T. officinale complex seeds is generally 80-90% (Falkowski et al., 1989), and Martinková and Honek (1997) reported 94% germination on moist filter paper, 28 d after collection. Most seeds germinate within 1.5 months after dispersal (Ogawa, 1978). However, von Hofsten (quoted by King (1966)) estimated that, of 10,000-20,000 seeds/m² dispersed into a meadow, only 50-125 would establish successfully. Germination is impaired after passage through the digestive tracts of cattle, with germination of 52%, 31% and 22% after retention in faeces for 5 h, 24 h and 48 h, respectively (Falkowski et al., 1989, 1990). Germination is also influenced by individual seed weight (Tweney and Mogie, 1999). There is variation in the weight of seeds produced in a single inflorescence and the heavier seeds have a greater probablility of germination (Tweney and Mogie, 1999).
Seeds of T. officinale complex germinate over a wide range of temperatures, from 5 to 35°C (Mezynski and Cole, 1974; Washitani, 1984), with less germination at higher temperatures (Martinková and Honek, 1997). Germination does not need vernalization or cold stratification. Most reports indicate that seeds of T. officinale complex germinate best under alternating temperatures and light. Collins (2000) found 85-94% of seeds, from two populations from London, Ontario, Canada, germinated under the following regimes: 15°C/5°C with 9 h light, 25°C/10°C with 14 h light, 35°C/20°C with 15 h light and 25°C/10°C in total darkness. However, other temperature regimes in total darkness led to reduced germination, with only 75% at 35°C/20°C and 43% at 15°C/5°C. Cross (1931) found 75-76% of seeds, collected from Ottawa, Ontario, Canada germinated in alternating temperatures of 30°C/20°C, while only 60-65% germinated at a constant temperature of 18-20°C. Mezynski and Cole (1974) reported maximum germination of fresh seeds, collected from Maryland, USA, at an alternating temperature of 20°C for 16 h and 10°C for 8 h (while seeds stored for 30 d germinated best at 20°C/15°C). Maguire and Overland (1959) found that 92% of seeds, collected from Washington State, USA, germinated at alternating temperatures (30°C/20°C) in alternating light and darkness, but when kept in darkness, there was only 72% germination. Also, at a constant temperature of 15°C in dark, only 4% germination was recorded, but when alternating light and dark was added to the same constant temperature, 96% germination was observed. Two UK studies (Williams, 1983; Thompson, 1989) reported similar results. Seed germination was greater, faster and more uniform in light than in dark (Isselstein, 1992; Letchamo and Gosselin, 1996), although dark germination increased with length of storage (Isselstein, 1992). These different findings may reflect variability between biotypes (van Loenhoud and Duyts, 1981). In a Swedish study, Noronha et al. (1997) used cold, dark stratification to show that seeds of T. officinale complex have an inducible light requirement for germination. Ecologically, this inducible light requirement is important for preventing the germination of buried seeds in conditions unfavourable to seedling development (Noronha et al., 1997).
There are varying reports of the viability of seeds of T. officinale complex after storage. Seeds reportedly germinate best after 3 months dry storage, but germination success then decreases with age (Russwurm and Martin, 1977), certainly after 1 year in storage, and rarely germinates after 3 years dry storage at 20°C (AJ Richards, University of Newcastle, UK, personal observation). In another study, following burial of T. officinale complex seeds for varying time periods, a small number of seeds (1%) were still viable for up to 9 yr after burial (Burnside et al., 1996). Typically, 1-6% of T. officinale complex seeds remained viable 4 yr after burial in soil, and soil storage for 5 yr or longer resulted in little detectable viability (Chepil, 1946; Roberts and Neilson, 1981). Von Hofsten (1954) found that seeds remained viable longer in the soil (up to 20-30 yr) than when dry-stored indoors. Depth of seed burial was negatively correlated to establishment success (Bostock and Benton, 1983), and Russwurm and Martin (1977) found seed can readily germinate under 2 cm of soil. Storage of seeds at room temperature decreased seed viability, compared to storage at 4°C (Letchamo and Gosselin, 1996). Mezynski and Cole (1974) reported that percentage germination decreased during 30 d storage of seeds at -15°C or 22°C, compared to fresh seeds. Al-Hially (1991) found that the rate of germination increased after 90 d storage.
The position of T. officinale complex seeds affects germination, with greatest germination occurring when there is good contact between the substrate and the scar of attachment of the seed, allowing water uptake (Sheldon, 1974). The hygroscopic pappus can move to alter the seed position as humidity changes and, in high humidity, the pappus closes hygroscopically (Sheldon, 1974). The backward-pointing hairs and teeth of the seed may play a role in firmly anchoring it during seedling establishment (Sheldon, 1974). Increasing soil compaction decreases seed germination, radicle penetration and seedling establishment, partly due to removal of microsites (Sheldon, 1974).
