Scientific name

Striga hermonthica (Delile) Benth.

Synonyms

Striga senegalensis Benth.; Buchnera hermonthicaDelile

Common names

Purple witchweed, witchweed, ekeyongo (Ekegusii), emoto (Ateso), Hayongo (Luo), kituha (Sukuma)

Family

Scrophulariaceae

Origin

The origin of Striga hermonthica is unclear. It may have originated in north-east Africa.

Naturalised distribution (global)

Striga hermonthica is widely naturalised in Africa and has spread to the Arabian peninsula.

Introduced, naturalised or invasive in East Africa

Striga hermonthica affects farmlands in Kenya, Uganda and Tanzania.

Habitat

Croplands and cultivation, notably in cereal crops in low moisture and low fertility soils. It is rarely found in natural vegetation.

Description

Herbaceous plant to 30 cm, sometimes creeping ( prostrate or decumbent).

The stem of Striga hermonthica is four sided (quadrangular). It causes characteristic yellowish blotches in the foliage about 1 cm long by 0.5 cm wide.

Striga hermonthica is an obligate parasite (it cannot develop independently from a suitable host). Its host range is normally limited to grasses. Its stems are hairy, hard, quadrangle-shaped and fibrous.

The leaves are narrow-oblanceolate (lance-shaped but broadest above the middle and tapering toward the base) and gradually decreasing towards the tip. The leaf size is 2.5 to 8 cm long and up to 2 cm wide.

The flowers of Striga hermonthica occur in a dense spike. Each spike may contain 6-10 open flowers.

Seeds are tiny dust-like seeds in a pod and are dispersed by wind, water and as a contaminant of farm implements and crop seeds.

Reproduction and dispersal

Striga hermonthica reproduces by seed. The small seeds are wind dispersed, can be moved with runoff following heavy rains, on the feet of man and livestock, on farm implements and in animal faeces following their ingestion of the seed.

Similar species

From the CABI Invasive Species Compendium: 'Striga hermonthica is most often confused with Striga aspera, a common plant in West Africa, occurring on rice, maize and many wild grass hosts. S. aspera has slightly smaller flowers but is readily distinguished (in West Africa) by the bend in the corolla tube occurring two thirds to three quarters up, rather than about half way. Also the bracts in S. aspera are narrower, 1-2 mm only and without a fringe of hairs. S. aspera is rare in East Africa, but there it is less readily distinguished from the local S. hermonthica on the basis of the corolla tube, and the bract character is more important. Other Striga species have different coloured flowers and/or more ribs on the calyx tube. Striga asiatica, a white-, yellow- or red-flowered parasite of cereals in Africa and Asia, has 10-14 calyx ribs. Those with 15 calyx ribs include Striga angustifolia, Striga forbesii and Striga latericea.

Striga forbesii differs from S. hermonthica in its 15 calyx ribs, salmon-pink flowers and broader, coarsely toothed leaves. It is widespread but sporadic across Africa and Madagascar, sometimes attacking cereals and sugarcane, especially in Zimbabwe and Tanzania; also on wild hosts, especially Setaria and Echinochloa spp. Biology and ecology are generally similar to those of S. hermonthica, as are the damaging effects on crops, but germination factors may be different.

Striga latericea is very similar to S. forbesii but is a perennial with deeper brick-red flowers and an underground stolon system, parasitic on a range of wild hosts in East Africa and on sugarcane in Ethiopia.

Economic and other uses

The plant has medicinal properties. However, this use cannot compensate for this plant's overall negative impacts.

Environmental and other impacts

Striga hermonthica the largest and most destructive of the Striga species. It is one of the most serious weeds in Africa. It can parasitise important agricultural crops such as corn, sorghum, millets and maize. The host plant's nutrients are depleted and energy is spent supporting the parasitic Striga. Infestations of witch weed reduce yields and contaminate crops. Damage is particularly severe under conditions of low rainfall and poor soil fertility. S. hermonthica is not known to have major impact on natural vegetation.

S. hermonthica has been listed as a noxious weed in almost all Australian states.

Management

The precise management measures adopted for any plant invasion will depend upon factors such as the terrain, the cost and availability of labour, the severity of the infestation and the presence of other invasive species. Some components of an integrated management approach are introduced below.

The best form of invasive species management is prevention. If prevention is no longer possible, it is best to treat the weed infestations when they are small to prevent them from establishing (early detection and rapid response). Controlling the weed before it seeds will reduce future problems. Control is generally best applied to the least infested areas before dense infestations are tackled. Consistent follow-up work is required for sustainable management.

Hand-pulling is commonly used as a control method but can often be ineffective especially if the crop has set seed. It is vital to ensure that Striga hermonthica is not allowed to set seed because of the large numbers of seed produced and their persistence in the soil.

More effective are integrated approaches with hand-pulling only used in combination with other methods such as crop rotation, growing less susceptible varieties, irrigation, manuring or otherwise improving soil fertility, intercropping with leafy species, and the planting of trap crops and catch crops.

Trap crops stimulate the germination of S. hermonthica but do not allow parasitisation. For suitable crops include cotton, cowpea and soybean but it will take several seasons to achieve a significant reduction in the S. hermonthica infestation.

'Catch-crops' are susceptible species which are encourage germination of the weed but are then destroyed before it has time to set seed.

Chemical control can be effective but can be problematic due to its cost, the fact that it is most effective after weed emergence and may do little to prevent the damage and increase yield in the short term, and the risk of damage to non-cereal crops. Application before witchweed emergence can be effective but is unreliable.

Work has been undertaken to find effective biological control agents against S. hermonthica and other Striga species but to date there has been no documented success.

Legislation

Not listed as a noxious weed by the state or governments in Kenya, Tanzania and Uganda.

References

CABI Invasive Species Compendium online data sheet. Striga hermonthica (witchweed). CABI Publishing 2011. www.cabi.org/ISC. Accessed March 2011.

Global Invasive Species Database online data sheet. Striga hermonthica. www.issg.org/database. Accessed March 2011.

Editors

Agnes Lusweti, National Museums of Kenya; Emily Wabuyele, National Museums of Kenya, Paul Ssegawa, Makerere University; John Mauremootoo, BioNET-INTERNATIONAL Secretariat - UK.

Acknowledgments

This fact sheet is adapted from The Environmental Weeds of Australia by Sheldon Navie and Steve Adkins, Centre for Biological Information Technology, University of Queensland. We recognise the support from the National Museums of Kenya, Tropical Pesticides Research Institute (TPRI) - Tanzania and Makerere University, Uganda. This activity was undertaken as part of the BioNET-EAFRINET UVIMA Project (Taxonomy for Development in East Africa).

Contact

BioNET-EAFRINET Regional Coordinator: [email protected]