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Striga
Scientific classification Edit this classification
Kingdom: Plantae
Clade: Tracheophytes
Clade: Angiosperms
Clade: Eudicots
Clade: Asterids
Order: Lamiales
Family: Orobanchaceae
Tribe: Buchnereae
Genus: Striga
Lour.
Synonyms
  • Campuleia Thouars
  • Microsyphus C.Presl
  • Psammostachys C.Presl
  • Camphyleia Spreng.

Striga, commonly known as witchweed,[1] is a genus of parasitic plants that occur naturally in parts of Africa, Asia, and Australia. It is currently classified in the family Orobanchaceae,[2] although older classifications place it in the Scrophulariaceae.[3] Some species are serious pathogens of cereal crops, with the greatest effects being in savanna agriculture in Africa. It also causes considerable crop losses in other regions, including other tropical and subtropical crops in its native range and in the Americas. The generic name derives from Latin strī̆ga, "witch".[4]

Witchweeds are characterized by bright-green stems and leaves and small, brightly colored and attractive flowers.[5] They are obligate hemiparasites of roots and require a living host for germination and initial development, though they can then survive on their own.[6]

The number of species is uncertain, but may exceed 40 by some counts.[6][7]

Hosts and symptoms

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Although most species of Striga are not pathogens that affect agriculture, some species have devastating effects upon crops, particularly those planted by subsistence farmers.[8] Crops most commonly affected are maize, sorghum, rice and sugarcane.[5][9] 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.[10] It is capable of significantly reducing yields, in some cases wiping out the entire crop.[5]

Host plant symptoms, such as stunting, wilting, and chlorosis, are similar to those seen from severe drought damage, nutrient deficiency, and vascular disease.[5][10][11]

Lifecycle

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Plant roots with connected Striga plant

Each plant is capable of producing between 90,000[12] and 500,000 seeds, which may remain viable in the soil for over 10 years.[13] Most seeds produced are not viable.[12] An annual plant, witchweed overwinters in the seed stage.[5] Its seeds germinate in the presence of host root exudate, and develop haustoria which penetrate host root cells.[5] Host root exudate contain strigolactones, signaling molecules that promote Striga seed germination.[14] A bell-like swelling forms where the parasitic roots attach to the roots of the host.[10] The pathogen develops underground, where it may spend the next four to seven weeks before emergence, when it rapidly flowers and produces seeds.[10] Witchweed seeds spread readily via wind and water, and in soil via animal vectors.[10] The chief means of dispersal, however, is through human activity, by means of machinery, tools, and clothing.[10][13]

Haustoria development

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Once germination is stimulated, the Striga seed sends out an initial root to probe the soil for the host root. The initial root secretes an oxidizing enzyme that digests the host root surface, releasing quinones.[15] 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.[16] 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.[16][17] Within 48–72 hours, the haustorium has penetrated the host root cortex.[16] Finger-like structures on the haustorium, called oscula (from Latin osculum, "little mouth") penetrate the host xylem through pits in the membrane.[17] The oscula then swell to secure their position within the xylem membrane. Striga sieve tubes develop along with the oscula. Shortly after the host xylem is penetrated, Striga sieve tubes develop and approach the host phloem within eight cells.[18][19] This eight cell layer allows for nonspecific nutrient transport from the host to the Striga seedling.[18][19] Within 24 hours after tapping the host xylem and phloem, the Striga cotyledons emerge from the seed.[16]

Environment

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Temperatures ranging from 30 to 35 °C (86 to 95 °F) in a moist environment are ideal for germination.[10] Witchweed will not develop in temperatures below 20 °C (68 °F). Agricultural soils with a light texture and low nitrogen levels tend to favor development.[20] Still, witchweed has demonstrated a wide tolerance for soil types if soil temperatures are favorably high.[5] Seeds have been shown to survive in frozen soil of temperatures as low as −15 °C (5 °F), attesting to their aptitude as overwintering structures.[5]

Soil temperature, air temperature, photoperiod, soil type, and soil nutrient and moisture levels do not greatly deter the development of witchweed.[5] 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.[5]

Management

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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.[10] To prevent witchweed from spreading it is necessary to plant uncontaminated seeds and to clean soil and plant debris off of machinery, shoes, clothing, and tools before entering fields.[10][20] If populations are low, hand weeding before seeds are produced is an option.