Vegetative growth is limited to the formation of multi-rosetted clumps and root fragmentation. The regenerative capacity of T. officinale complex roots has been examined by Naylor (1941) and Khan (1973). Vegetative propagule weight was positively correlated to establishment success (Bostock and Benton, 1983). Root segments that were 1.25 mm in diameter had to be at least 6-10 mm in length to regenerate, and segments as short as 2 mm could regenerate only if they were more than 4 mm in diameter (Warmke and Warmke, 1950). The minimum length for shoot regeneration was 1.5 mm and for root regeneration 2 mm (Khan, 1969). More shoots and roots regenerated from longer root segments than from shorter ones (Khan, 1969). Regenerative capacity decreased as fragment volume decreased (down the length of the root) (Mann and Cavers, 1979). Planting cuttings in an inverted or horizontal plane, rather than the normal planting orientation resulted in a decline in regeneration and survival, and an increase in regeneration time (Mann and Cavers, 1979). Planting depth did not consistently influence regenerative capacity or timing (Mann and Cavers, 1979). Root fragments were highly vigourous and capable of producing new plants even when covered by 5-10 cm of soil (Falkowski et al., 1989; 1990), which may be due to their high sugar content (~11% of dry matter). Therefore, successful physical removal must include the entire root system.
T. officinale complex is perennial, but often rather short-lived. Plants can aestivate, overwinter as a reduced basal rosette under snow cover or as seed (Cyr et al., 1990). Colonizers in a seral succession are often killed by later competition.
T. officinale complex can tolerate a broad range of climatic conditions (Simon et al., 1996) and is distributed in almost every temperate and subtropical region of the world (Holm et al., 1997). It is tolerant of a very wide range of conditions, from 90 to 2780 mm annual precipitation and 4.3 to 26.6°C average annual temperature. It shows a wide range of adaptability to light, being able to grow vigorously in full sunlight, or in diffused light in the shade of trees or buildings (Longyear, 1918). It can grow in a wide range of soils (Simon et al., 1996) but it flourishes best in moist, good quality loam (Jackson, 1982). Soil moisture determines its local distribution, with well-watered areas of lawns being especially favourable for its growth (Longyear, 1918). However, it has been recorded pushing up through concrete, hanging from eavestroughs of houses, and growing from cracks in old stone walls (Jackson 1982). Established T. officinale complex plants are very resistant to drought, while young plants are very sensitive and have a limited chance of invading coarse-textured, shallow or rapidly-drying soils (von Hofsten, 1954; PB Cavers, University of Western Ontario, London, Canada, personal communication). It grows in soils ranging in pH of 4.2-8.2 (Holm et al., 1997). It can tolerate moderate salinity and can occur in submaritime sites and on the bare verges of salted roadside verges in Europe and the USA. It also shows differential tolerance of heavy metals (Zhuikova et al., 1999). It has been shown to respond positively to K (Tilman et al., 1999), P (Zaprzalka and Peters, 1982) and N (Lihan and Jezikova, 1991) and negatively to Na and Mg (Panak et al., 1991) in the soil. It may grow at sea level or up to an elevation of 6000 m, where it can be found in association with subalpine or alpine native Taraxacum species (Longyear, 1918; Holm et al., 1997). In hilly terrain, T. officinale complex occurs more often on ridges than in hollows but this may be due to differential herbivory (e.g. slugs, rodents, grasshoppers) on the gradient, rather than differential effects of competition (Reader, 1992).
T. officinale complex forms mycorrhizal associations (Truszkowska, 1951; Hawker et al., 1957). Three vesicular-arbuscular mycorrhizal fungi, Glomus mosseae (Gange et al., 1994), G. intraradices (Gange, 1999) and Pythium ultimum (Hawker et al., 1957), have been reported on T. officinale complex. Infection by G. mosseae and G. intraradices conferred some degree of resistance in T. officinale complex roots to larvae of the black vine weevil Otiorhynchus sulcatus (Gange et al., 1994; Gange, 1999). Either fungus inoculated alone caused a significant reduction in larval growth and survival, but when both fungi were inoculated together, the effect disappeared, possibly due to competition between the fungi (Gange, 1999). There have been some recent studies on the mycorrhizal interactions of T. officinale complex, including the effect of elevated carbon dioxide levels on mycorrhizal function (Staddon et al., 1999); the effect of non-host and host plants on mycorrhizal colonization (Fontenla et al., 1999); and the effect of T. officinale complex on mycorrhiza inoculum potential, soil aggregation and yield of maize (Kabir and Koide, 2000).
