Senna obtusifolia (sicklepod)
- Summary of Invasiveness
- Taxonomic Tree
- Notes on Taxonomy and Nomenclature
- Plant Type
- Distribution Table
- History of Introduction and Spread
- Habitat List
- Hosts/Species Affected
- Host Plants and Other Plants Affected
- Biology and Ecology
- Rainfall Regime
- Soil Tolerances
- Natural enemies
- Notes on Natural Enemies
- Means of Movement and Dispersal
- Pathway Vectors
- Plant Trade
- Impact Summary
- Environmental Impact
- Impact: Biodiversity
- 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
- Senna obtusifolia (L.) Irwin & Barneby
Preferred Common Name
Other Scientific Names
- Cassia obtusifolia L.
- Cassia tora L.
- Cassia tora var. obtusifolia (L.) Haines
- Emelista tora (L.) Britton & Rosa
- Senna tora (L.) Roxb.
International Common Names
- Spanish: ejotillo; sambran (Spain); yerba hedionda (Cuba)
- French: pistache marron
Local Common Names
- : coffeeweed; habucha; peanut weed
- Australia: Chinese senna; coffee weed; Java bean; sicklepod senna
- Bolivia: aya-poroto; mamuri
- Brazil: fedegoso; fedegoso-branco; mata pasto; matapasto liso
- Colombia: bicho; bichomacho; chilinchil
- Cuba: guanina
- Dominican Republic: brusca cimarrona; brusca hembra
- El Salvador: comida de murcielago; frijolillo
- French Guiana: cafe zerb pian
- Guatemala: ejote de invierno; ejotil
- Madagascar: voamahatsara
- Mauritius: cassepuante; herbe pistache
- Paraguay: taperva; taperva moroti; taperva sayju
- Puerto Rico: dormidera
- Venezuela: chiquichique
- CASOB (Cassia obtusifolia)
Summary of InvasivenessTop of page S. obtusifolia and S. tora are tall subshrubs that produce a massive seed bank and are readily dispersed by livestock. They are primarily highly economically detrimental to a number of agricultural crops especially in North America and Australasia. S. obtusifolia is spreading and displacing native vegetation in the Australasia/Pacific regions. In places it produces impenetrable monotypic stands. In Australia it is thought to increase the risk of damage to forest by fire.
Taxonomic TreeTop of page
- Domain: Eukaryota
- Kingdom: Plantae
- Phylum: Spermatophyta
- Subphylum: Angiospermae
- Class: Dicotyledonae
- Order: Fabales
- Family: Fabaceae
- Subfamily: Caesalpinioideae
- Genus: Senna
- Species: Senna obtusifolia
Notes on Taxonomy and NomenclatureTop of page Many recent floras use the new nomenclature which puts many former Cassia spp. including C. obtusifolia and C. tora, into the genus Senna, and the new classification of Irwin and Barneby (1982) is used here. However, where acknowledging these two species as separate (following Irwin and Barneby 1982), in terms of their agronomic importance and control, there is probably little difference between S. obtusifolia and S. tora, and both are included together for the purpose of this datasheet. Thus, whereas S. tora (and C. tora) are included here as non-preferred scientific names, they are not strictly synonyms.
There has been much debate on the classification of S. obtusifolia. Linnaeus (1753), De Wit (1955) and Randell (1988) recognize Cassia obtusifolia and Cassia tora as separate species but others (Bentham, 1871) recognized them as the same species, and Haines (1922) claims that they are intraspecific taxa within the same species. The Botanical Laboratory of the United States Department of Agriculture advises that Cassia obtusifolia is a synonym of Cassia tora and this was accepted by Holm et al. (1997).
Characteristics proposed to separate S. obtusifolia and S. tora include glands on the leaf rachis, length of flower pedicel, length and width of petals, degree of fruit curvature, seed coat features, chemical content and smell of crushed foliage (see Morphology section). Randell (1995) studied the taxonomy and evolution of S. obtusifolia and S. tora and concluded that S. tora probably evolved in Asia from plants of S. obtusifolia. Randell (1988) separated the two taxa on the following basis: S. obtusifolia: petioles 1.5-2 cm long; fruiting pedicels 2-3 cm long; anthers with short beaks; seed areole narrow, not longitudinal. S. tora: petioles 2-4.5 cm long; fruiting pedicels to 1.5 cm long; anthers truncate, beakless; seed areole broad, longitudinal.