Striga in the United States has been controlled through the use of several management strategies, including quarantines imposed on affected areas, control of movement of farm equipment between infected and uninfected 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. However, each mature Striga plant can produce tens of thousands of tiny seeds, which may remain dormant in the soil for many years.[21] 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.[22] Cotton, sunflower and linseed are also effective trap crops.[10] Planting silverleaf desmodium (Desmodium uncinatum), as is done in push-pull intercropping, inhibits Striga seed germination and has worked effectively intercropped with maize.[23][24]

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.[20] Coating maize seeds with fungi or a herbicide also appears to be a promising approach. An example is TAN222, the "Striga-resistant" maize variety which is coated with the systemic herbicide imazapyr, to which it is resistant. Any witchweed seeds sprouting when this maize is in the seedling stage are poisoned when their haustoria embed in the seedling's roots.[25][26]

Several sorghum varieties have high levels of resistance in local conditions, including 'N-13', 'Framida', and 'Serena'.[27][28] 'Buruma', 'Shibe', 'Okoa' and 'Serere 17' millet cultivars are considered to be resistant in Tanzania.[28] Some maize varieties show partial resistance to witchweed, including 'Katumani' in Kenya.[28] In a number of rice cultivars, including some cultivars of NERICA (New Rice for Africa), effective pre- and post- attachment resistance mechanisms have been identified.[29][30][26] 'StrigAway'™ herbicide-resistant, herbicide-impregnated maize has been shown to reduce the seed bank by 30% in two seasons.[26]

Importance

[edit]

Maize, sorghum, and sugarcane crops affected by witchweed in the United States have an estimated value well over $20 billion.[5] Furthermore, witchweed is capable of wiping out an entire crop.[10] It is so prolific that in 1957 the US Congress allocated money in an attempt to eradicate witchweed. Because of Striga, the Animal and Plant Health Inspection Service (APHIS) of the U.S. Department of Agriculture established a research station and control methods.[13] Through infestation mapping, quarantine, and control activities such as contaminated seed destruction, the acreage parasitized by witchweed has been reduced by 99% since its discovery in the United States.[13] APHIS has even offered cash rewards those who identify and report the weed, and encourages landowners to check their own acreage.[13]

Parasitizing important economic plants, witchweed is one of the most destructive pathogens in Africa.[11] Witchweed affects 40% of Africa's arable savanna region, resulting in up to $13 billion lost every year.[28] Striga affects 40 million hectares (98,842,153 acres) of crops in sub-Saharan Africa alone.[26] In parts of Africa, the witchweed infestation is so severe that some farmers must relocate every few years.[31] 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.[31]

Species

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Common crop parasites

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Striga asiatica
Striga bilabiata
  • 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

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The following species are recognised in the genus Striga:[32]


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See also

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References

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Further reading

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[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Striga

View on Grokipedia
from Grokipedia
Striga is a of approximately 40 species of root hemiparasitic in the family , commonly known as witchweeds, that primarily parasitize grasses () such as cereals and cause severe crop losses in . These annual spend much of their life cycle underground, with tiny seeds that can remain dormant in the soil for up to 10 years, germinating only in response to chemical signals like strigolactones exuded by host . Upon germination, the parasite forms a specialized to penetrate the host's root vascular system, extracting water and nutrients via the ; Striga species have also acquired host genes through while retaining some photosynthetic capability above ground. The genus is predominantly distributed across , where about 80% of species occur, with additional presence in parts of and , originating from regions like Ethiopia's Semien Mountains and Sudan's Nubian Hills. Notable species include S. hermonthica and S. asiatica, which infest crops like , , millet, , and , leading to 20–100% yield reductions and affecting over 300 million people; S. gesnerioides uniquely targets dicotyledonous hosts such as . A single Striga can produce 50,000 to 500,000 seeds, exacerbating infestations and making control challenging through methods like hand weeding, , or breeding resistant varieties. Economically, Striga inflicts annual losses exceeding $1 billion USD, particularly on smallholder farmers in over 40 African countries, contributing to food insecurity and . Research focuses on disrupting signals, enhancing host resistance, and integrated management strategies to mitigate its impact, as the parasite's evolutionary adaptations, including host gene theft, complicate eradication efforts.

Taxonomy and Description

Genus Overview

Striga is a of flowering in the family , within the order . Prior to molecular phylogenetic revisions in the 1990s and 2000s, the genus was classified under the family , but subsequent analyses confirmed its placement in as a monophyletic group. The comprises approximately 40 of parasites, with the majority native to tropical and subtropical regions of , , and . These evolved from non-parasitic ancestors in the lineage, with the origin of the family dated to 50–44 million years ago and the emergence of around 49–41 million years ago based on and fossil-calibrated phylogenetic evidence. Striga species are typically annual or perennial herbs growing 15–80 cm tall, featuring erect stems with opposite or alternate narrow, linear leaves that are often sessile and pubescent. Their inflorescences form terminal spikes of small, tubular flowers in shades of pink, purple, or white, adapted for by . Each produces numerous tiny measuring 200–300 μm in length, which can remain viable in the soil for 10–15 years, facilitating long-term persistence in host environments. As hemiparasites, Striga species retain for partial autotrophy but rely heavily on host plants for water and nutrients via connections formed by specialized haustoria. A key trait is the production and response to strigolactones, plant hormones that serve as signaling molecules to trigger germination in the presence of suitable host .