T. officinale complex has an impact on the relationship between the ladybird beetle Coleomegilla maculata and its prey, pea aphids (Acyrthosiphon pisum) in lucerne (Harmon et al., 2000). It is thought that the omnivorous ladybirds are attracted to dandelion as a pollen resource, and the pea aphids suffer greater predation because they are in the same area.
T. officinale complex provides an alternative host to several important viruses, so that it can act as a reservoir and contagion for insect- and nematode-borne disease. These include Tomato ring spot virus, spread by Xiphinema americanum (Mountain et al., 1992; Ramsdell et al., 1993 and many earlier reports); Tomato spotted wilt virus and Cucumber mosaic cucumovirus which infect many agricultural, ornamental and greenhouse crops (Murphy et al., 1999; Groves et al., 2002); Beet western yellows virus in Brassica napa (Polák and Májková, 1992); Beet pseudo-yellows virus in Cucumis melo (Soria et al., 1991); Lettuce mosaic virus in Iraq (Shawkat et al., 1982); Dandelion yellow mosaic virus in lettuce (Bos et al., 1983); Lettuce pseudoyellows virus, which infects Lactuca sativa and Cucumis sativus (van Dorst et al., 1980) and Cherry rasp leaf virus (Hansen et al., 1974). It may also act as an alternative host for boll weevils (Haynes and Smith, 1992), cabbage looper, yellow-striped armyworm (Dussourd and Denno, 1994), green peach aphid (Kaakeh and Hogmire, 1991), larvae of the apple moth pest Lacanobia subjuncta (Landolt, 2002), Pseudomonas viridiflava, which causes bacterial streak and bulb rot of onion (Gitaitis et al., 1998), and other microorganisms.
Air TemperatureTop of page
|Parameter||Lower limit||Upper limit|
|Mean annual temperature (ºC)||4||27|
|Mean maximum temperature of hottest month (ºC)||40|
|Mean minimum temperature of coldest month (ºC)||-20|
RainfallTop of page
|Parameter||Lower limit||Upper limit||Description|
|Mean annual rainfall||90||2780||mm; lower/upper limits|
Rainfall RegimeTop of page
Soil TolerancesTop of page
Special soil tolerances
Natural enemiesTop of page
Notes on Natural EnemiesTop of page
Numerous fungi have been reported on T. officinale complex including rusts, powdery mildews and others (Tompkins and Hansen, 1950; Pape, 1954; von Hofsten, 1954; Anon., 1957; Anon., 1960; Conners, 1967; Müller, 1969; Shaw, 1973; Kuusi et al., 1984; Ginns, 1986; von Hinrichs, 1988; Farr et al., 1989; Riddle et al., 1991; Stojanovic et al., 1993; Ellis and Ellis, 1997; Neumann Brebaum and Boland, 1999). Bacteria (Rhodehamel and Durbin, 1985; Sterk et al., 1987; Leite et al., 1997; Gitaitis et al., 1998), viruses (Ryjkoff, 1943; Kassanis, 1947; Thomas, 1949; Blattny et al., 1954; Misiga et al., 1960; Tuite, 1960; Valenta et al., 1961; Duffus, 1965; Cech and Branisová, 1973; Brcák, 1979; Johns, 1982; Bos et al., 1983; Gracia et al., 1983; Dijkstra et al., 1985; Soria et al., 1991; Powell et al., 1992; Fuchs et al., 1994; Brunt et al., 1996) and phytoplasmas (Dale, 1972; Terlizzi et al., 1994; Arzone et al., 1995; Firrao et al., 1996; Skoric et al., 1998, Viczián et al., 1998; Wang and Hiruki, 2001) have also been recorded on T. officinale complex.
Means of Movement and DispersalTop of page
After flowering, the scapes of T. officinale complex elongate significantly, allowing enhanced wind dispersal of seeds (Radosevich and Holt 1984). The seeds have pappi that further aid in dispersal by wind (Lovell and Rowan 1991). The seed settling velocity may be a useful surrogate for the measurement of dispersal ability (Andersen 1992) and the average settling velocity of seed-pappus units of T. officinale complex is 2.37 km h-1 (Andersen 1993). Sheldon and Burrows (1973) found that the distance travelled by seed-pappus units of T. officinale complex increased with increasing wind speed. Wind speeds of 5.47, 10.94, and 16.41 km h-1 resulted in distances travelled of 0.76, 1.52, and 2.27 m, respectively. Von Hofsten (1954) estimated the dissemination distance of seeds was 200-500 m. Seeds are also dispersed by water, especially via irrigation ditches (Salisbury 1961; Radosevich and Holt 1984). Seeds can survive in water for up to 9 months (Comes et al., 1978).
Seeds are dispersed in the excreta of animals such as cattle, horses and birds (Salisbury 1961; Mt. Pleasant and Schlather 1994).
