Prota 3: Dyes and tannins/Colorants et tanins
Φsterr. Bot. Zeitschr. 25: 11 (1875).
Scrophulariaceae (APG: Orobanchaceae)
n = 20
Striga orobanchoides (R.Br.) Benth. (1836).
Witchweed, cowpea witchweed (En). Striga (Fr). Kichawi nke (Sw).
Origin and geographic distribution
Striga gesnerioides is widespread all over tropical Africa, the Indian Ocean islands and tropical Asia. It has been introduced to Florida.
The sap of Striga gesnerioides is used to dye skins blue-black in Mali. In Kenya root sap is used to colour the fingers an indigo blue. The flowers yield a pink colour which can be used for painting. As fodder Striga gesnerioides is useless; cattle will not graze it and only camels browse it reluctantly when nothing else is available. In Botswana the stem is said to be used as toothbrush. In traditional medicine, the powdered plant is sprinkled on wounds or, made into a paste with some fat, applied against swollen neck glands.
The dyeing properties may derive, as does the cytotoxicity of the plant, from the presence of iridoid glucosides, which have been isolated and characterized from other Striga species. An ethanolic extract of whole Striga gesnerioides plants showed antifertility effects in male rats. Ethanolic and water extracts showed antihistaminic and mast cell stabilizing activity in rats. The flavones apigenin and luteolin, isolated from Striga gesnerioides, showed anti-inflammatory and antispasmodic activities.
Succulent, greenish yellow annual herb up to 35 cm tall, usually branching from the base, glabrous or minutely puberulent; each plant with a single, large, tuberous primary haustorium 13 cm in diameter, and with numerous adventitious roots emerging from subterranean scales; stem square but obtusely angled. Leaves opposite, appressed to the stem, scale-like, 510 mm Χ 23 mm. Inflorescence a terminal bracteate spike. Flowers bisexual, zygomorphic, 5-merous, not fragrant, sessile; calyx tubular with 5 teeth at apex, 46 mm Χ 2 mm; corolla tubular, 2-lipped, up to 15 mm long, bent in upper part of tube, pale blue to dark purple, upper lobes 2, fused, sharply recurved, up to 2.5 mm long, lower lobes 3, spreading, c. 3 mm long; stamens 4, 2 longer and 2 shorter; ovary superior, tubular, 2-celled, style terete, stigma 2-fid. Fruit an ovoid capsule 12 mm Χ 3 mm, many-seeded. Seeds very small, dust-like, with prominent encircling ridges.
Striga comprises about 40 species in the tropics and subtropics of the Old World with occasional introductions elsewhere. At least 11 Striga species attack crops, including all the important tropical cereals (maize, sorghum, pearl millet, finger millet, upland rice), sugar cane and many grasses grown for fodder. Currently 4 species are considered major problems in tropical agriculture: Striga hermonthica (Del.) Benth. in cereals, Striga asiatica (L.) Kuntze in cereals, Striga gesnerioides in a variety of dicots (e.g. cowpea) and Striga aspera (Willd.) Benth. in cereals and grasses and they are all widespread in tropical Africa. Most Striga species are used in traditional medicine, and occasionally for dyeing; for example Striga asiatica is used as a mordant for Cochlospermum tinctorium Perr. ex A.Rich., Striga aspera is used in Nigeria as a black facial stain, and Striga hermonthica is used as a dye itself, but also as a mordant in indigo dyeing to deepen the colour.
Striga gesnerioides is an obligate root parasite with low photosynthetic activity. Its seeds germinate in response to specific germination stimulants exuded by host roots. After germination, an haustorium is formed in the root of the host plant through differentiation of the root apex. A vascular connection is subsequently established between the 2 plants, allowing the parasite to absorb water and nutrients that are essential for its development. Control of the parasite is difficult to achieve because of the intimate association between the parasitic weed and its host. Striga gesnerioides as a whole has a very broad host range. It is a major pest in cowpea, parasitizing the roots and often considerably reducing its growth and yield. In the wild it parasitizes leguminous weeds, particularly Tephrosia and Indigofera in West Africa, but also Convolvulaceae (e.g. in Sudan Merremia, Ipomoea and Jacquemontia) and Euphorbiaceae. Due to its autogamous nature, many different strains have evolved that can attack few or only a single plant species. These strains vary in their morphology and their distribution often overlaps. In West Africa there are at least five different strains that parasitize cowpea and each one varies in the number of cultivars it can attack. Tobacco and sweet potato are occasionally attacked. Strains attacking tobacco in Zimbabwe do not affect cowpea. Bambara groundnut is not affected; evidence of the occurrence of Striga gesnerioides on groundnut is conflicting. The cultivation of crops such as cowpea in monoculture and without fallow years or rotation crops results in a rapid increase of favourable circumstances for growth of Striga which, on poor soils, can reduce yield by more than 50%.
Striga gesnerioides is found in open grassland, savanna with scattered trees, on rocky hills and most abundantly as a weed in crops.
Genetic resources and breeding
As a widespread parasitic weed Striga gesnerioides is not liable to genetic erosion, and breeding research is directed to resistant host plants. Striga gesnerioides is very variable and comprises many morphologically identical strains, but as each one is adapted to specific cowpea cultivars, control is very difficult and research complicated. Much research is being done however, and promising results have been obtained towards breeding resistant cowpea cultivars, developing chemicals which force Striga seed to germinate, or prevent it from doing so, and developing biological control methods, e.g. through insects feeding on Striga (e.g. butterfly larvae of Junonia orithya feed on stems and fruits, the weevil larvae of Smicronyx umbrinus on fruits).
Striga gesnerioides as source of a dye will remain of minor importance. All research effort is focused on controlling its damage to crops. Promising results have been obtained in developing biological control methods.
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Correct citation of this article:
Jansen, P.C.M., 2005. Striga gesnerioides (Willd.) Vatke In: Jansen, P.C.M. & Cardon, D. (Editors). PROTA 3: Dyes and tannins/Colorants et tanins. [CD-Rom]. PROTA, Wageningen, Netherlands.