Morphological Characteristics

Striga plants exhibit a distinctive morphology adapted to their hemiparasitic , featuring erect stems that emerge from the after underground attachment to host roots. The stems are typically quadrangular and often ridged, measuring up to 100 cm in height depending on the species, such as S. hermonthica, which can reach this maximum, while S. asiatica is shorter at 10–30 cm. They are usually glabrous to slightly pubescent, with colors ranging from bright green to purplish, and may be branched or unbranched, supporting the plant's upright growth and . Leaves are or alternate, sessile or subsessile, and reduced in to linear-lanceolate scales, typically 1–5 cm long and 1–4 mm wide, with entire margins and minimal pubescence. In hemiparasitic species like S. hermonthica, content is present but reduced, often resulting in a yellowish-green coloration that reflects limited independent . These leaves lack a well-developed layer and contain few chloroplasts per cell, adaptations that underscore the plant's reliance on host-derived nutrients. The forms a spike or lax , bearing sessile or subsessile flowers with a tubular, bilabiate corolla 5–15 mm long, featuring vivid colors such as pink or light purple in S. hermonthica, red in S. asiatica, or purple in S. gesnerioides to attract pollinators. Fruits develop as loculicidal capsules, 7–20 mm long, each containing 100–500 dust-like seeds. These exarillate seeds measure 0.2–0.3 mm long with a reticulate coat featuring spiraling ridges, and they exhibit mechanisms requiring 3–6 months of after-ripening post-dispersal to break primary before conditioning can induce . Anatomically, mature Striga plants have a reduced , with few primary and adventitious ones capable of forming secondary attachments, emphasizing their parasitic dependence. is facilitated by specialized connections established via haustoria, which penetrate host to form direct vascular bridges for and uptake, lacking connections. Early seedlings may form mycorrhizal associations with arbuscular fungi for initial acquisition in prior to host attachment, though this is transient.

Biology and Parasitism

Lifecycle Stages

Striga seeds exhibit an extended period of , remaining viable in the for up to 10–20 years, which allows the parasite to persist in the absence of suitable hosts. Initial after-ripening for several months is required to break primary , preventing untimely . Conditioning follows, where seeds must experience moist and temperature fluctuations between 25–35°C for 1–2 weeks to activate responsiveness to germination stimulants; this phase synchronizes the parasite's cycle with the onset of the rainy season in tropical environments. Without conditioning, seeds enter secondary , further enhancing their . Germination is triggered specifically by root exudates from host plants, such as strigolactones (e.g., sorgolactone released by ), at concentrations as low as 10⁻¹⁶ M. Optimal temperatures for this process range from 20–40°C, with the emerging within 24 hours and extending 1–3 mm. The germinated has a limited window of 3–7 days to locate and attach to a host before it desiccates and dies, ensuring the parasite's dependence on the host for survival. Upon host root contact, the tip differentiates into a , a specialized attachment organ that penetrates the host's cortex within 48–72 hours using enzymatic degradation and mechanical force to establish connections for nutrient and water uptake. Initially, the parasite relies on its own for autotrophic growth while forming the underground. This penetration phase marks the transition to obligate , with the enabling resource withdrawal from the host. Following penetration, Striga undergoes vegetative growth for 4–8 weeks underground, developing shoots and adventitious roots before emerging above ground. Flowering occurs 6–12 weeks after attachment, typically 4 weeks post-emergence, leading to seed set within 2–3 months of initial contact. A single mature plant can produce 50,000–500,000 tiny seeds, dispersed via wind and rain to replenish the . In the final senescence phase, the parasite detaches from the host after , completing its lifecycle in 3–4 months under tropical conditions. Host damage accumulates throughout the attachment but intensifies during the parasite's flowering stage, when resource demands are highest, often resulting in severe stunting and yield losses.

Haustorium Formation

Haustorium formation in Striga species begins with the induction phase, where host-derived haustorium-inducing factors (HIFs), such as like 2-hydroxybenzoic acid and quinones like 2,6-dimethoxybenzoquinone (DMBQ), are perceived by the of the parasitic . These HIFs trigger signaling cascades involving (ROS) generation and redistribution, activating downstream within hours of exposure. For instance, in S. hermonthica, phenolic signals lead to the upregulation of PLETHORA transcription factors and PIN-FORMED transporters, arresting and initiating prehaustorium differentiation. During , the tip undergoes rapid swelling and localized in the , cortex, , and pericycle, transforming into a multicellular prehaustorium over 2–4 days. This process involves the formation of intrusive cells at the apex, which secrete cell wall-loosening enzymes, including xyloglucan endotransglucosylases/hydrolases (XETs) and endoglucanases, to facilitate tissue remodeling in the parasite and prepare for host invasion. The developing differentiates into functional regions, including a search zone for host contact, a neck for penetration, and an attachment zone for vascular connection, enabling directed growth toward the host root surface. Penetration follows, with haustorial intrusive cells exhibiting directed polar growth and enzymatic degradation of the host root's cortical cell walls, allowing into the cortex and establishment of contact with the host . This results in the formation of xylem-xylem bridges that connect the parasite's vascular system to the host's, facilitating the unidirectional transfer of and minerals without direct connections. The haustorium's invasive cells avoid mechanical rupture by navigating intercellular spaces and loosening host cell walls via targeted hydrolases. Once established, the serves as the primary conduit for resource acquisition, supplying the majority of the parasite's water and mineral needs from the host through these bridges. Genetic studies since 2010 have identified orthologs in Striga to genes involved in root invasion and development, such as those regulating formation and signaling, highlighting evolutionary co-option for . Variations in haustorium morphology occur across , with hemiparasitic Striga species forming smaller, lateral adapted for root attachment and partial autotrophy, in contrast to the larger, terminal of holoparasitic relatives like that support full host dependency. Post-2020 research has advanced understanding of signaling pathways regulating haustorium development in parasitic plants.

Hosts and Impacts

Primary Hosts

Striga species primarily parasitize gramineous crops, with S. hermonthica and S. asiatica targeting key cereals such as sorghum (Sorghum bicolor), (Zea mays), (Pennisetum glaucum), and (Oryza sativa), as well as (Saccharum officinarum). These parasites attach to the roots of these hosts, deriving water and nutrients while causing significant yield reductions in subsistence farming systems across . Wild grasses, including species like Andropogon spp., also serve as common hosts, maintaining Striga populations in natural ecosystems. In addition, S. gesnerioides primarily targets dicotyledonous crops such as cowpea (Vigna unguiculata) and other legumes, leading to substantial losses in pulse production in Africa. Host specificity in Striga is largely determined by the exudation of strigolactones from host roots, which stimulate seed germination; gramineous hosts like cereals release higher levels of these compounds, making them particularly susceptible to S. hermonthica, which preferentially infects Poaceae family members. In contrast, S. gesnerioides targets non-gramineous dicotyledonous hosts such as cowpea (Vigna unguiculata). Genomic analyses of Striga receptors, such as the HTL/KAI2 paralogs, reveal that variations in ligand-binding pockets enable precise matching to strigolactone profiles from different hosts, confirming the molecular basis of this specificity. Non-crop hosts, particularly wild African savanna grasses, act as reservoirs for Striga seeds, sustaining populations between cropping seasons and facilitating persistence in uncultivated areas. Bridge hosts like Setaria spp. play a critical role in transmission, allowing Striga to move from wild habitats into agricultural fields by serving as intermediate vectors for seed dispersal and germination. Historically, pre-1900 records document Striga associations with native African flora, including indigenous grasses, prior to the intensification of colonial agriculture, which expanded cultivation of susceptible staple crops and amplified infestations. Recent genomic studies from the have further elucidated host range mechanisms through receptor-ligand interactions, while climate models indicate potential expansions into new hosts under changing conditions; for instance, S. hermonthica is increasingly reported on (Eragrostis tef) in , driven by warmer temperatures and shifting rainfall patterns that favor parasite establishment on this emerging cereal crop.

Symptoms and Damage

Striga infestation manifests early in the host plant's development, typically within 2-4 weeks of haustorial attachment to the roots, through subtle yet progressive symptoms such as , in lower leaves, and reduced tillering. These initial signs arise from the parasite's diversion of water, nutrients, and photosynthates from the host, often resulting in 20-50% yield reductions even at low parasite densities. As the infection advances, visible indicators become more pronounced, including the emergence of Striga shoots above the surface—often several per host plant—coinciding with host and overall drought-like stress. The haustorial invasion disrupts host vasculature, leading to internal discoloration and impaired nutrient transport, which exacerbates the host's decline. Physiologically, Striga infection significantly impairs host function, with reductions in photosynthetic rates of up to 46% in and 31% in due to decreased and content. This is compounded by altered hormone balance, including elevated (ABA) levels in the host's sap, which further limits water uptake and promotes . Weakened hosts become susceptible to secondary by pathogens and pests, amplifying damage through compounded stress. Yield impacts vary by parasite density, host variety, and infection timing, with high densities exceeding 10 plants per square meter causing up to 100% loss. In , yields typically decline by 30-90% under infestation, while experiences 20-80% reductions, reflecting the parasite's severe resource drain. Over multiple seasons, persistent Striga populations build up a long-lived , with seeds remaining viable for over a decade, intensifying infestations and perpetuating yield losses in subsequent crops.

