Senna obtusifolia (sicklepod)

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).

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).