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The symptoms of attack by S. asiatica may be apparent some time before the weed emerges, hence, the common name 'witchweed'. At an early stage, these symptoms are indistinguishable from those caused by drought, i.e. wilting and curling of the leaves, but they are strong indicators of S. asiatica if they occur when the soil is still moist. The infected plant may also show stunting from quite an early stage and pronounced scorching of the leaf borders and finally of the whole leaf area may occur at a later stage, hence, the common name 'fireweed' and equivalents in other languages.
As shoot growth is reduced, root growth is increased in plants infected by S. asiatica (Patterson, 1990).
Regulatory control has been enforced in the USA for many years and by the end of the twentieth century was reported to be finally nearing success with 93% of the originally infested area declared Striga-free by the end of 1994 (English et al., 1995) and 98% by 1999 (Patterson, 1999). Some pockets, however, were persisting still in 2011 (Iverson et al., 2011).
The eradication programme employed in the USA involved the application of herbicide, not only to kill the emerged weed in the cereal crop (maize) but also to control alternative grass hosts in the rotational broad-leaved crops such as tobacco and soyabeans. The spraying programme in maize, which was repeated several times per season, was based on an intensive survey procedure and was supplemented by injection of ethylene gas to cause suicidal germination. Fumigation with various fumigants was used to finally clear small infestations but these have since been banned for agricultural use. A strict local quarantine was enforced, including vehicle washing and restriction on the movement of many classes of agricultural produce.
Cultural Control and Sanitary Measures
The complex germination requirements, and specialised physiology of S. asiatica, together with certain ecological preferences, can be exploited to some degree in cultural control methods.
Preference for relatively dry conditions can be countered by using irrigation wherever available. Preference for infertile soil conditions can be countered by improving soil fertility via the use of nitrogenous and phosphate manures, green manuring (see Chibudu, 1998) or rotation with leguminous crops. The use of Crotalaria green manure crops has proved especially effective in suppressing S. asiatica in rice in Tanzania (Riches, 2005; Kayeke et al., 2007; Rodenburg et al., 2010). Treatments providing such longer-term improvement in soil fertility are generally preferable to the use of inorganic fertilisers, but in Angola N at 60 kg/ha in the form of ammonium sulphate markedly reduced S, asiatica emergence and greatly increased maize yield. (Dovala and Monteiro, 2014) and Kabambe et al. (2007) concluded that fertiliser was the most important contributor to increased maize yield in Malawi. Urea proved helpful in rice in Tanzania, (Riches et al., 2005) but the cost discouraged most farmers from adopting it.
Most early studies failed to show benefit from application of phosphate (e.g. Farina et al.,1985) but more recent research has shown that phosphate may be as important as nitrogen in reducing exudation of strigolactones (e.g. Lopez-Raez et al., 2008; Chen et al., 2009). Jamil et al. (2013; 2014a) have now shown that ‘micro-dosing’ – the application of diammonium phosphate at 2-4 g per planting hole - can reduce emergence of Striga hermonthica and increase yield in sorghum and in millet. Subsequently Jamil et al. (2014b) have shown that similar benefit can be gained more economically by soaking crop seed (rice, sorghum and millet) in potassium phosphate. In one corresponding result on S. asiatica modest doses of di-ammonium phosphate have provided good results when applied in the ridge before planting maize in Malawi (Shaxson and Riches, 1998).
Preference for low humidity and high transpiration rates can be countered by dense crop planting, inter-cropping with leafy species or later planting when rains have set in more fully and continuously, provided care is taken not to prejudice the growth of the cereal crop to any great extent. Conversely, early planting may be beneficial in more temperate zones when temperatures are sub-optimal for the weed. Under dry conditions it has been shown in Ethiopia that a combination of tie-ridging with skip-row planting (omitting every 3rd row) provided better tolerance of Striga species. Rotation out of susceptible cereals is desirable wherever possible to allow the seedbank to decline, though the longevity of the seed means that one or two break crops provide only partial control. Some alternative crop species, however, can act as 'trap-crops', stimulating germination of the parasite but failing to allow parasitisation. Germination is stimulated but the parasite seedlings fail to penetrate the root beyond the cortex (Hood et al., 1998). These are ideal crops to include in rotation, but must be planted at a time and a density to ensure optimum germination. For S. asiatica, suitable crops include cotton, cowpea, soyabean, pearl millet (where not normally attacked) but even these may need to be grown for several seasons to achieve a useful reduction of the infestation. The choice of crop variety may also be significant (Iwo and Uwah, 2007; YongqingMa, 2015).