Distribution and Ecology

Geographic Distribution

Striga are primarily native to , with the core range encompassing savanna and semi-arid regions from through . The genus originates from regions such as Ethiopia's Semien Mountains and Sudan's Nubian Hills. S. hermonthica predominates in areas from to , , and , while S. asiatica extends across similar African zones into southern and eastern Asia and . S. gesnerioides is concentrated in the and countries like , , and . Infestations affect an estimated 26 million hectares of cropland across , contributing to annual grain yield losses valued at over $1 billion and impacting the livelihoods of approximately 100 million people dependent on subsistence farming. Hotspots include northern , where up to 85% of and fields are infested with S. hermonthica; Ethiopia's northern and eastern regions; and Sudan's zones, as documented in mapping efforts by organizations like ICRISAT and FAO. The parasite's spread is predominantly human-mediated through contaminated crop seeds, farm equipment, and trade, such as during 19th-century colonial agricultural exchanges; natural dispersal via wind or water is limited to short distances under 1 km. Historically, Striga occurrences were localized before the , but intensified farming practices associated with post-1960s agricultural expansions led to broader dissemination across cereal-growing areas. S. asiatica was introduced to the in the via contaminated seeds in the , infesting up to 175,000 hectares before a multi-decade eradication program greatly reduced the infestation to about 1,000 hectares as of 2024, at a total cost of about $250 million. Projections indicate potential northward range shifts in due to warming temperatures.

Environmental Influences

Striga species thrive in soils with low fertility, particularly those deficient in and , where nutrient scarcity promotes and parasite establishment. These parasites are commonly associated with sandy or light-textured soils, though they can also infest heavier clays and vertisols under low-nutrient conditions. High levels of or in the inhibit by suppressing the production of strigolactones, the signaling molecules exuded by host that trigger Striga attachment. in the range of 5 to 8 supports viability and , with neutral to slightly alkaline conditions often correlating with higher infestation densities. stress further enhances , allowing dormant Striga seeds to persist in the for up to 10–20 years, depending on environmental conditions and species, exacerbating long-term infestation risks in arid environments. Climatically, Striga flourishes in warm temperatures between 25°C and 35°C, which are optimal for preconditioning, , and development. Annual rainfall of 500 to 1500 mm, typical of semi-arid s, facilitates the wet-dry cycles necessary for activation, while excessive above 600 mm may dilute stimulants and reduce banks. The genus is highly frost-sensitive, restricting its natural distribution to tropical and subtropical regions where freezing temperatures are absent. These climatic preferences align Striga with semi-arid savanna ecosystems, where seasonal droughts and moderate rains sustain its lifecycle without host dependence during . Biotic factors significantly modulate Striga prevalence beyond host interactions. Soil microbes, such as Fusarium oxysporum f. sp. strigae, degrade seed coats and reduce viability by up to 50-80% in infested fields, offering natural suppression in microbially diverse soils. Insect pollinators, including bees, facilitate cross-pollination and seed production in flowering Striga plants, enhancing reproductive success in open habitats. Competition from co-occurring weeds can alter Striga density; dense weed covers shade the soil, lowering temperatures below the 30-35°C threshold for emergence and thereby limiting parasite outbreaks. Recent models from the project expansions of Striga's suitable range by 2050, driven by rising temperatures and more frequent erratic rainfall patterns that extend heat-moisture coupling events into higher elevations and marginal areas. Elevated atmospheric CO2 levels may indirectly boost host susceptibility by altering root exudate profiles, potentially increasing release and parasite attachment rates. Agricultural practices profoundly influence Striga seed bank dynamics independent of direct host effects. Continuous of cereals builds up seed banks exponentially, as repeated host availability sustains high densities over seasons. In contrast, crop rotation with non-host trap crops like induces suicidal without attachment, reducing seed banks by approximately 70% after one to two cycles in rotation systems.