Seeds can be effectively dispersed by mechanical disturbance such as mowing and tillage. T. officinale complex can also be dispersed by tillage with a plough or disc, because of its ability to regenerate vegetatively from root sections (von Hofsten 1954). However, Falkowski et al. (1990) reported ploughing to be an effective method of eliminating T. officinale because the lower parts of roots were less viable than the upper parts, which were buried by ploughing. T. officinale complex can also contaminate hay, and so can be a potential seed source for other parts of the farm (Tardif, 1997).
The intentional introduction of T. officinale complex for its many beneficial uses is likely.
Pathway VectorsTop of page
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|
|Stems (above ground)/Shoots/Trunks/Branches||weeds/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)|
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
It is rarely a problem in pastures and forage, as it forms a popular and nutritious supplement to the diet of most livestock. The leaves harvested as silage do not reduce the feeding value of the crop and may form a healthier feed than pure grass. However, when present in dense populations, T. officinale complex can cause slower drying and moulding of hay because of its high water content, can cause non-productive spots in the pasture due to its rosette form, has undesirably high levels of some phenols, and can be a potential seed source for other parts of the farm (Tardif, 1997; Klimes et al., 2003). Doll (1984) found that forage from an alfalfa crop with a T. officinale complex infestation of 13-31% dry weight, required an additional day to dry to the same level as forage free of T. officinale complex. Its leaves dried faster than stems or ribs and this can cause a loss in dry matter yield during mechanical haymaking (Isselstein and Ridder, 1993).
Taraxacum officinale complex provides an alternative host to several important viruses, so that weeds can act as a reservoir and contagion for insect- and nematode-borne disease. These include tomato ring spot virus, spread by Xiphinema americanum (Mountain et al., 1992; Ramsdell et al., 1993 and many earlier reports); tomato spotted wilt virus and cucumber mosaic cucumovirus which infect many agricultural, ornamental and greenhouse crops (Murphy et al., 1999; Groves et al., 2002); beet western yellows virus in Brassica napa (Polák and Májková, 1992); beet pseudo-yellows virus in Cucumis melo (Soria et al., 1991); dandelion virus with many potential hosts (Dijkstra et al., 1985); lettuce mosaic virus in Iraq (Shawkat et al., 1982); dandelion yellow mosaic virus in lettuce (Bos et al., 1983); lettuce pseudoyellows virus, which infects Lactuca sativa and Cucumis sativus (van Dorst et al., 1980) and cherry rasp leaf virus (Hansen et al., 1974). It may also act as an alternative host for boll weevils (Haynes and Smith, 1992), cabbage looper, yellow-striped armyworm (Dussourd and Denno, 1994), green peach aphid (Kaakeh and Hogmire, 1991), larvae of the apple moth pest Lacanobia subjuncta (Landolt, 2002), Pseudomonas viridiflava, which causes bacterial streak and bulb rot of onion (Gitaitis et al., 1998), and other microorganisms (refer to Natural Enemies table).
Taraxacum is commonly abundant in orchards, but there is no indication that this reduces yield. However it is perceived as a serious competitor to flowering apple and pear trees for honeybee visits, and much time and expense is spent to remove them (see Social Impact below).
While T. officinale complex can compete effectively for water against shallow-rooted crops (Hartwig, 1990), it fails against vigorous grass (Lolium) leys (Ogg, 1983), although Wardle and Peltzer (2003) found it had a significant competitive advantage over Lolium perenne. It is mostly an annoying weed of gardens and amenity grasslands, frequently found in urban and anthropogenic sites, and municipalities spend considerable amounts on pesticides for its control on public property (Surgeoner and Roberts, 1992; Schnick et al, 2002).
Environmental ImpactTop of page
Impact: BiodiversityTop of page
Threatened SpeciesTop of page
Social ImpactTop of page
Risk and Impact FactorsTop of page
- Invasive in its native range
- Proved invasive outside its native range
- Highly adaptable to different environments
- Tolerates, or benefits from, cultivation, browsing pressure, mutilation, fire etc
- Highly mobile locally
- Has high reproductive potential
- Has propagules that can remain viable for more than one year
- Negatively impacts agriculture
- Negatively impacts human health
- Negatively impacts tourism
- Reduced amenity values
- Reduced native biodiversity
- Competition - monopolizing resources
- Competition - smothering
- Pest and disease transmission
- Highly likely to be transported internationally accidentally
- Highly likely to be transported internationally deliberately
UsesTop of page
T. officinale complex is quite a nutritious forage (Dutt et al., 1982; Marten et al., 1987; Wilman and Riley, 1993) It is an excellent pasture feed for dairy cattle, improving milk flow and quality (Jackson, 1982) and has been cited as a cure for catarrh in livestock (Kostuch and Kopec, 1997). It contains carotenoids, vitamins A, C and D, polysaccharides, organic acids, proteins, sugars, pectin, choline and minerals, especially K (Neamtu et al., 1992), andhas only trace amounts of essential oils and a low amount of tannin that might affect quality or palatability (Falkowski et al., 1990). The plant can contain as much protein as white clover (Bockholt et al., 1995) and is a valuable feed, based on its fat and carbohydrate content (Spatz and Baumgartner, 1990). Bergen et al. (1990) found that T. officinale complex had protein and mineral contents high enough to exceed the established requirements for cattle, and that cattle consumed dandelion as readily as, or sometimes in preference to, grass pasture. However, Falkowski et al. (1990) reported that it was not eaten readily by most domestic animals because of its bitterness.
