Striga gesnerioides (cowpea witchweed)
- Taxonomic Tree
- Notes on Taxonomy and Nomenclature
- Distribution Table
- Risk of Introduction
- Habitat List
- Hosts/Species Affected
- Host Plants and Other Plants Affected
- Growth Stages
- List of Symptoms/Signs
- Biology and Ecology
- Natural enemies
- Notes on Natural Enemies
- Plant Trade
- Detection and Inspection
- Similarities to Other Species/Conditions
- Prevention and Control
- Links to Websites
- Distribution Maps
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PicturesTop of page
IdentityTop of page
Preferred Scientific Name
- Striga gesnerioides (Willd.) Vatke (1875)
Preferred Common Name
- cowpea witchweed
Other Scientific Names
- Buchnera gesnerioides Willd. (1800)
- Buchnera hydrabadensis Roth. (1821)
- Buchnera orobanchoides R.Br. (1814)
- Striga orobanchoides R.Br. Benth. (1836)
- STRGE (Striga gesnerioides)
Taxonomic TreeTop of page
- Domain: Eukaryota
- Kingdom: Plantae
- Phylum: Spermatophyta
- Subphylum: Angiospermae
- Class: Dicotyledonae
- Order: Scrophulariales
- Family: Orobanchaceae
- Genus: Striga
- Species: Striga gesnerioides
Notes on Taxonomy and NomenclatureTop of page Originally described as Buchnera gesnerioides by Wildenow in 1801, it was independently named B. orobanchoides by R Brown in 1814. In 1836, Bentham used the specific name orobanchoides when creating the new combination Striga orobanchoides which was widely used until quite recently. However, Vatke's combination, using the earlier specific name to make the combination Striga gesnerioides, is now accepted as the more valid.
DescriptionTop of page S. gesnerioides differs markedly from most other Striga species, in being totally parasitic, without expanded leaves and with a pale-green or yellowish colour. In vigorous plants, as on cowpea, the stems branch mainly below the soil and emerge as a cluster of generally unbranched, fleshy, erect shoots 10-20 cm high, with scale leaves only a few millimetres long (Parker and Riches, 1993). On other hosts, shoots may be single. Much of the shoot comprises the spike-like inflorescence. Flowers, generally in opposite pairs, subtended by bracts 4-6 mm long, are sessile with a tubular calyx, also 4-6 mm long with five ribs and corolla 5-15 mm long with corolla lobes expanding to about 5 mm across. Flower colour in forms attacking cowpea is usually mauve but occasionally white, whereas in other forms it may be reddish, purple or even yellow. The capsule, up to 5 mm long, develops several hundred minute seeds about 0.25 mm long, not readily distinguishable from those of S. asiatica (see Musselman and Parker, 1981). Seed production per plant was estimated to be over 60,000 (Hartman and Tanimonure, 1991).
S. gesnerioides also differs from most other Striga species in developing a substantial haustorium at least several millimetres across, about 1 cm on tobacco and often up to 3-4 cm in diameter on cowpea. The root system is rudimentary.
Chromosome number (2n) = 40.
DistributionTop of page
S. gesnerioides is widely distributed in Africa, from Morocco and Egypt, south to South Africa, also in Arabia and widely in India (up to 2000 m) and Sri Lanka. Holm et al. (1979) indicate occurrence in Australia, but there is no more recent confirmation of this. It also occurs locally in Florida, USA.
A record of S. gesnerioides in Japan (Holm et al., 1979; EPPO, 2014) published in previous versions of the Compendium is unreliable. No original source was provided for the record in Holm et al. (1979). According to Spallek et al. (2013), S. gesnerioides is not present in Japan.
Across most of its range it occurs only on wild hosts. It is only in the western African countries of Senegal, Mali, Togo, Benin, Burkina Faso, Ghana, Nigeria, Niger, Cameroon and Chad that it constitutes a serious weed problem on cowpea. In Zimbabwe, South Africa and Ethiopia it occurs only very locally on tobacco and/or sweet potato.