In some regions, the choice of trap crop may be influenced by their susceptibility to parasitisation by Alectra vogelii. In Malawi, Kabambe et al. (2008a) found certain soyabean varieties, pigeon pea and velvet bean (Mucuna pruriens) to be suitable where A. vogelii occurred precluding the use of the susceptible cowpea. Moe recently Alectra- resistant cowpea varieties have become available (Kabambe et al., 2014). 'Catch-crops' are susceptible crop or forage species which are used to encourage germination of the weed but are then destroyed before it has time to set seed. These are rarely popular with the farmer but may be suitable in some ecologies, especially if there is a bi-modal rainfall when the less reliable season can perhaps be devoted to such a crop.
Inter-cropping with e.g. cowpea can result in significant reductions in S. asiatica and increases in yield of maize (Chivinge et al., 2001; Atera et al., 2013). The benefit is mainly shown under wet conditions, when the inter-crop further reduces transpiration of the parasite. Under dry conditions, cereal crop yield may be reduced and economics then depend on the productivity and value of the intercrop. In the case of S. hermonthica, the intercrop is more effective when planted intra-row rather than inter-row and in Mozambique within-row pigeon pea was more effective than N and P in increasing yield and suppressing Striga asiatica in maize (Rusinamhodzi et al., 2012).
Using the perennials Desmodiumuncinatum or D. intortum as the intercrop, as in the so-called ‘push-pull- technique (designed to control stem-borer, the Desmodium intercrop repels the moths while a surrounding stand of grass attracts them) appears to have even greater benefit, suppressing S. hermonthica almost completely in maize and greatly enhancing yield, especially as soil fertility is built up over a period of years (Khan et al., 2000; 2006; 2007). In South Africa Reinhardt and Tesfamichael (2011) confirm that a combination of D. uncinatum with added nitrogen can provide 100% suppression of S. asiatica in sorghum. In Angola, comparable results against S. asiatica in maize have been obtained using intercrops of Desmodium uncinatum (cv. D. Silver leaf), Cajanus cajan, Mucuna pruriens, species of Tephrosia. And species of Crotalaria. Trao-cropping with Tripsacum laxum has also proved useful in those trials.
In certain situations, transplanting may be an effective means of reducing Striga attack (Dawoud et al., 1996).
Mechanical methods of destruction are not generally satisfactory. If the weed is hoed at ground level, it is almost certain to regenerate from buds below ground and hence hoeing needs to be repeated at quite frequent (2-3 week) intervals. However, good results have been reported with mid-season ridging (Kasembe and Chivinge, 1997). Hand-pulling has a more durable effect but is not practicable for dense infestations and can only be considered for mopping up scattered populations before they build up or after they have been greatly reduced by other methods. Hoeing or hand-pulling are much easier in row-planted crops than in broadcast plantings.
Organisms considered to have potential as biocontrol agents for S. asiatica and/or other Striga species have included the gall-weevil, Smicronyx spp. the agromyzid fly, Ophiomyia Strigalis, the moths, Eulocastra argentisparsa and Eulocastra undulata, the plume moth, Stenoptilodes taprobanes, the powdery mildew Sphaerotheca fuliginea and other fungi including Drechslera longirostrata, Phoma and Cercospora species (Greathead, 1984). Evans (1987) considered Cercosporastrigae the most promising of the fungi. Several different species of Fusarium have been shown to have potential against both S. hermonthica and S. asiatica (Abbasher et al., 1995; 1996). None of these organisms have yet been successfully utilised, but the possibilities for using Fusarium oxysporum are still considered promising (Beed et al., 2007). The strain Foxy 2, widely tested on S. hermonthica is confirmed also to be active against S. asiatica (Elzein et al., 2002). More virulent strains are now being tested along with a novel means of on-farm culturing and it is hoped there could be commercialisation before too long (Koltai, 2015). The only attempts to control S. hermonthica have been with the introduction of Smicronyx albovariegatus and Eulocastra argentisparsa from India into Ethiopia in 1974 and 1978, but there is no evidence that these organisms ever established.
Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources: