Species: S. angustifolia - S. asiatica - S. densiflora - S. forbesii - S. gesnerioides - S. hermonthica - S. masuria
Striga, commonly known as witches weed, is a genus of 28 species of parasitic plants that occur naturally in parts of Africa, Asia, and Australia. Witchweeds are characterized by bright-green stems and leaves and small, brightly colored flowers. Furthermore, they are obligate parasites of roots and therefore require a living host for survival. The genus is classified in the family Orobanchaceae, although older classifications place it in the Scrophulariaceae. 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. Crops most commonly affected are corn, sorghum, and sugarcane. Three species cause the most damage: Striga asiatica, S. gesnerioides, and S. hermonthica.
Hosts and Symptoms
Witchweed parasitizes corn, millet, sorghum, sugarcane, rice, legumes, and a range of weedy grasses. It is capable of significantly reducing yields, in some cases wiping out the entire crop.
Host plant symptoms such as stunting, wilting, and chlorosis are similar to those seen from severe drought damage, nutrient deficiency, and vascular disease.
Photo of plant roots with connected Striga plant
Each plant is capable of producing up to 500,000 seeds, which may remain viable in the soil for over 10 years. An annual plant, witchweed overwinters in the seed stage. Its seeds germinate in the presence of host root exudate and develop haustoria which penetrate host root cells. Host root exudate contain strigolactones, signaling molecules that promote Striga seed germination. A bell-like swell forms where the parasitic roots attach to the roots of the host. The pathogen colonizes underground where it may spend the next 4-7 weeks before emergence, when it rapidly flowers and produces seeds. Witchweed seeds spread easily by wind, water, and soil via animal vectors. The chief means of dispersal, however, is through human interaction, by means of machinery, tools, and clothing.
Temperatures ranging from 30-35°C in a moist environment are ideal for germination. Witchweed will not develop in temperatures below 20°C. Agricultural land with light soil and low Nitrogen levels tend to favor the development of witchweed. Still, witchweed has demonstrated a wide tolerance for soil type if soil temperatures are favorably high. Seeds have been shown to survive in frozen soil of temperatures as low as -15°C, attesting to their aptitude as overwintering structures.
Soil temperature, air temperature, photoperiod, soil type, and soil nutrient and moisture levels do not greatly deter the development of witchweed. These findings suggest that witchweed, though it has been limited to the Carolinas in the United States, that the pathogen would successfully infect the massive corn crops of the midwest.
Management of witchweed is difficult because the majority of its life cycle takes place below ground. If it is not detected before emergence, it is too late to reduce crop loss. To prevent witchweed from spreading, growers should plant uncontaminated seeds and clean soil and plant debris off of machinery, shoes, clothing, and tools before entering fields. If there is little witchweed, hand weeding before seeds are produced is an option. If there is a lot then a more intense management plan will need to be used.
Striga in the United States has been 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. 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 . Planting silverleaf (Desmodium uncinatum) has worked in maize crops. Cotton, sunflower, linseed, and witchweed resistant maize are also effective trap crops.
Increasing Nitrogen levels in the soil, growing tolerant varieties,and trap-cropping, planting crops that are susceptible but are harvested before witchweed seed is produced, can also be used. Coating maize seeds with fungi or a herbicide also appear to be promising approaches.
There are several sorghum varieties that have high levels of resistance in local conditions, including ‘N-13‘, ‘Framida‘, and ‘Serena‘. 'Buruma', 'Shibe', 'Okoa' and 'Serere 17' are millet cultivars that are considered to be resistant in Tanzania. Some corn varieties show partial resistance to witchweed, including ‘Katumani’ in Kenya. Some crop cultivars, especially corn, are resistant to herbicides that kill witchweed when the haustoria attaches to the crop. STRIGAWAY® maize has been shown to reduce the seed bank of Striga by 30% in two seasons.
Corn, sorghum, and sugarcane crops affected by witchweed in the United States have an estimated value well over $20 billion. Furthermore, witchweed is capable of wiping out an entire crop. In fact, it is so prolific that in 1957 Congress allocated money in an attempt to eradicate witchweed. Thus, the Animal and Plant Health Inspection Service (APHIS) of the U.S. Department of Agriculture established a research station and control methods. Through infestation mapping, quarantine, and control activities such as contaminated seed destruction, the acreage parasitized by witchweed has been reduced 99% since its discovery in the United States. APHIS has even offered cash rewards for those identify and report the weed, and encourages landowners to check their own acreage.
Parasitizing important economic plants, witchweed is one of the most destructive pathogens in Africa. In fact, Witchweed affects 40% of Africa’s arable savanna region, resulting in up to $13 billion lost every year . Striga affects 40 million hectares of crops in sub-Saharan Africa alone. The witchweed infestation is so bad in parts of Africa that some farmers have to relocate every few years. Furthermore, the majority of crops in Africa are grown by subsistence farmers who can’t afford expensive witchweed controls, who therefore suffer much as a result of this pathogen.
Common Witchweed Species
Photo of a Striga plant (Witchweed)
Striga asiatica: 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: Striga hermonthica (purple witchweed) is also a parasite that affects grasses, particularly sorghum and pearl millet in subsaharan Africa.
Cowpea witchweed: 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).
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.
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10. ^ a b c California Department of Food and Agriculture. Witchweed. http://www.cdfa.ca.gov/phpps/ipc/weedinfo/striga.htm
11. ^ Mourik, T. (2007) Striga hermonthica seed bank dynamics process quantification and modelling. PhD thesis, Wageningen University. http://library.wur.nl/wda/dissertations/dis4240.pdf
12. ^ 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
13. ^ Radford, Tim (September 18, 2003). "Perfect maize, in three simple steps". The Guardian (London). http://education.guardian.co.uk/higher/research/story/0,,1044085,00.html. Retrieved May 24, 2010.
14. ^ a b c d "Purple Witchweed." Infonet-biovision. N.p., 14 Sep 2009. Web. 7 Dec 2010. <http://www.infonet-biovision.org/default/ct/112/pests>.
15. ^ a b c "Controlling Witchweed in Sub-saharan Africa." Web. 7 Dec 2010. <http://www.aatf-africa.org/UserFiles/File/Controlling%20witchweed%20in%20SSA_AATF_Annual-Report_2005.pdf>
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* Hoffmann Gérard., 1994. Contribution to the study of parasitic Phanerogams of Burkina Faso and Mali : some aspects of their ecology, biology and control technics. Doctorate thesis N° 941IX3 0083, University of Aix-Marseille 3 (France), 242 pages, 282 ref. INIST-CNRS, Cote INIST : T 95870
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* Khan, Z.R., Midega, C.A.O., Hassanali, A., Pickett, J.A., Wadhams, L.J., 2007. Assessment of Different Legumes for the Control of Striga hermonthica in Maize and Sorghum.Crop Science 47:730-734
* Moore T. H. M.; Lane J. A.; Child D. V.; Arnold G. M.; Bailey J. A.; Hoffmann Gérard, 1995, New sources of resistance of cowpea (Vigna unguiculata) to Striga gesnerioides, a parasitic angiospermEuphytica vol. 84, n°3 : 165-174