T. officinale complex is commonly used as a salad green (Gao, 1995; Letchamo and Gosselin, 1995). In Toronto, Canada alone, 155 tonnes of leaves (valued at $CN595,000) were marketed in 1988 and 1989 (Letchamo and Gosselin, 1995). It has been used to replace lettuce in Nordic countries during winter, and is also blanched as a substitute for chicory. Extracts from T. officinale complex have been used in cheese preparation, due to its milk clotting and proteolytic properties (Akuzawa and Yokoyama, 1988). Taraxacum officinale complex plant parts are used in a wide variety of soups, main courses, desserts and beverages (Gail, 1994). Beverages include tea (from dried leaves), wine (from inflorescences or leaves), beer, a coffee substitute (using dried roots that have been roasted and ground) and it is added by Scandinavians to flavour schnapps (Gail, 1994; Dalby, 1999; Khan, 2001). In all these forms it is markedly and invariably diuretic. Recently, a program was initiated in Québec to introduce organic production of T. officinale complex for commercial processing of the roots (Letchamo and Gosselin, 1995). Significantly higher chlorophyll content, and root and shoot growth were found under hydroponic culture of T. officinale (Letchamo and Gosselin, 1995). Debudding the plants under hydroponic medium increased root yield by 131% compared with the control (Letchamo and Gosselin, 1995). The inflorescences are also an excellent source of nectar for honey, however, the nectar is more commonly used by beekeepers in the brood nest for spring colony buildup (Gail, 1994; Dalby, 1999).
T. officinale complex has been tested as a soil amendment for organically grown herbs, but it was not as useful as some other weeds (Li, 1996). It also possesses allelopathic properties and can suppress some fungal pathogens, such as Fusarium oxysporum f. sp. radicis-lycopersici Jarvis & Shoemaker, and nematodes (Jarvis, 1989; Falkowski et al., 1990; Alvarez et al., 1998). Fresh above-ground material of T. officinale complex lowered population densities of infective, second-stage juveniles of the nematode Meloidogyne hapla Chitwood and, therefore, increased carrot yield in the greenhouse (Alvarez et al., 1998). Taraxacum officinale complex has also been used to improve the mycorrhizal colonization of maize, and this resulted in the maize having higher P content, shoot dry weight and yield compared to that provided by winter wheat as a cover crop (Kabir and Koide, 2000). It also improved soil aggregation. Taraxacum officinale complex is also useful as a persistent species in the protection of the escape of dust from power plant cinder dumps and slag heaps in Poland (Kitczak et al., 1999). Alcoholic extracts have been successfully used as an insecticide against spider-mite (Tomczyk and Szymanska, 1995). When Taraxacum was grown as a salad crop in Germany, control against other broad-leaved weeds was best achieved by isoproturon applied at the end of March (Koch, 1980) or by pronamide (Leprince, 1971).
T. officinale complex is generally recognized as a good indicator of environmental pollution and is often used as a biomonitor because it is an abundant, widely distributed plant, that accumulates elements including As, Br, Cd, Ce, Co, Cr, Cs, Cu, Fe, Hg, Mn, Mo, Pb, Sb, Se, Ti, V and Zn (Kuleff and Djingova, 1984; Djingova et al., 1986; Simon et al., 1996; Winter et al., 2000; Malawska and Wilkomirski, 2001a; Djingova and Kuleff, 2002). The accumulation of elements by T. officinale complex depends not only on their soil concentration, but also on its sorption capacity and organic matter content; and the leaves, roots, scapes and inflorescences accumulate different concentrations of metals (Capecka and Gaweda, 2001). Recently T. officinale complex was used to evaluate trace metal bioavailability in abandoned industrial sites, community gardens and parks in urban Montréal, Québec, Canada (Marr et al., 1999). With increasing levels of pollution, traits such as the length and weight of seeds decrease, but the number of seeds increases, as an adaptation to survive unfavourable conditions (Savinov, 1998). The findings of Zhuikova et al. (2002) contradicted this however, as they found that seed weight increased and in some cases, seed number decreased, with increasing levels of pollution. The authors also found that the number of unfilled seeds generally increased with increasing pollution. The work of Keane et al. (2001) indicates that T. officinale complex may not be a particularly effective tool for for quantifying levels of environmental metal contamination in some situations due to the effect of other soil, plant, seasonal and/or other environmental factors on metal uptake from soil and accumulation. Yet preliminary studies on its potential use in phytoremediation via repeated croppings has been promising (Pichtel et al., 2000). Taraxacum officinale complex is considered a good bioindicator of environmental pollution with polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), dibenzo-p-dioxins, dibenzofurans, andsome chlorobenzenes where the soil or atmosphere has been contaminated with oil derivatives and other toxic substances (Bohme et al., 1999; Malawska and Wilkomirski, 2001a,b).