Economic Importance

Agricultural Effects

Striga infestation profoundly disrupts cereal crop production, particularly affecting staple grains like and in . Annual yield losses attributable to Striga are estimated at approximately 8.6 million tons of and millet grain across the region, with and bearing the brunt due to their susceptibility as primary hosts. In smallholder farms, where infestation levels are often high, yield reductions can range from 30% to 100%, severely limiting availability and farmer . These losses stem from the parasite's and theft, compounded by its production, which stunts host growth even at low densities. The economic toll of Striga on African is substantial, with annual costs estimated at $7-13 billion USD during the , primarily from foregone production and associated market impacts. These figures exacerbate food insecurity for over 300 million people reliant on affected crops. At the farm level, Striga forces smallholders to allocate substantially more labor time to manual weeding, diverting efforts from other productive activities and increasing drudgery, particularly for women. Infested fields often prompt shifts to less nutritious alternative crops, such as , while prolonged fallowing to break the parasite's accelerates soil degradation through and nutrient depletion. Regionally, Striga hotspots amplify these effects, with the Sahel zone in countries like and experiencing abandonment of sorghum cultivation due to chronic infestation covering up to 70% of cropland. In , including and , the parasite impacts approximately 1.4 million hectares of farmland, leading to widespread yield collapses and migration from agriculture. Indirect consequences include in infested fields, as intensification displaces native and , and heightened reliance on herbicides, which fosters weed resistance and environmental contamination.

Global Burden

Striga infestation poses a severe threat to across , where it primarily attacks staple crops such as , , millet, and , leading to widespread and . The parasite affects the livelihoods of over 300 million people, many of whom rely on smallholder farming for subsistence, perpetuating cycles of by forcing land abandonment and reducing household availability. By diminishing yields of nutrient-dense cereals that form the dietary backbone for millions, Striga exacerbates undernutrition, particularly in rural communities where alternative food sources are limited. The economic burden disproportionately impacts smallholder farmers, who constitute the majority of those affected and face substantial income losses from reduced harvests and increased labor demands. These farmers, often operating on marginal lands, experience net revenue declines ranging from 6% to 85% depending on severity and environmental factors, deepening in agrarian societies. Gender disparities compound this issue, as women, who typically manage weeding and post-harvest tasks in Striga-prone fields, bear a heavier and have less access to control technologies, further limiting household resilience. International policy efforts have targeted Striga through collaborative initiatives led by organizations like ICRISAT and , including farmer field schools established in the early to promote integrated control practices and build community awareness. Since 2010, substantial investments in agricultural —exceeding hundreds of millions from philanthropic and multilateral sources—have supported Striga-resistant varieties and biocontrol methods, aiming to bolster food systems in affected regions. Recent advancements as of 2025 include the identification of key genes (SbSLT1/2) in that confer resistance, reducing yield losses by 49-52% in field trials. These programs underscore the parasite's role in broader development challenges, linking reduced crop productivity to outcomes such as higher child stunting rates; for instance, areas with chronic yield losses show elevated prevalence, with general improvements correlating to a 13.6% reduction in stunting likelihood per 10% yield gain. Looking ahead, without scaled interventions, Striga's burden is projected to intensify due to and expanding cultivation on infested lands, potentially amplifying food insecurity for millions more by 2030. However, successes in integrated , such as push-pull systems and resistant hybrids, have demonstrated potential for substantial reductions in infestation, with some approaches achieving over 75% fewer emerged plants in field trials across , including in where adoption has improved local yields and farmer incomes.

Management Strategies

Cultural and Agronomic Methods

Crop rotation with non-host trap crops, such as , sunflower, or like and , induces suicidal of Striga seeds without allowing parasite attachment and growth, thereby depleting the . Field studies have shown that such rotations can reduce Striga seed banks by 30-50% after one to two seasons, with cumulative declines of up to 50-80% over 2-3 years when integrated into longer cycles. This approach not only suppresses Striga but also improves overall farm productivity by diversifying crops and breaking pest cycles. Intercropping cereals like or with such as or Desmodium species releases low levels of strigolactones, reducing Striga stimuli, while allelochemicals from these intercrops further inhibit parasite development. Field trials in western demonstrated 65-95% reductions in Striga emergence and counts when maize was intercropped with Desmodium uncinatum or Desmodium intortum, particularly when the legume was cut at 18 weeks after planting to optimize suppression. These systems also enhance soil nitrogen through legume fixation, supporting cereal yields without external inputs. Soil fertility management plays a key role in Striga suppression by altering parasite-host dynamics. Application of fertilizers at rates of 60-120 kg N/ha delays Striga by 2-4 weeks and reduces overall infestation levels, as higher nutrient availability strengthens host vigor and limits parasite . Organic amendments, such as or residues, promote microbial activity that accelerates Striga seed decay in the , further contributing to a 20-50% decline in viable seeds over time. Manual weeding through hand-pulling Striga plants before seed set is a direct cultural method effective at low densities, preventing and reducing future populations by up to 70% in subsequent seasons. However, it remains labor-intensive, typically allowing one person to cover only 0.1-0.5 ha per day depending on field conditions and weed density, making it impractical for large-scale or heavily infested areas. In the 2020s, push-pull systems have emerged as an integrated agronomic advance, combining Desmodium intercropping (the "push" component that repels Striga and pests via allelochemicals) with border plantings of Napier grass (the "pull" that attracts and traps pests). This approach has achieved 80-99% Striga control in fields while boosting yields by 1-2 t/ha, and as of 2025, it has been adopted by more than 350,000 smallholder farmers in , with ongoing efforts to expand to 1 million households by 2030 through addressing social-psychological adoption factors. These systems benefit farmers through improved and multiple harvests.

