Striga (plant)

Striga, commonly known as witchweed, is a genus of 28 species^[1] of parasitic plants that occur naturally in parts of Africa and Asia. The genus is classified in the family Orobanchaceae,^[2] although older classifications place it in the Scrophulariaceae.^[3] Although most species of Striga are not pathogens that affect human agriculture, some species have devastating effects upon crops, particularly those planted by subsistence farmers^[4]. Three species cause the most damage: Striga asiatica, S. gesnerioides, and S. hermonthica. Striga asiatica has a very wide geographic distribution, from Africa through southern and eastern Asia to Australia. Since the 1950s it is also known from the United States. This introduction, likely a result of human activity, resulted in an infestation of corn (maize, Zea) across many counties in North and South Carolina. The United States Department of Agriculture and state agencies imposed a quarantine on this area to control its spread - a process that was apparently successful. Striga hermonthica (purple witchweed) is also a parasite that affects grasses, particularly sorghum and pearl millet in subsaharan Africa. Cowpea witchweed, as its name implies, is a parasite of cowpea (Vigna unguiculata), which is not a grass but a member of the legume family (Fabaceae or Leguminosae). This species was also accidentally introduced into Florida in the United States where it was found parasitizing Indigofera hirsuta (hairy indigo, another legume).

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Striga in the United States was controlled through the use of several management stategies, including quarantines imposed on affected areas, control of movement of farm equipment between infected and noninfected areas, herbicide application, and imposed "suicidal germination." For the latter, in fields not yet planted in crops, seeds present in the soil are induced to germinate by injecting ethylene gas which mimics the natural physiological response tied to host recognition. Because no host roots are available, the seedlings die. Unfortunately, each Striga plant can produce tens of thousands of tiny seeds and these can remain dormant in the soil for many years ^[5]. Thus, such treatments do not remove all seeds from the soil. Moreover, this method is expensive and not generally available to many farmers in developing nations of Africa and Asia.

Another method called trap cropping involves planting a species in an infested field that will induce the Striga seeds to germinate but will not support attachment of the parasite. This method has been used in sorghum plantations by planting Celosia argentea between the sorghum ^[6]. Planting silverleaf (Desmodium uncinatum) has worked in maize crops.^[7] Coating maize seeds with fungi or a herbicide also appear to be promising approaches.

Species

The species in this genus:

• Striga aequinoctialis W. Africa.
• Striga angolensis Angola.
• Striga angustifolia E. Africa, Asia, Indonesia.
• Striga asiatica (Asiatic witchweed) Africa, Arabian peninsula, India, Burma, China, Indonesia, Philippines, Malaysia, New Guinea, Australia (introduced?), USA (introduced).
• Striga aspera Africa.
• Striga bilabiata Africa.
• Striga brachycalyx Africa.
• Striga chrysantha Central Africa.
• Striga dalzielii W. Africa.
• Striga elegans Angola, Malawi, South Africa, Zimbabwe.
• Striga forbesii Africa, Madagascar.
• Striga gastonii Chad and Central African Republic.
• Striga gesnerioides (cowpea witchweed) Africa, Arabian peninsula, India, USA (introduced).
• Striga gracillima Tanzania.
• Striga hallaei Gabon, Democratic Rebublic of Congo.
• Striga hermonthica (purple witchweed) Senegal to Ethiopia, Democratic Republic of Congo and Tanzania, Angola, Namibia.
• Striga hirsuta Madagascar.
• Striga junodii SouthAfrica, Mozambique.
• Striga klingii W. Africa, Nigeria, Ghana, Cameroon, Togo.
• Striga latericea E. Africa, Ethiopia, Somalia.
• Striga lepidagathidis Senegal, Guinea, Guinea Bissau.
• Striga lutea Sudan, Ethiopia.
• Striga macrantha W. Africa, Nigeria, Ivory Coast, Togo.
• Striga passargei W. & C. Africa, Arabian peninsula (?).
• Striga pinnatifida Ethiopia.
• Striga primuloides Ivory Coast, Nigeria.
• Striga pubiflora Somalia.
• Striga yemenica Ethiopia.

References

1. ^ Kamal I. Mohamed, Lytton John Musselman, Charles R. Riches. The Genus Striga (Scrophulariaceae) in Africa. Annals of the Missouri Botanical Garden, Vol. 88, No. 1 (2001) [1]
2. ^ Nelson D. Young, Kim E. Steiner, Claude W. dePamphilis (Autumn, 1999). "The Evolution of Parasitism in Scrophulariaceae/Orobanchaceae: Plastid Gene Sequences Refute an Evolutionary Transition Series". Annals of the Missouri Botanical Garden 86 (4): pp. 876-893. doi:10.2307/2666173.
3. ^ For example, Integrated Taxonomic Information System as of 16 Sep 2007
4. ^ Nickrent, D. L. and Musselman, L. J. 2004. Introduction to Parasitic Flowering Plants. The Plant Health Instructor. DOI: 10.1094/PHI-I-2004-0330-01 [2]
5. ^ Mourik, T. (2007) Striga hermonthica seed bank dynamics process quantification and modelling. PhD thesis, Wageningen University. http://library.wur.nl/wda/dissertations/dis4240.pdf
6. ^ J.R. Olupot et al.: The effectiveness of Celosia argentia (Striga chaser) to control Striga on sorghum in Uganda. Crop Protection 22/-/2003. S. 463-8. Online-Version
7. ^ "Perfect maize, in three simple steps", The Guardian, September 18, 2003.