Upadhyaya and Singh (1986) claim that S. tora and S. obtusifolia differ in their anthraquinone content and that they also fail to hybridize. Most material described as S. tora from Africa and America (north, south and central) is S. obtusifolia but both species are found in Asia and Australia. The confused nomenclature is apparent in the literature; S. tora is often cited when it should be called S. obtusifolia but it is often not possible to be confident about the correct identification where the species co-exist.
The original genus Cassia is from the Greek kasia, derived by Dioscorides (1st century AD) from the Hebrew quetsi'oth, denoting 'fragrant shrubs'; obtusifolia combines the Latin obtusos, meaning 'blunt', with folius, a leaf, and refers to the rounded leaf apices (Parsons and Cuthbertson, 1992).
DescriptionTop of page S. obtusifolia and S. tora are erect, bushy, annual or short-lived perennial herbs growing to heights of 1.5 to 2.5 m (Parsons and Cuthbertson, 1992: Holm et al., 1997). The stems are obtusely angled to cylindrical, smooth, often highly branched. The robust taproot is about 1 m long, with several descending laterals. Unlike many legumes, the roots of S. obtusifolia and S. tora do not support nodules of nitrogen-fixing bacteria.
Two stipules, about 15 mm long, are present where the alternate leaves join the stem. The first true leaves are pinnate with two pairs of leaflets. Leaves in mature plants are even-pinnately compound, 8 to 12 cm long, with three pairs of leaflets. The leaves of S. tora are rank-smelling when crushed (Holm et al., 1997) but S. obtusifolia is less pungent. Leaflets are obovate to oblong-obovate with asymmetrical bases, increasing in size from the base to the apex of the leaf, up to 6 cm long and 4 cm wide. The tips of the leaflets are bluntly oval to round, with a very small point at the tip of the main vein. A small, rod-like gland is situated on the rhachis between the lower pair of leaflets but, in S. tora, a second gland is present between the middle pair of leaflets. A second gland is sometimes present in S. obtusifolia but only on lower leaves (Brenan, 1967).
Flowers are solitary or in pairs, in leaf axils, on pedicels 1-3 cm long (1 cm in S. tora). The calyx has five free, unequal sepals, keeled on the back. The corolla has five free, spreading, yellow petals, obovate to obovate-oblong, narrowed at the base and rounded at the tip, except for the standard (uppermost petal) which has two lobes. There are 10 stamens of which seven are fertile and three are staminodes. The ovary has numerous ovules. In S. obtusifolia, the stigma is oblique with an acute rim; in S. tora it is straight with two rolled back lips (ACTA, 1986a, b). The fruit is a brownish-green, slender, curved, compressed pod, 10 to 25 cm long and 2 to 6 mm wide, containing 25 to 30 seeds. Pods are slightly indented between the seeds. There are two major variants of S. obtusifolia in the Americas, differing primarily in pod type. Plants from the Antilles and the USA have pods 3.5-6 mm in diameter, as do African specimens and those from India, Indo Malaya and China (Irwin and Barneby, 1982). In South America and the Philippines, the pod is narrower (2-3.5 mm) in diameter and strongly curved. Seeds are rhomboidal, 4 to 5 mm long, shiny and yellowish brown to dark red. In S. obtusifolia, the areole (marking on the seed coat) is very narrow (0.3 to 0.5 mm wide); in S. tora it is large (1.5 to 2 mm wide) (Brenan, 1967).
Plant TypeTop of page Annual
DistributionTop of page S. obtusifolia is a native to tropical South America but has become widespread throughout the tropics and subtropics. However, the extent of its original distribution in the neotropics is unknown. It has been generally confused with S. tora, a species confined to Asia from India to China and Fiji, with the possible exception of one Congo specimen and a single specimen from Mafia Island, Tanzania (Brenan, 1967). In the USA the range of S. obtusifolia is similar to that of 150 years ago although infestations are still increasing (Teem et al. 1980). In northern Queensland, Australia, it infests about 600,000 ha and is still spreading. Although S. obtusifolia can become widespread in Australia, a model predicts that its optimal habitat will be coastal Queensland and New South Wales. This model also provides a predicted world distribution map that highlights regions such as much of southern Europe, South-East Africa, Madagascar and North New Zealand as having a high potential to support populations of the species. There is also some evidence that a cold tolerant biotype may be evolving and this would further increase the potential range of the weed (Mackey et al., 1997).
