Senna obtusifolia
(sicklepod)

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 and is invasive in parts of East Africa. In places it produces impenetrable monotypic stands. In Australia it is thought to increase the risk of damage to forest by fire.

Annual
Biennial
Broadleaved
Seed propagated
Shrub
Woody

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 (where Witt and Luke (2017) have recorded this species as invasive), 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).

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).

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.

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).

Reproductive biology

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).

Ecology

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).

Environmental Requirements

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).

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).

Animal feed, fodder, forage

• Fodder/animal feed

Medicinal, pharmaceutical

• Traditional/folklore

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).

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

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).

Biological Control

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).

Chemical Control

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).