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: 23 Apr 2020
|Continent/Country/Region||Distribution||Last Reported||Origin||First Reported||Invasive||Reference||Notes|
|Benin||Present||Hepper (1963); EPPO (2020)|
|Botswana||Present||Hepper (1990); EPPO (2020)|
|Burkina Faso||Present||M'Boob (1994); EPPO (2020)|
|Burundi||Present||M'Boob (1994); EPPO (2020)|
|Cabo Verde||Present||Hepper (1963); EPPO (2020)|
|Cameroon||Present||GRIN (2000); EPPO (2020)|
|Central African Republic||Present||GRIN (2000)|
|Chad||Present||Parker and Riches (1993); EPPO (2020)|
|Congo, Democratic Republic of the||Present||Parker and Riches (1993); EPPO (2020)|
|Congo, Republic of the||Present||EPPO (2020)|
|Egypt||Present, Widespread||Holm et al. (1979); EPPO (2020)|
|Eswatini||Present||Wells et al. (1986)|
|Ethiopia||Present, Widespread||Parker and Riches (1993); EPPO (2020)|
|Ghana||Present||Hepper (1963); EPPO (2020)|
|Guinea||Present, Widespread||Holm et al. (1979); EPPO (2020)|
|Kenya||Present||Holm et al. (1979); EPPO (2020)|
|Lesotho||Present||Wells et al. (1986)|
|Malawi||Present||Hepper (1990); EPPO (2020)|
|Mali||Present||Hepper (1963); EPPO (2020)|
|Mauritania||Present||Parker and Wilson (1986); EPPO (2020)|
|Morocco||Present||Parker and Wilson (1986); EPPO (2020)|
|Mozambique||Present||Hepper (1990); EPPO (2020)|
|Namibia||Present||Wells et al. (1986)|
|Niger||Present||Hepper (1963); EPPO (2020)|
|Nigeria||Present, Widespread||Hepper (1963); EPPO (2020)|
|Senegal||Present||Parker and Riches (1993); EPPO (2020)|
|Sierra Leone||Present||GRIN (2000)|
|South Africa||Present, Widespread||Parker and Riches (1993); EPPO (2020)|
|Sudan||Present||Holm et al. (1979); EPPO (2020)|
|Togo||Present||Hepper (1963); EPPO (2020)|
|Zambia||Present||Hepper (1990); EPPO (2020)|
|Zimbabwe||Present, Widespread||Parker and Riches (1993); EPPO (2020)|
|Cambodia||Present, Widespread||Holm et al. (1979); EPPO (2020)|
|-Tamil Nadu||Present||Parker and Riches (1993)|
|Japan||Absent, Unconfirmed presence record(s)||Spallek et al. (2013); Holm et al. (1979); EPPO (2020)|
|Oman||Present||Musselman and Hepper (1988)|
|Saudi Arabia||Present, Widespread||Musselman and Hepper (1988); Parker and Riches (1993); EPPO (2020)|
|Sri Lanka||Present, Widespread||Holm et al. (1979); EPPO (2020)|
|Yemen||Present||Parker and Wilson (1986); Musselman and Hepper (1988); EPPO (2020)|
|United States||Present, Localized||Holm et al. (1979); EPPO (2020)|
|-Florida||Present||Parker and Riches (1993); EPPO (2020)|
|Australia||Present, Widespread||Holm et al. (1979); EPPO (2020)|
Risk of IntroductionTop of page All Striga species are listed as prohibited imports into Australia, Israel, Russia and USA.
HabitatTop of page S. gesnerioides is a plant of the semi-arid tropics, associated both with agriculture and with undisturbed natural vegetation. In southern Africa, Hepper (1990) indicates occurrence in Mopane woodland, rocky grassland and cultivated ground. In West Africa, it is apparently able to grow under quite wet conditions but fails to attack aquatic species growing in water 10 cm deep (Porteres, 1948). A survey by Cardwell and Lane (1995) suggested an association of S. gesnerioides with sandy soils.