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.
History of Introduction and SpreadTop of page Our knowledge of the introduction, early distribution and spread of the two Senna taxa around the world is largely unknown. In Australia S. tora was first recorded in 1871 but the earliest collection was made at Port Darwin (Northern Territory) in 1888. It escaped from green manure trial plots in Queensland in 1917-18 and is now only sparingly naturalised. S. obtusifolia was first recorded as a weed in the Northern Territory in 1961 and from Queensland in 1963, but is thought to have been introduced to Australia in the early 1940s around Darwin. It was declared a noxious plant in Queensland under the Rural Lands Protection Act in 1981 (Parsons and Cuthbertson, 1992; Mackey et al., 1997). In Hawaii, S. obtusifolia was thought to have been naturalised since at least the 1960s but the recent discovery of a 1940 herbarium specimen points at a pre-World War 2 introduction (Staples et al., 2003).
HabitatTop of page S. obtusifolia and S. tora are found in cropped land, pastures, roadsides, waste land, woodland and natural grasslands. They can grow in a range of soil types, including heavy-textured and well aerated or sandy soils. They grow at an optimum temperature of 25°C and in areas with annual precipitation ranging from 640 mm to 4290 mm, with an optimum of 1520 mm (Holm et al., 1997). While growth is best in moist conditions, there is good tolerance of dry soils (Hoveland and Buchanan, 1973). Good growth is sustained with a soil pH of between 4.6 and 7.9 (Murray et al., 1976).
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)|
|Terrestrial ‑ Natural / Semi-natural||Natural grasslands||Present, no further details||Harmful (pest or invasive)|
Hosts/Species AffectedTop of page S. obtusifolia and S. tora are weeds of a wide range of crops including: cereals, fibre crops (cotton and jute), legumes (especially soyabean), pastures, sugarcane, cotton, groundnut, tobacco, tree crops (citrus, coconut, rubber, etc.) and vegetables. Most crops are likely to be infested when grown within the habitat range of these weeds.
Host Plants and Other Plants AffectedTop of page
|Arachis hypogaea (groundnut)||Fabaceae||Main|
|Capsicum annuum (bell pepper)||Solanaceae||Other|
|Cocos nucifera (coconut)||Arecaceae||Main|
|Coffea arabica (arabica coffee)||Rubiaceae||Other|
|Colocasia esculenta (taro)||Araceae||Main|
|Fragaria ananassa (strawberry)||Rosaceae||Other|
|Glycine max (soyabean)||Fabaceae||Main|
|Helianthus annuus (sunflower)||Asteraceae||Other|
|Hevea brasiliensis (rubber)||Euphorbiaceae||Other|
|Ipomoea batatas (sweet potato)||Convolvulaceae||Other|
|Manihot esculenta (cassava)||Euphorbiaceae||Other|
|Musa textilis (manila hemp)||Musaceae||Other|
|Nicotiana tabacum (tobacco)||Solanaceae||Other|
|Oryza sativa (rice)||Poaceae||Main|
|Phaseolus lunatus (lima bean)||Fabaceae||Other|
|Phaseolus vulgaris (common bean)||Fabaceae||Main|
|Saccharum officinarum (sugarcane)||Poaceae||Main|
|Sorghum bicolor (sorghum)||Poaceae||Main|
|Vigna unguiculata (cowpea)||Fabaceae||Other|
|Zea mays (maize)||Poaceae||Main|
Biology and EcologyTop of page Genetics
In the New World there are two forms of S. obtusifolia. The first originating from the Caribbean and also found in the USA has a uniglandular extra-floral nectary on the upper surface of the rachis (and 2n=28) and the other from northern South America has two extra-floral nectaries (and 2n=26) (Irwin and Barneby, 1982). In Paraguay both chromosome numbers (2n=26 and 2n=28) have been reported (Anon., unda). Senna obtusifolia and S. tora are caryologically distinct even though both have n=13 but some forms of obtusifolia have n=12, 13, 14 and some Indian forms of tora have n=13, 14. It has been suggested that S. tora derived from the Caribbean form of S. obtusifolia and could have arisen via aneuploid loss from obtusifolia (Randell, 1995; Mackey et al., 1997). There is also some evidence that a cold-tolerant biotype may be evolving and this would further increase the potential range of the weed (Mackey et al., 1997).
Physiology and phenology
Germination can occur between 13 and 40°C (Misra, 1969) and at any time of the year provided moisture is available (Parsons and Cuthbertson, 1992). Seedlings can emerge from a soil depth of 12.7 cm but not from 15 cm (Teem et al., 1980). Emergence from 12.7 cm takes 9 days but 63% emergence takes place within 3 days when seeds are only 2.5 cm deep. Seedling growth is best between 30 to 36°C (Teem et al., 1980). In Australia, seedling growth is slow in early spring but increases rapidly at temperatures over 24°C, primary root growth is optimum at 32°C (Parsons and Cuthbertson, 1992). Holm et al. (1997) state that optimum root growth occurs at 25°C.
Intraspecific competition increases plant height, whilst branching decreases as plant density rises from 1 to 40 plants/m² (Singh, 1969). Photoperiod dramatically affects growth. In India, as the photoperiod increased from 6 to 15 hours plants grew taller. Continuous light, however, results in short plants (Misra, 1969). Turner and Karlander (1975) found that 6-12 hours of light induces 100% flowering and pods are only produced when plants receive between 8 and 11 hours of light (Misra, 1969). Photoperiod responses may vary around the world but it appears that S. tora and S. obtusifolia are short-day plants. Retzinger (1983) recorded seed production of 2800 to 8200 seeds/plant resulting in an enormous seed bank in the soil.
Flowers first appear after 43-84 days depending on ecotype and climate. Being a short-day plant, short photoperiods accelerate reproductive development and 16 h light prevents flowering. Short days are not only necessary for flower induction but also for bud development (Mackey et al., 1997).
Pollen is released through the vibration of the flowers by bees (buzz pollination) and it is thought that self-pollination is probably the norm. The dehiscent pod can disperse seeds up to 5 m. Further dispersal can occur via water or in mud attached to the feet or fur of animals, or to shoes and machinery. When ingested by cattle, horse or goats it may survive the passage of the gut (Mackey et al., 1997).
Seeds of S. tora and S. obtusifolia have hard seed coats which need to be mechanically damaged to break dormancy. In dry storage, seeds lose their viability quite rapidly (Doll et al., 1976); seeds stored for three years had an overall germination of 22%. Nine-year-old seed had 9% germination (Ewart, 1908) and 10% of seed buried in the soil for 30 months germinated (Egley and Chandler, 1978). Baskin et al. (1998) report that 90% of seeds are green, hard-coated and dormant while 10% are brown and non-dormant. After scarification, dry heat at 80-100°C, or alternating temperatures, the green seeds would germinate in either light or dark conditions. Seeds are readily scarified by fire (Mackey et al., 1997).
In Minas Gerais, Brazil, S. obtusifolia is commonly found in early succession following fire in secondary forest, but does not become dominant (Martins et al., 2002), and is an important woody plant component of Roraima Savannahs (Miranda et al., 2002). In northern Australia S. obtusifolia abundance is positively correlated with buffalo activity (Braithwaite et al., 1984).
S. obtusifolia can grow in a wide range of soil types. It also tolerates much variation in soil pH and nutrients. Plant growth can be affected by soil phosphorus and potassium, for instance, low soil potassium concentrations result in stunted growth. In North America extreme temperatures limit the range of the plant, whereas in Australia it is thought to be limited by soil moisture availability and dry stress. Disturbance, either through logging and road construction or from feral animals (cattle and pigs) and storms, increases open forest and woodland susceptibility to invasion (Mackey et al., 1997).
Seeds germinate between 18 and 36°C and seedling growth occurs at the temperature range of 18-39°C, but the threshold for leaf production is around 13°C (Mackey et al., 1997).
RainfallTop of page
|Parameter||Lower limit||Upper limit||Description|
|Mean annual rainfall||640||4290||mm; lower/upper limits|
Rainfall RegimeTop of page Summer
Soil TolerancesTop of page
Natural enemiesTop of page
|Natural enemy||Type||Life stages||Specificity||References||Biological control in||Biological control on|
Notes on Natural EnemiesTop of page In view of the very close taxonomic affinities of S. tora and S. obtusifolia, the natural enemies of these two species will be interchangeable (Cock and Evans, 1984). The principal records of natural enemies found in the literature for S. tora and S. obtusifolia refer to the Bruchidae (Coleoptera), a relatively small family, consisting mostly of oligophagous seed feeding beetles, most of which specialize on the Leguminosae (Cock and Evans, 1984). Recent work carried to identify potential biocontrol agents has resulted in the production of a substantial list of insects associated with S. obtusifolia (Palmer and Pullen, 2001). Investigations on the impact of the introduced ant Pheidole megacephala in Australia's Northern Territory has shown that the ants tend S. obtusifolia extrafloral nectaries and indicated that the ants facilitated the invasion. Plants growing in uninfested areas suffered from clear signs of insect herbivory and the plants were small and isolated. In contrast, in areas infested by the introduced ants the individuals were larger, often in dense stands, and suffered from no or limited leaf damage (Hoffmann et al., 1999).
