Striga

Although most species of Striga are not pathogens that affect agriculture, some species have devastating effects upon crops, particularly those planted by subsistence farmers. Crops most commonly affected are maize, sorghum, rice and sugarcane. Three species cause the most damage: Striga asiatica, S. gesnerioides, and S. hermonthica. Witchweed parasitizes maize, 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. ==Lifecycle==

Each plant is capable of producing between 90,000 Most seeds produced are not viable. An annual plant, witchweed overwinters in the seed stage. A bell-like swelling forms where the parasitic roots attach to the roots of the host. If the quinone product is at the appropriate concentrations, a haustorium will develop from the initial root. The haustorium grows toward the host root until it makes contact with the root surface, establishing parasitic contact in relatively short order. Within 12 hours of initial haustorium growth, the haustorium recognizes the host root and begins rapid cell division and elongation. The haustorium forms a wedge shape and uses mechanical force and chemical digestion to penetrate the host root, pushing the host cells out of the way. Within 48–72 hours, the haustorium has penetrated the host root cortex. This eight cell layer allows for nonspecific nutrient transport from the host to the Striga seedling. Within 24 hours after tapping the host xylem and phloem, the Striga cotyledons emerge from the seed. ==Environment==

Temperatures ranging from in a moist environment are ideal for germination. Still, witchweed has demonstrated a wide tolerance for soil types if soil temperatures are favorably high. Seeds have been shown to survive in frozen soil of temperatures as low as , 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, while limited to the Carolinas in the United States, seem to suggest that the pathogen could successfully infect the massive corn crops of the American Midwest. ==Management==

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 losses. Thus, such treatments do not remove all seeds from the soil. Moreover, this method is expensive and not generally available to 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 rows. Cotton, sunflower and linseed are also effective trap crops. Increasing nitrogen levels in the soil, growing Striga-tolerant varieties, trap-cropping, and planting susceptible crops harvested before witchweed seed is produced, are proven tactics. Several sorghum varieties have high levels of resistance in local conditions, including 'N-13', 'Framida', and 'Serena'. 'Buruma', 'Shibe', 'Okoa' and 'Serere 17' millet cultivars are considered to be resistant in Tanzania. 'StrigAway'™ herbicide-resistant, herbicide-impregnated maize has been shown to reduce the seed bank by 30% in two seasons. == Importance ==

Maize, sorghum, and sugarcane crops affected by witchweed in the United States have an estimated value well over $20 billion. The majority of crops in Africa are grown by subsistence farmers who cannot afford expensive witchweed controls, who therefore suffer much as a result of this pathogen. ==Species==

Common crop parasites '' '' • 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 on corn (maize) 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 gesnerioides, 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). • Striga hermonthica (purple witchweed) is also a parasite that affects grasses, particularly sorghum and pearl millet in sub-Saharan Africa (Senegal to Ethiopia, Democratic Republic of Congo and Tanzania, Angola, Namibia). Species list The following species are recognised in the genus Striga: • Striga aequinoctialis - West Africa • Striga alba • Striga angolensis - Angola • Striga angustifolia - East Africa, Asia, Indonesia • Striga asiatica - (Asiatic witchweed) Africa, Arabian peninsula, India, Burma, China, Indonesia, the Philippines, Malaysia, New Guinea, Australia (introduced?), USA (introduced) • Striga aspera - Africa • Striga barthlottii • Striga baumannii • Striga bilabiata - Africa • Striga brachycalyx - Africa • Striga chrysantha • Striga crispata • Striga curviflora • Striga dalzielii - West Africa • Striga densiflora • Striga dewevrei • Striga diversifolia • Striga elegans - Angola, Malawi, South Africa, Zimbabwe • Striga ellenbergeri • Striga flava • Striga forbesii - Africa, Madagascar • Striga fulgens • Striga gastonii • Striga gesnerioides - (cowpea witchweed) Africa, Arabian peninsula, India, USA (introduced) • Striga glumacea • Striga gracillima • Striga hallaei • Striga hermonthica - Senegal to Ethiopia, Democratic Republic of Congo and Tanzania, Angola, Namibia • Striga indica • Striga junodii - South Africa, Mozambique • Striga kamalii • Striga klingii - West Africa, Nigeria, Ghana, Cameroon, Togo • Striga latericea - East Africa, Ethiopia, Somalia • Striga lepidagathidis • Striga linearifolia • Striga lutea • Striga macrantha - West Africa, Nigeria, Ivory Coast, Togo • Striga magnibracteata • Striga masuria • Striga micrantha • Striga multiflora • Striga musselmanii • Striga parviflora • Striga passargei - West and Central Africa, Arabian peninsula • Striga pinnatifida • Striga primuloides - Ivory Coast, Nigeria • Striga pubiflora - Somalia • Striga schlechteri • Striga spanogheana • Striga squamigera • Striga strigosa • Striga sulphurea • Striga yemenica ==Gallery==

Image:Striga densiflora (Agya) in Hyderabad, AP W IMG 9828.jpg|Striga densiflora in Hyderabad, India Image:Striga densiflora (Agya) in Hyderabad, AP W IMG 9826.jpg|Striga densiflora in Hyderabad Image:Striga densiflora (Agya) in Hyderabad, AP W IMG 9825.jpg|Striga densiflora in Hyderabad ==See also==