Habitat ListTop of page
Hosts/Species AffectedTop of page S. gesnerioides, as a species, has a wide host range, including annual, perennial and woody species, mainly in Dicotyledonae but also some grass species. The main economic host is cowpea, with tobacco and sweet potato also affected very locally. The most commonly affected hosts occur in the families Acanthaceae, Convolvulaceae, Euphorbiaceae, Fabaceae and Solanaceae (Parker and Riches, 1993). Other genera providing occasional hosts, as listed by Porteres (1948) and others, include Sansevieria (Agavaceae), Commiphora (Burseraceae), Cleome (Capparaceae), Helianthus (Asteraceae), Bergia (Elatinaceae), Echinochloa, Panicum, Rottboellia, Brachiaria, Paspalum, Andropogon, Cymbopogon, Hyparrhenia, Oryza, Setaria, Hyparrhenia, Eleusine (Poaceae), Dysophylla (Lamiaceae), Pterodiscus (Pedaliaceae), Cissus (Vitaceae).
Individual biotypes of S. gesnerioides have a much narrower range of hosts, however. The types attacking cowpea are rarely detected on any wild host. An exception is the occurrence of a Nigerian biotype on Indigofera spicata and I. tinctoria as well as cowpea in pot experiments by Igbinnosa and Okonkwo (1991); another type with deeper purple flowers and more slender stems, which overlaps in distribution, attacks Tephrosia (Fabaceae), Jacquemontia and Merremia species (Convolvulaceae). Although the host range of individual biotypes is generally very limited, the hosts attacked by a single biotype may come from diverse plant families. The form occurring in Florida, USA, has only five or six known hosts but these include sunflower (Asteraceae), sweet potato (Solanaceae), Jacquemontia tamnifolia (Convolvulaceae) and Alysicarpus vaginalis (Fabaceae) in addition to the main host Indigofera hirsuta (Fabaceae) (Upton, 1979). Other forms are apparently specific (or nearly so) to tobacco, or to Euphorbia species, while individual biotypes can also differ in the varieties of cowpea that they can parasitize, as noted under Control. These specificities apparently depend on factors involved after attachment, rather than at the germination stage, but the precise mechanisms are not yet fully understood.
References from before 1957, cited here and in other sections, are usefully abstracted in the compilation by McGrath et al. (1957).
Host Plants and Other Plants AffectedTop of page
|Euphorbia abyssinica||Euphorbiaceae||Wild host|
|Indigofera hirsuta (hairy indigo)||Fabaceae||Wild host|
|Ipomoea batatas (sweet potato)||Convolvulaceae||Other|
|Jacquemontia tamnifolia (Smallflower morningglory)||Wild host|
|Nicotiana tabacum (tobacco)||Solanaceae||Other|
|Oryza glaberrima (African rice)||Poaceae||Other|
|Vigna unguiculata (cowpea)||Fabaceae||Main|
Growth StagesTop of page Flowering stage, Fruiting stage, Vegetative growing stage
SymptomsTop of page Symptoms on cowpea are not always obvious in the early stages of infection but show themselves gradually as veinal chlorosis, reduced growth, poor fruiting and eventually chlorosis and withering of foliage. Uprooting reveals the substantial yellowish haustorium, 1-3 cm in diameter at the point of attachment of the Striga plant.
List of Symptoms/SignsTop of page
|Leaves / yellowed or dead|
|Whole plant / dwarfing|
|Whole plant / early senescence|
Biology and EcologyTop of page The biology of S. gesnerioides is generally very similar to that of S. hermonthica and S. asiatica. It is an obligate parasite with minute seeds, unable to establish without the help of a host plant. Germination depends on a period of moist conditioning and exposure to germination stimulants in host root exudates, the most important of which is alectrol (so named because it also stimulates germination of the related parasite Alectra vogelii) (Muller et al., 1992). This is closely related to the lactones, strigol and sorgolactone, which stimulate S. hermonthica and S. asiatica. S. gesnerioides is also stimulated by ethylene but is relatively insensitive to the strigol analogues GR 7 and GR 24 (Igbinnosa and Okonkwo, 1992; Maass, 1999). Prolonged conditioning in the absence of stimulant results in a secondary 'wet dormancy' (Reid and Parker, 1979; Maass, 1999). Germination temperature appears to be uncritical over the range 23-33°C. Germination is slower than in other species, taking 2-3 days.