Means of Movement and DispersalTop of page Natural dispersal
S. obtusifolia is spread via the gut of domestic livestock and by motor vehicles (Neldner et al., 1997).
In Australia's Queensland S. obtusifolia escaped from green manure trial plots (Neldner et al., 1997).
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|
|True seeds (inc. grain)||seeds|
Impact SummaryTop of page
|Fisheries / aquaculture||None|
ImpactTop of page In Georgia, USA, S. obtusifolia was identified as the most troublesome weed statewide averaged over all crops. It was present in all crops surveyed and across the State's climate gradient (Webster and Macdonald, 2001). If S. tora and S. obtusifolia are left uncontrolled for 2-4 weeks after planting, crop yields are dramatically reduced (Holm et al., 1997). Cotton yields were reduced by 25% when this weed was present at a density of 1.1 plants/m of row (Buchanan and Burns, 1971) and each plant per 15 m of row reduced cotton yield by 40 kg/ha (Buchanan et al., 1980). Murray et al. (1976) concluded that 1, 2 and 3 plants/0.3 m of row reduced yields of cotton by 11, 23 and 46%, respectively. Soyabean yields were reduced by 92 kg/ha for each S. obtusifolia plant/m of row (Thurlow and Buchanan, 1972). Seeds of S. tora were found to be one of the commonest contaminants of leguminous cover crop seeds imported into Malaysia (Tasrif et al., 1991). In Australian sugarcane growing regions where sicklepod is becoming an increasing problem, costs to chemically control the pest are readily increasing. Dense infestations of S. obtusifolia in Queensland can reduce the cattle carrying capacity up to nearly 100%. It is also a problem in young forestry plantation where expensive spraying programmes have to be carried out. In Australia it was estimated that in 1997 S. obtusifolia control cost around $1 million per year (Mackey et al., 1997).
S. tora or S. obtusifolia is an alternative host for the pests Etiella zinckenella in India (Subba Rao et al., 1976) and Aphis craccivora in India (Patel and Patel, 1972) and Uganda (Davies, 1972). In Venezuela, S. obtusifolia is a reservoir for Tobacco mosaic virus which is spread by Myzus persicae (Debrot, 1974). It is also a source of Colletotrichum capsici which causes anthracnose on tomato fruit and cotton seedlings (McLean and Roy, 1991) and of S. fragariae which causes anthracnose on strawberries (Howard and Albregts, 1973). Although the plant is not palatable, cattle may occasionally eat it when little other forage is available and poisoning may result (Mackey et al., 1997).
Cattle will not feed on the growing plant of S. obtusifolia, although they will eat it in silage and also the dry seed pods (Cock and Evans, 1984). Seeds of S. obtusifolia are harmful to chickens due to the presence of a trypsin inhibitor, but this is inactivated by boiling, which converts the seeds into a good source of protein (Cock and Evans, 1984).
In Benin farmers have reported S. obtusifolia as alternative host of cowpea pests (Kossou et al., 2001).
Environmental ImpactTop of page It is the most serious invasive plant of the Cape York Peninsula grassland where it colonised and displaced large areas of Imperata cylindrica grasslands (Neldner et al., 1997). In the biodiversity important islands of the Galapagos, S. obtusifolia is suspected to be causing significant ecological change (Tye et al., 2002). In northern Australia the spread of the species is thought to increase the risk of rain forest degradation by wild fires during the dry season (Russell-Smith and Bowman, 1992).
Impact: BiodiversityTop of page In both Australia and the Galapagos S. obtusifolia is spreading in protected areas (Hoffmann et al., 1999; Tye et al., 2002), but the impact of the plant on biodiversity and ecosystem function is largely unknown, but for its ability to displace native species (Mackey et al., 1997).