T. officinale complex is also useful as an experimental subject in classroom practical work. As it is very common, easily recognizable and perennial, it is easy to obtain and is ideal for studying germination, gravitropism, auxin effects, water potential measurement, polarity of root sections, morphological variation and plant cell structure (Oxlade and Clifford, 1999).
Other species of Taraxacum were used to provide a substitute for rubber in the former USSR during the second war, when special extraction plants were built and thousands of hectares were used to cultivate Taraxacum (Krotov, 1945).
Uses ListTop of page
- Host of pest
Human food and beverage
- Beverage base
- Honey/honey flora
- Spices and culinary herbs
Similarities to Other Species/ConditionsTop of page
Prevention and ControlTop of page
Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.Cultural Control
Encroachment of T. officinale complex into turfgrass is greater in turf receiving limestone applications and tended to decrease with increasing phosphorus rates (Turner et al., 1979). However, encroachment appeared to be more related to competition than the nutritional requirements of weeds since encroachment tended to decrease as the yield of turfgrass clippings increased (Turner et al., 1979). Phosphorus levels had a greater impact on growth of T. officinale complex than pH, with an application of 84 kg superphosphate ha-1 increasing the yield of T. officinale complex ten-fold compared to no added superphosphate, while added nitrogen had no effect (Zaprzalka and Peters, 1982). Root growth was more responsive to differences in phosphorus and pH than top growth (Zaprzalka and Peters, 1982). Tilman et al. (1999) showed that T. officinale complex is relatively inefficient at metabolising potassium, so that grasslands with low potassium availability tend to have small populations (Tilman et al., 1999). T. officinale complex had a higher requirement for potassium and its biomass was more limited by potassium than any of five common grass species tested, suggesting that it is a poorer competitor for potassium than these grasses. Also, T. officinale complex density and abundance were positively correlated with potassium levels in its tissues (Tilman et al., 1999).
T. officinale complex control was greater in plots of Kentucky bluegrass treated with 600 kg N/ha than when treated with 300 kg N/ha, even though both levels are very high (Johnson and Bowyer, 1982). This supported the findings of Zahnley and Duley (1934) who found that after they had fertilized and produced a dense growth of grass, T. officinale complex seedlings did not become established readily and growth was repressed by competition from the grass. Competition was further increased by cutting the grass at a greater height, which tended to shade the ground and retard the development of T. officinale complex (Zahnley and Duley, 1934).
Occurrence of T. officinale complex is greater in crop rotation systems with a high frequency of broadleaf crops (four rotations with broadleaf crops in 3 of 4 yr) compared to a low broadleaf frequency of broadleaf crops (three rotations with broadleaf crops in 2 of 4 yr) (Stevenson and Johnston, 1999). On the semi-arid Canadian prairies, T. officinale complex was present in all crop rotations examined; continuous winter wheat, winter wheat-fallow, winter wheat-spring canola, winter wheat-lentil or flax (Blackshaw et al., 1994). Ominski et al. (1999) found that populations of T. officinale were greater in crop rotations of lucerne and cereals, than in continuous cereal crops.
Green manure and cover crops have been trialled for their ability to control dandelion. The presence of crown vetch as a living mulch reduced T. officinale complex numbers by 74% in no-tillage corn (Hartwig, 1989). Autumn seeded cover crops of autumn rye, winter wheat and annual rye in the fallow year reduced the density of T. officinale complex in a fallow-wheat cropping sequence (Moyer et al., 2000). Yellow sweetclover, grown as a green manure fallow replacement crop, controlled dandelion, as well as other perennial and annual weeds (Blackshaw et al., 2001). Weed suppression was similar whether yellow sweetclover was harvested as hay or its residues were incorporated or left on the soil surface, suggesting that suppression may be in part due to allellopathic compounds released from decomposing yellow sweetclover. Mulches of wood sawdust, coarse bark and hay have also been used for the effective control of T. officinale complex (Rifai et al., 1999; Kaufmane and Libek, 2000).