Chemical and Biological Controls

Chemical controls for Striga primarily involve applications targeted at the parasite's early attachment stages, minimizing damage to host crops like and . Imazapyr seed coating on imidazolinone-resistant (IR) varieties is a widely adopted method, where low doses of the (typically less than 30 g per ) are applied directly to seeds before planting. This approach allows the systemic to translocate to Striga haustoria upon attachment, effectively killing the parasite while the resistant host remains unharmed. Field trials across have demonstrated efficacy in reducing Striga emergence and increasing yields by 1.0 to 3.0 tons per in infested fields. However, concerns over potential Striga resistance to have emerged since the mid-2010s, prompting recommendations to integrate this method with other strategies for durability. Systemic herbicide applications, such as or delivered via the host plant's whorl, provide another targeted option by exploiting the parasite's haustorial connections for uptake. These low-dose regimes (e.g., approximately 100 ml per of formulated product) enable translocation to Striga attachments without severely impacting the host, particularly when applied post-. Studies indicate that such applications can suppress Striga emergence by delaying attachment and reducing parasite , though varies with timing and environmental conditions. Biological controls leverage microbial agents to target Striga seeds in the , with f. sp. strigae strain Foxy 2 being a prominent example developed for mycoherbicide use. This is typically applied as a drench or granular formulation at planting, infecting and reducing Striga seed viability through . Trials since the 2000s have shown variable efficacy, with some combined applications with resistant hosts achieving substantial reductions in Striga emergence, though standalone use in East African fields has often been limited. The strain's safety for non-target fungi, including arbuscular mycorrhizal taxa, supports its integration into sustainable systems. Natural products derived from allelopathic plants offer eco-friendly alternatives, with aqueous extracts from species like and inhibiting Striga seed by disrupting biochemical signals. Eucalyptus leaf extracts, rich in compounds such as 1,8-cineole, can reduce germination rates by 80-90% in lab assays, while Crotalaria extracts show similar suppressive effects when applied as soil amendments. When integrated with , these extracts achieve up to 80% overall Striga control in field settings by depleting seed banks over seasons. Challenges in these controls include variable field efficacy due to soil and climate factors, as well as the need for precise application to avoid host . Recent studies highlight progress in integrated herbicide-biocontrol approaches for Striga management in and . While no specific EPA approvals for Striga-targeted bioherbicides were noted in , ongoing regulatory progress for microbial agents underscores their growing viability.