Social ImpactTop of page In Australia no beneficial effects have been identified, whereas in parts of Africa and Asia both taxa are used in traditional medicine (Mackey et al., 1997). As S. obtusifolia often produces dense and tall monotypic stands, it is likely to hinder human movement in infested areas. In Australia's Iron Range National Park it is feared that if S. obtusifolia infestations increase markedly, the number of visitations may decrease (Mackey et al., 1997).
Risk and Impact FactorsTop of page Invasiveness
- 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
- Damaged ecosystem services
- Ecosystem change/ habitat alteration
- Negatively impacts agriculture
- Negatively impacts animal health
- Reduced native biodiversity
- Competition - monopolizing resources
- Pest and disease transmission
- Produces spines, thorns or burrs
- Highly likely to be transported internationally accidentally
- Difficult to identify/detect as a commodity contaminant
- Difficult to identify/detect in the field
- Difficult/costly to control
UsesTop of page Fermented leaves of S. obtusifolia are used as a meat substitute in Sudan (Dirar, 1984) and as a mineral and vitamin supplement by certain tribes in Kenya and Senegal (Becker, 1986). Leaves of S. tora and S. obtusifolia are high in protein (14.4%) and are highly palatable to poultry (Murty, 1962) but excessive consumption can be detrimental. Modest quantities of S. tora and S. obtusifolia seeds can be used as dietary supplements in livestock but too much can have adverse effects (Holm et al., 1997). Seeds can be used as a substitute for coffee and as a mordant in dyeing (Ambasta, 1986; Staples et al., 2003; DeFilipps et al., 2004). 'Kawal' meat substitutes can be made from S. obtusifolia leaves (Dirar, 1984).
S. tora and S. obtusifolia are reported to have a number of medicinal uses, indeed, S. tora is reputed to have been used for medicinal purposes as early as 4000 BC (Nickell, 1960). The whole plant, especially the root, has purgative (Ambasta, 1986) and antihelminthic properties and the leaves are used to treat ringworm (Ambasta, 1986) and other skin diseases (Cock and Evans, 1984). Quisumbing (1951) records that S. tora is used as a vermifuge and purgative in the Philippines and to treat dysentery and opthalmia in Indo-China. In French Guiana root tincture is rubbed on rheumatic areas and the leaf infusion is effective for renal calculi (DeFilipps et al., 2004).
Uses ListTop of page
Animal feed, fodder, forage
- Fodder/animal feed
Similarities to Other Species/ConditionsTop of page There were over 600 species of Cassia before the division of Irwin and Barneby (1982), of which S. occidentalis somewhat resembles S. tora and S. obtusifolia in morphology, distribution, ecology and biology. However, S. occidentalis is normally perennial, and the leaves differ in having 4 to 6 pairs of leaflets (only 3 in S. tora and S. obtusifolia) and the leaflets are ovate or oblong-lanceolate, with a pointed tip (unlike the blunt or rounded tips of S. tora and S. obtusifolia).
Prevention and ControlTop of page
Control of S. tora and S. obtusifolia is difficult and can be obtained only with a sustained combination of all available methods. Although repeated discing of summer fallows favours germination and emergence, and tends to reduce seed numbers in the soil (Bridges and Walker, 1985), cultivation usually spreads rather than controls these weeds. Hence, single plants should be grubbed out before flowering. Hand pulling is difficult because of the deep, curved taproot, and plants can regrow from underground buds in the crown region (Holm et al., 1997). Larger colonies can be slashed but this does not eliminate S. tora and S. obtusifolia. Slashing reduces plant vigour which, with a programme of top dressing and restricted grazing, enables re-establishment of native pastures (Parson and Cuthbertson, 1992). As shading severely limits S. obtusifolia growth, late emerging seedlings can be somewhat suppressed by young soyabean if the rows are narrow enough for rapid canopy closure (Nice et al., 2001).
Zero-tillage land management can lead to increased seed populations compared with conventionally tilled plots (Vencill and Banks, 1994).
Various mulching treatments can be used to control S. tora and S. obtusifolia: rye mulch is effective in sunflower and soyabeans (Brecke and Schilling, 1996), giving up to 90% early control (Worsham, 1991). Polypropylene fabric mats completely inhibit the growth of S. obtusifolia when placed over glasshouse flats (Martin et al., 1987). Browne et al. (1989) have demonstrated the potential for controlling S. obtusifolia by soil solarization with clear plastic but they concede that this may only be economical for domestic gardens and small areas of horticultural crops.
Competitive crops offer possibilities for suppressing the growth of S. tora and S. obtusifolia, for example, Shaw et al. (1997) compared different soyabean cultivars and found that cultivar 9592 Pioneer was more effective in reducing shoot height than Asgrow 5979 when no herbicide treatment was used
In regions where S. obtusifolia is still spreading, such as northern Australia, it has been suggested that closing or relocating roads and restricting the movement of cattle to uninvaded areas are measures that may limit range expansion (Neldner et al., 1997).
S. obtusifolia has been a target weed for biological control, particularly in the USA. Alternaria cassiae, formulated as a mycoherbicide, has given >96% control of S. obtusifolia and increased the yields of soyabean (Parsons and Cuthbertson, 1992). Granular formulations of A. cassiae mycelia with sodium alginate + kaolin, applied pre-emergence (using approximately 3 kg conidia/500 kg formulation), gave 50% control of S. obtusifolia in soyabeans within 14 days and significantly increased crop yield (Walker, 1983). In greenhouse trials, an inoculum concentration of 10,000 spores/ml of A. cassiae gave 100% control of S. obtusifolia (Boyette and Walker, 1985). Another species, Alternaria alternata, infecting S. obtusifolia has been discovered widening the range of suitable pathogens to be evaluated to control the species using bioherbicides (Mello et al., 2001). A strain of Fusarium oxysporum isolated from S. obtusifolia has potential as a mycoherbicide (Boyette et al., 1993). Pseudocercospora nigricans has also been identified as a potential biological control agent (Hofmeister and Charudattan, 1987). In a review of possibilities for the biological control of S. tora and S. obtusifolia, Cock and Evans (1984) suggested that the bruchid Sennius instabilis, which attacks S. obtusifolia in tropical America, should be considered for introduction against S. tora in the Old World, and that three fungi (Pseudocercospora nigricans, Pseudoperonospora cassiae and Ravenelia berkeleyii) should be evaluated for possible use as mycoherbicides or classical biological control agents.
Walker and Tilley (1997) identified Myrothecium verrucaria as a potential mycoherbicide agent although it does affect a number of plant species including some economically important crops. Müller-Schärer et al. (2000) have reported that early results indicated that a multiple-pathogen strategy consisting of four pathogens applied in a single, post-emergence spray was feasible without loss of efficacy or host specificity. Following a survey of the phytophagous arthropod fauna in Central America, two species, Mitrapsylla albalineata (Homoptera: Psyllidae) and Conotrachelus sp. 'Morelos' (Coleoptera: Curculionidae), have been brought to Australia for further investigations as potential biocontrol agents (Palmer and Pullen, 2001).
Herbicides that give control of S. tora and S. obtusifolia, either alone or in mixtures with other products include: 2,4-D amine (rice); 2,4-DB (groundnuts, soyabean); acifluorfen (groundnuts, mung bean, soyabean); atrazine (maize, sorghum); butylate (maize); chlorimuron (soyabean); chloroxuron (carrots, onions, soyabean); clopyralid (barley, oats, wheat); dicamba (maize); dichlorprop (cereals); diuron (cotton, oats, soyabean); EPTC (castor, citrus, flax, maize, Phaseolus beans, potato, sorghum, sugar beet, sunflower, sweet potato); flumetsulam (soyabean); fluometuron (cotton); fluridone (cotton); glufosinate (soyabean); glyphosate and glyphosate trimesium (land preparation, minimum tillage, tree crops, vines); imazaquin (groundnut, soyabean); linuron (cotton, potato, soyabean); metribuzin (soyabean); MSMA (cotton); norflurazon (cotton); oxyfluorfen (cotton); pendimethalin (soyabean); picloram (grassland); primisulfuron (maize); prometryn (cotton); pyridate (groundnuts); and vernolate (groundnuts) (Anon., 1998).
Hicks et al. (1998) show that a mixture of pyridate and 2,4-DB acts synergistically on S. obtusifolia without increased damage to groundnut. S. obtusifolia occurs in flushes, seedlings emerge after rainfall events, and few herbicides, including etribuzin, imazaquin, chlorimuron, flumetsulam, and glyphosate, provide adequate control and their efficacy can depend heavily on environmental conditions (Buehring et al., 2002).
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