Striga (plant) - AbsoluteAstronomy.com

Striga, commonly known as witches weed, is a genus

Genus

In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

of 28 species of parasitic plant

Parasitic plant

A parasitic plant is one that derives some or all of its sustenance from another plant. About 4,100 species in approximately 19 families of flowering plants are known. Parasitic plants have a modified root, the haustorium, that penetrates the host plant and connects to the xylem, phloem, or...

s 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 parasite

Obligate parasite

An obligate parasite is a parasitic organism that cannot complete its life cycle without dependence on its host.-See also:*Obligate intracellular parasite*Parasitism*Parasitic plant*Facultative parasite...

s of roots and therefore require a living host for survival. The genus is classified in the family Orobanchaceae

Orobanchaceae

Orobanchaceae, the broomrape family, is a family of flowering plants of the order Lamiales, with about 90 genera and more than 2000 species. Many of these genera were formerly included in the family Scrophulariaceae sensu lato...

, although older classifications place it in the Scrophulariaceae

Scrophulariaceae

Scrophulariaceae, the figwort family, are a family of flowering plants. The plants are annual or perennial herbs with flowers with bilateral or rarely radial symmetry. Members of the Scrophulariaceae have a cosmopolitan distribution, with the majority found in temperate areas, including...

. 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

Wilting

Wilting refers to the loss of rigidity of non-woody parts of plants. This occurs when the turgor pressure in non-lignified plant cells falls towards zero, as a result of diminished water in the cells...

, and chlorosis are similar to those seen from severe drought damage, nutrient deficiency, and vascular disease.

Disease Cycle

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

Exudate

An exudate is any fluid that filters from the circulatory system into lesions or areas of inflammation. It can apply to plants as well as animals. Its composition varies but generally includes water and the dissolved solutes of the main circulatory fluid such as sap or blood...

and develop haustoria which penetrate host root cells. Host root exudate contain strigolactone

Strigolactone

Strigolactones are plant hormones that have been implicated in inhibition of shoot branching. Strigolactones are carotenoid-derived and trigger germination of parasitic plant seeds and stimulate symbiotic mycorrhizal fungi...

s, 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.

Environment

Temperatures ranging from 30 to 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

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

Ethylene

Ethylene is a gaseous organic compound with the formula . It is the simplest alkene . Because it contains a carbon-carbon double bond, ethylene is classified as an unsaturated hydrocarbon. Ethylene is widely used in industry and is also a plant hormone...

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 crop

Trap crop

A trap crop is a plant that attracts agricultural pests, usually insects, away from nearby crops. This form of companion planting can save the main crop from decimation by pests without the potential issues and controversy involved in using pesticides....

ping 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
Celosia argentea
Celosia argentea, commonly known as plumed cockscomb, is a plant of tropical origin, and is known for its very bright colors.-Description:...

between the sorghum. Planting silverleaf (Desmodium
Desmodium
Desmodium is a genus in the flowering plant family Fabaceae, sometimes called tick-trefoil, tick clover or beggar lice. There are dozens of species and the delimitation of the genus has shifted much over time....

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.

Importance

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

Aphis

Aphis may refer to:* Aphis, a genus of aphid species* Animal and Plant Health Inspection Service , organizational unit of the USDA* HMS Aphis , Royal Navy insect class gunboat...

) 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

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

Maize

Maize known in many English-speaking countries as corn or mielie/mealie, is a grain domesticated by indigenous peoples in Mesoamerica in prehistoric times. The leafy stalk produces ears which contain seeds called kernels. Though technically a grain, maize kernels are used in cooking as a vegetable...

, 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
Striga hermonthica
Striga hermonthica, commonly known as purple witchweed, is a parasitic plant belonging to the genus Striga. It is devastating to "major crops such as sorghum and rice . In subsaharan Africa in addition to S. bicolor, and O...

(purple witchweed) is also a parasite that affects grass

Grass

Grasses, or more technically graminoids, are monocotyledonous, usually herbaceous plants with narrow leaves growing from the base. They include the "true grasses", of the Poaceae family, as well as the sedges and the rushes . The true grasses include cereals, bamboo and the grasses of lawns ...

es, particularly sorghum

Sorghum

Sorghum is a genus of numerous species of grasses, one of which is raised for grain and many of which are used as fodder plants either cultivated or as part of pasture. The plants are cultivated in warmer climates worldwide. Species are native to tropical and subtropical regions of all continents...

and pearl millet

Pearl millet

Pearl millet is the most widely grown type of millet. Grown in Africa and the Indian subcontinent since prehistoric times, it is generally accepted that pearl millet originated in Africa and was subsequently introduced into India. The center of diversity, and suggested area of domestication, for...

in subsaharan Africa.

Cowpea witchweed: Cowpea witchweed, as its name implies, is a parasite of cowpea

Cowpea

The Cowpea is one of several species of the widely cultivated genus Vigna. Four cultivated subspecies are recognised:*Vigna unguiculata subsp. cylindrica Catjang...

(Vigna unguiculata), which is not a grass but a member of the legume family (Fabaceae

Fabaceae

The Fabaceae or Leguminosae, commonly known as the legume, pea, or bean family, is a large and economically important family of flowering plants. The group is the third largest land plant family, behind only the Orchidaceae and Asteraceae, with 730 genera and over 19,400 species...

or Leguminosae). This species was also accidentally introduced into Florida in the United States where it was found parasitizing Indigofera
Indigofera
Indigofera is a large genus of about 700 species of flowering plants belonging to the family Fabaceae.The species are mostly shrubs, though some are herbaceous, and a few can become small trees up to tall. Most are dry-season or winter deciduous. The leaves are pinnate with 5–31 leaflets and the...

hirsuta (hairy indigo, another legume).

Species

The species in this genus:

Striga aequinoctialis W. Africa.
Striga angolensis Angola.
Striga angustifolia E. Africa, Asia, Indonesia.
Striga asiatica
Striga asiatica
Asiatic witchweed is a hemiparasitic plant in the Scrophulariaceae family. It is native to Asia and sub-Saharan Africa, but has been introduced into other parts of the world including Australia and the United States...

(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
Striga hermonthica
Striga hermonthica, commonly known as purple witchweed, is a parasitic plant belonging to the genus Striga. It is devastating to "major crops such as sorghum and rice . In subsaharan Africa in addition to S. bicolor, and O...

(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.

External links

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