Attachment and penetration of the host root do not appear to differ from the other main species (see Reiss and Bailey, 1998), but the physiology of the established parasite differs in showing very low rates of photosynthesis. The effects on the host also differ in that there is no change in root:shoot ratio. Host roots beyond the point of attachment tend to abort. Host photosynthesis may be reduced to some degree but the greatest damaging effect is attributed to the removal of metabolites from the host (Graves et al., 1992; Hibberd et al., 1996). Fertilization is autogamous (Musselman et al., 1991).
A survey by Cardwell and Lane (1995) suggested an association of S. gesnerioides with sandy soils.
Natural enemiesTop of page
Notes on Natural EnemiesTop of page S. gesnerioides in West Africa is frequently affected by several gall-forming Smicronyx species, one of which, S. dorsomaculatus, has recently been described as a new species (Anderson and Cox, 1997). The galls caused by Smicronyx species occur variously in the capsules and in the base of the stem. Markham (1985) concluded that stem galls and plant pathogens, including Macrophomina phaseolina, often significantly reduced vigour and seed production of S. gesnerioides in Mali. It has also been noted that use of insecticides in cowpea can result in the destruction of Smicronyx and an increase in vigour of the parasite (Parker and Riches, 1993).
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|
|Bulbs/Tubers/Corms/Rhizomes||seeds||Yes||Pest or symptoms not visible to the naked eye but usually visible under light microscope|
|Fruits (inc. pods)||seeds||Yes||Pest or symptoms not visible to the naked eye but usually visible under light microscope|
|Growing medium accompanying plants||seeds||Yes||Yes||Pest or symptoms not visible to the naked eye but usually visible under light microscope|
|True seeds (inc. grain)||seeds||Yes||Pest or symptoms not visible to the naked eye but usually visible under light microscope|
|Plant parts not known to carry the pest in trade/transport|
|Stems (above ground)/Shoots/Trunks/Branches|
ImpactTop of page S. gesnerioides is a severe pest of cowpea across many countries of West Africa, causing significant loss of yield and continuing to spread and intensify in some areas. In a farm survey in the Kano district, northern Nigeria, at least 25% of farmers reported severe infestation of S. gesenerioides in cowpea (Bottenburg, 1995), and Emechebe et al. (1991) reported that many farmers' fields across northern Nigeria had been 'completely blighted'. Ouedraogo (1989) reported a comparable situation in Burkina Faso. In a trial to assess crop loss, yields across a number of cowpea varieties averaged 30% lower and were 56% lower in the most susceptible parasitized by S. gesnerioides (Aggarwal and Ouedraogo, 1989).
Detection and InspectionTop of page Where infestation is suspected, from previous history or from symptoms of chlorosis, uprooting the crop can reveal the nodules of young Striga seedlings. These can vary in size from a few millimetres to over 2 cm in diameter and are somewhat irregular in shape.
A technique for detecting the seeds of Striga spp. as contaminants of crop seed is described by Berner et al. (1994). This involves sampling the bottom of sacks, elutriation of samples in turbulent flowing water and collection of seeds and other particles on a 90-µm mesh sieve. Striga seeds are then separated from heavier particles by suspension in a solution of potassium carbonate of specific gravity 1.4 in a separating column. Sound seeds collected at the interface are then transferred to a 60-µm mesh for counting. However, the seeds of S. gesnerioides are not readily distinguished from those of other Striga species such as S. asiatica or S. hermonthica (see Musselman and Parker, 1981).
Similarities to Other Species/ConditionsTop of page S. gesnerioides is not readily confused with other Striga species, due to the lack of expanded leaves (see Parker and Riches, 1993, for a key to agriculturally important species).
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
No cultural control methods have been widely developed or adopted. Crop rotation should be effective over the long term but is rarely practicable and there has been little research on the potential for trap crops, although pigeon pea, velvet bean (Mucuna species), sorghum and soyabean have been suggested (Wild, 1948; Igbinnosa and Okonkwo, 1991; Berner and Williams, 1998). Cardwell and Lane (1995) also note an apparent absence of S. gesnerioides where cotton is grown. The use of manures or fertilizer to increase soil fertility has apparently much less impact on this species than on those attacking cereal crops (Parker and Riches, 1993). Hand-pulling is difficult and liable to uproot the crop itself, though it should be used where new sporadic infestations are discovered.