The timing and frequency of harvesting also influences the degree of dandelion infestation. Moyer et al. (1999) found that the growth-stage-based harvest times of alfalfa affected encroachment by T. officinale complex and the resultant alfalfa quality. For example, when alfalfa was harvested at the vegetative or prebud stage, it contained 25% T. officinale complex by weight, but when it was harvested at any other stage such as flowering, it contained <2.5%.
Flame and hot water have been investigated as control methods for T. officinale complex in pear and apple orchards. Ferrero et al. (1994) found that flaming favours the growth of T. officinale complex; however a later study by Rifai et al. (1999) found that flaming was an effective control method, although the developmental stage was crucial with small plants being controlled by a single flaming, while larger plants required four treatments. There have also been differing results with hot water. Kurfess and Kleisinger (2000) found the application of hot water (85-95°C) to T. officinale complex provided good control, while Rifai et al. (1999) found hot water (as steam at 150°C) was ineffective.
When T. officinale complex seeds were exposed to radiation, including radiation in the zone of the Chernobyl accident, there was an increase in the proportion of abnormal seedlings, an increase in the frequency of chromosomal aberrations in the cells of meristematic tissues, an increase in stability with respect to further radiation, and a change in the rate of growth and development (Pozolotina, 1996; 2001). Dai and Upadhyaya (2002), found that ultraviolet-B radiation inhibited radicle elongation and decreased leaf area, but did not influence seed germination, shoot elongation or plant biomass. The potential use of electromagnetic radiation in the form of microwaves has been studied for T. officinale complex control on railway tracks in Europe (Kunisch et al., 1992). Plants were killed by microwave treatment for 16 s, which increased the soil temperature by more than 40°C in controlled environments (Kunisch et al., 1992).
Mechanical removal of T. officinale complex plants has been of limited value, as the long taproot must be entirely removed. The plant responds positively to repeated cutting (Dahmen and Kuhbauch, 1990; Teyssonneyre et al., 2002) or heavy grazing (Rogalski et al., 1997Harker et al., 2000) where 'gaps' in the crop cover are created. In addition, debudding or defoliation of the plant can result in a shift of the shoot-root ratio, favouring root growth and exacerbating the problem (Letchamo and Gosselin, 1995). Struik (1967) studied the reaction of T. officinale complex to different grassland management regimes; mown, heavily grazed, lightly grazed, and uncut. As the degree of defoliation increased, plant radius decreased, length of the longest leaf decreased, root length decreased, leaf number increased and plant form changed from upright to slanting and appressed. The cover rate of T. officinale complex in grasslands was significantly higher grazed and mown treatments (Popolizio et al., 1994; Klimes et al., 2003). Taraxacum officinale complex is able to rapidly replace a large quantity of leaves relative to other organs during short periods (every 30-50 d) between mowing (Sawada et al., 1982), partly due to increased light availability (von Hofsten, 1954). Cutting the scapes of T. officinale complex in bud or in flower resulted in the production of seeds that were non-viable, while cutting the scapes after the seeds had ripened resulted in 91% seed germination (Gill, 1938).
Tillage frequency has been shown to have an impact on germination and establishment of T. officinale complex. Densities are usually higher in minimum and zero-till treatments than in conventional till (Légère et al., 1993; Blackshaw et al., 1994; Stevenson et al., 1998). In spring wheat in clay soil in northern Alberta, Canada, higher densities of T. officinale complex were also found under zero tillage systems than under other systems (Arshad et al., 1994). In eastern Canada, density was higher in alfalfa that had been seeded directly into grain stubble, as compared to conventionally-seeded alfalfa (Rioux, 1994). In sweet corn, T. officinale complex emergence was not affected by tillage, probably because of the short time between seed dispersal and germination (Mohler and Calloway, 1992). In the UK, the occurrence of T. officinale complex in plots sown with spring barley was inversely proportional to cultivation frequency. Plots that were never cultivated had a total of 196 T. officinale complex plants m-2. Yearly cultivation resulted in 72 plants m-2, quarterly cultivation resulted in 54 plants m-2, and monthly cultivation resulted in 33 plants m-2 (Chancellor, 1964).