Some herbicides have shown moderate promise for conventional pre-emergence application (see Parker and Riches, 1993), but the farmers affected by S. gesnerioides are not generally in a position to use these and there has been no field use. A more recent development has been the demonstration that S. gesenerioides may be controlled by application of the herbicide imazaquin to the crop seed itself (Berner et al., 1994) but it is uncertain whether this has yet been used in practice.
S. gesnerioides is often very heavily affected by Smicronyx gall-forming weevils and while there has been no attempt to exploit these for biological control it has been noted that this natural control can be adversely affected by insecticide use. The only effort towards biological control has been the testing of the ethylene-generating bacterium Pseudomonas syringae as a means of inducing suicidal germination (Berner et al., 1999). Promising progress in the development of mycoherbicides for control of S. hermonthica, based on Fusarium species, has recently been reported (Watson et al., 2000), and could have relevance for control of S. gesnerioides in the future.
The main control measure available for cowpea is varietal resistance. The most important source of resistance is the landrace B301, originally selected for its partial resistance to Alectra vogelii in Botswana (Parker and Riches, 1993). B301 fortunately shows high-level resistance to A. vogelii in West Africa (based on two dominant genes) as well as to S. gesnerioides (based on a single dominant gene) (Singh et al., 1993; Atokple et al., 1995). The resistance, or virtual immunity, of this line has been effective against all biotypes of the parasite in West Africa except that occurring locally in southern Benin. Lane et al. (1996) describe the existence of five known parasite biotypes, varying in their virulence on different 'resistant' varieties of cowpea. Two other sources of resistance, Suvita-2 and IT82D-849, have different single dominant genes for resistance to the Mali biotype, and a different pattern of response to the five parasite biotypes (Atokple et al., 1995). While B301 and IT82D-849 resist four of the known biotypes, Suvita-2 is resistant only in Mali. Fortunately, although each of these three sources is susceptible in southern Benin, other resistant lines, 58-57 and IT81D-994, are resistant to this Benin biotype (Lane et al., 1993) even though they show susceptibility to biotypes in Niger and Nigeria. Further lines with resistance to some biotypes include APL-1 and 87-2 (Moore et al., 1995) but none of these, other than B301, and to some extent IT81D-994, has cross-resistance to Alectra. The mechanisms of resistance are not fully understood but involve a failure of the parasite to develop normally following penetration of the parasite haustorium into the host root (Reiss et al., 1995).
Although B301 is an agronomically poor line, the simple dominance of the resistance character has allowed its ready transfer into more desirable varieties (Singh and Emechebe, 1991). IITA (International Institute for Tropical Agriculture) has now developed lines with resistance to Striga and Alectra, as well as to various other pests and diseases. Singh (1999) lists the most promising of these and indicates that IT90K-76 and IT90K-59, both with resistance from B301, have already been released in Nigeria and South Africa, respectively. Progress is also being made in the development of varieties with combined resistance to Alectra and all five biotypes of S. gesnerioides, using crosses between 58-57 and the B301-derived IT90K-76 (Singh and Emechebe, 1997). In Senegal, Cisse et al. (1995) report a useful degree of resistance in the variety Mouride, whereas the variety KN-1 demonstrates a degree of tolerance, being less damaged in spite of parasite development (Gworgwor, 1991).
Although there has been no evidence as yet for the breakdown of resistance after repeated trials of the varieties based on B301 and other lines, vigilance will be needed to detect any such breakdown and minimize the risks of a build-up of more virulent biotypes. Shawe and Ingrouille (1993) used isoenzyme techniques to demonstrate differences between the populations of S. gesnerioides (from Niger) which emerged on a susceptible variety and on Suvita-2 which is partially resistant to the Niger biotype, emphasizing the risk of selection for virulence in any but totally immune varieties.
ReferencesTop of page
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Wells M J, Balsinhas A A, Joffe H, Engelbrecht V M, Harding G, Stirton C H, 1986. A catalogue of problem plants in southern Africa incorporating the national weed list of South Africa. Memoirs, Botanical Survey of South Africa. v + 658pp.
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