T. officinale complex can survive after tillage with a plough or disc, because of its ability to regenerate from root sections (von Hofsten, 1954). Tillage must be deep enough to cut the root 10 cm below the crown, since even small pieces of root can propagate new plants (Warmke and Warmke, 1950; Mann and Cavers, 1979). Falkowski et al. (1990) reported ploughing to be an effective method of eliminating T. officinale complex because the lower parts of roots were less viable than the upper parts, which were buried by ploughing. Heavy duty cultivators are most successful, particularly if followed up by a rod weeder. Discers are usually ineffective in well-established stands. Mechanical cultivation is most effective in light soils in dry weather. In heavy soils and where soils are moist, root fragments regenerate readily and cultivation can exacerbate the problem. In these cases, crop rotation and/or chemical control is advocated (Legere et al., 1993).
For land clearance and pre-emergence control, glyphosate is typically used, either alone or in combination with a wide variety of other herbicides. In grasslands, for example, pastures, silage and hay-crops not undersown to legumes, and in sports turf, Taraxacum is usually controlled by 2,4-D, often accompanied by herbicides such as picloram, dicamba, dichlorprop, fluazifop-P, mecoprop, sethoxydim or triclopyr (Darwent and Lefkovitch, 1995; Anon., 1997). Other agents have been successfully used in grassland including isoxaben, metsulfuron, quinclorac, clopyralid, dithiopyr, fluroxypyr and imazaquin (Neal, 1990; Chandran et al., 1998). In lucerne (Zaprzalka and Peters, 1982), chlorsulfuron, hexazinone (Moyer et al 1990; Malik et al., 1993), terbacil and dichlobenil (Waddington, 1980; 1987) effectively control Taraxacum. Maleic hydrazide has been used to control T. officinale complex in apple orchards in the USA (Miller and Eldridge, 1989). Considerable success in 'organic' control has been achieved using corn gluten meal, a waste product from corn milling (Bingaman and Christians, 1995; Quarles, 1999).
Sheep and geese have been used for biological control of T. officinale complex, with sheep being more effective than geese in controlling the weed in Christmas tree plantations in North Carolina, USA. (Müller et al., 1999).
Phoma exigua and P. herbarum have been isolated from T. officinale complex in Ontario, Canada, and considered as potential biocontrol agents (Neumann Brebaum, 1998; Neumann Brebaum and Boland, 1999). Controlled-environment studies showed that young T. officinale plants were more susceptible to P. herbarum than older plants (Neumann and Boland, 2002). P. taraxaci was considered as a biocontrol agent for T. officinale complex in Sweden (von Hofsten, 1954). P. taraxaci spread by pycnospores and infected seeds, however, it was extremely variable with respect to its pathogenicity on T. officinale complex and its viability in soil. Von Hofsten (1954) also mentioned an unnamed 'ring-forming fungus' which released a substance that was highly toxic to T. officinale complex and other plants. Sclerotinia species have also been tested as biological control agents for T. officinale complex in Canada and New Zealand (Riddle et al., 1991; Waipara et al., 1993). Sclerotinia sclerotiorum and S. minor Jagger were evaluated in a controlled environment and in turfgrass swards for their virulence on T. officinale complex. Isolates of both species reduced the dry weight of plants in a controlled environment and reduced the number of plants in turfgrass swards. Heat-killed seeds of perennial ryegrass were suitable as both a growth substrate for Sclerotinia spp. and a delivery system to T. officinale complex (Riddle et al., 1991). A mycelium-on-wheat preparation has been used for S. sclerotiorum, while either a granular sodium alginate formulation or a mycelium-on-barley preparation has been employed to deliver S. minor (Ciotola et al., 1991; Brière et al., 1992; Waipara et al., 1993). Sclerotinia sclerotiorum caused localized infection on the leaf laminas and created basal necroses of 1-2 cm in length on tap roots of T. officinale complex (Burpee, 1992; Waipara et al., 1993). These necroses inhibited leaf regrowth from the root after defoliation (Burpee, 1992).
Integrated pest management strategies for T. officinale complex in turf include the selection of competitive turfgrass species, application of increased quantities of fertilizers, and mechanical control by mowing or removal. For example, studies in Ontario, Canada showed that Kentucky bluegrass was the least competitive of six turf species, perennial ryegrass was the most competitive and increased amounts of nitrogen fertilizer suppressed T. officinale complex in all turfgrass swards (Hall et al., 1992, Tripp, 1997). Grass competition under frequent close mowing did not prevent T. officinale complex from surviving and spreading (Timmons, 1950). Of four grasses tested, native buffalograss was the most competitive with T. officinale complex (Timmons, 1950). The size and average density of T. officinale complex plants can be reduced by decreasing the row spacing of grass crops (Darwent and Elliott, 1979). In Ontario, Canada, optimum control of T. officinale complex was obtained with 2,4-D applications in spring in combination with spudding (root cutting and shoot removal) (Mann, 1981). Also in Ontario, Canada, sublethal rates of 2,4-D were applied in combination with the potential bioherbicide S. minor for the control of T. officinale complex (Schinck et al., 2002).
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