Host Resistance Breeding

Host resistance breeding for Striga focuses on developing crop varieties, particularly in cereals like and , that minimize parasite attachment and damage through genetic modifications targeting stimulants, haustorial formation, and overall tolerance. Conventional approaches, initiated in the , emphasize selection and of landraces and wild relatives with low production to reduce Striga . For instance, in , breeders have targeted the LOW GERMINATION STIMULANT 1 (LGS1) locus, where s alter profiles by decreasing potent stimulants like 5-deoxystrigol and increasing inactive forms like orobanchol, effectively eliminating signals without yield penalties. The SRN39 variety exemplifies this, showing near-complete suppression of Striga emergence in field assays due to its LGS1 , with reductions in parasite attachment exceeding 90% compared to susceptible lines. efforts since the have incorporated such traits into elite lines, achieving progressive gains like 10% fewer emerged plants per selection cycle in programs. Quantitative trait locus (QTL) mapping has advanced these efforts by pinpointing genomic regions associated with resistance. In , the LGS1 gene on chromosome 5 regulates sulfation, conferring low germination stimulant activity and protecting yields by limiting initial parasite infection. For , QTL studies have identified multiple loci, including two on linked to incompatible host responses that reduce Striga attachment, explaining up to 20% of phenotypic variance in resistance traits. Genome-wide association studies (GWAS) in have further detected 24 SNPs across chromosomes 1–10 associated with reduced Striga damage and higher grain yield under infestation, with hybrids incorporating these QTLs providing 10–20% in Striga-infested fields. These mappings enable precise of resistance alleles, enhancing hybrid performance without compromising agronomic traits. Transgenic methods target strigolactone biosynthesis pathways for more robust resistance. (RNAi) silencing of genes like VP14, a 9-cis-epoxycarotenoid dioxygenase in the pathway, reduces strigolactone exudation from maize roots, achieving 40–80% lower Striga in bioassays compared to wild types. Similar approaches have silenced orthologs of MAX1, a enzyme downstream in the pathway, further disrupting stimulant production; in , five MAX1 homologs co-expressed with CCD7/CCD8 genes have been targeted to mimic these effects. Field trials of GM maize in since 2010, using root-specific promoters for RNAi constructs, demonstrated up to 80% resistance to Striga hermonthica in Kenyan sites, though faces regulatory hurdles including political opposition and high costs. Marker-assisted selection (MAS) leverages SNPs identified via GWAS to accelerate breeding for haustorial inhibition and low-stimulant traits. In maize, SNPs near ZmCCD1 on chromosome 9 reduce strigolactone levels, limiting haustorium initiation, while markers on chromosome 10 near amt5 enhance nitrogen-based defenses against attachment. For rice, facing Striga asiatica, MAS has incorporated SNPs for pre-attachment resistance from upland varieties, reducing parasite emergence by reinforcing root cell walls. Multi-parent advanced generation inter-cross (MAGIC) populations, developed in sorghum and maize between 2015 and 2025 using 8–19 founders, have facilitated fine-mapping of resistance QTLs, enabling pyramidization of multiple alleles for durable, broad-spectrum protection. Significant progress includes the release of over 50 Striga-resistant varieties across cereals since the 1990s, with ICRISAT programs in contributing drought-tolerant lines like those derived from SRN39 backcrosses. When combined with agronomic practices like , these varieties have delivered up to 90% Striga control and tripled sorghum yields in Kenyan trials, underscoring their role in integrated .

Species Diversity

Key Parasitic Species

Striga hermonthica, commonly known as purple witchweed, is the most widespread and agriculturally damaging species in the genus, primarily affecting cereal crops such as and across . As an obligate hemiparasite, it attaches to host via haustoria to extract and nutrients, leading to yield losses ranging from 40% to 100% in heavily infested fields, with even low infestation levels causing significant reductions. The features erect stems up to 80 cm tall with lanceolate leaves and distinctive purple to pinkish flowers arranged in terminal spikes. A single mature can produce up to 500,000 tiny seeds, which remain viable in soil for over 20 years, exacerbating its persistence and spread. Striga asiatica, or Asiatic witchweed, is a hemiparasitic species native to and parts of , where it infests cereals like , , and , as well as . It was accidentally introduced to the , first discovered in in 1956, but a comprehensive federal and state quarantine and control program has reduced infested areas from over 450,000 acres to less than 1% of that area as of 2024, though eradication efforts continue. Plants are smaller, typically 15-40 cm in height, with opposite linear leaves and small yellow to reddish flowers. Like other Striga species, it produces thousands of seeds per plant, contributing to its invasive potential in suitable environments. Striga gesnerioides, known as cowpea witchweed, targets dicotyledonous hosts including , , and other , making it a major threat to pulse crops in the of . This hemiparasite exhibits at least seven distinct races defined by their virulence on specific varieties, allowing it to overcome host resistance through specialized host-parasite interactions. Plants grow to 10-50 cm tall with scale-like leaves and pale pink to purple tubular flowers in dense spikes. Its adaptation to arid conditions enhances its prevalence in marginal farming areas, where it can cause complete crop failure under high infestation. Among other notable species, Striga forbesii primarily parasitizes wild grasses in southern and eastern , resulting in limited direct impact on major crops. It features salmon-pink flowers and a more robust compared to crop-infesting relatives. Striga angustifolia, a rarer species confined to parts of including and , occasionally affects local grasses but poses minimal agricultural concern due to its restricted distribution. Comparative studies highlight differences in parasitic mechanisms among key species; for instance, S. hermonthica develops more aggressive and extensive haustorial connections to hosts than S. asiatica, enabling greater extraction and host damage. Genomic sequencing efforts in the , including the assembly of the S. hermonthica revealing approximately 35,000 protein-coding genes and evidence of whole-genome duplications, have provided insights into haustorial development and host specificity, facilitating the design of targeted control strategies.

Complete Species List

The genus Striga encompasses 54 accepted species, all classified within the family , primarily as obligate or facultative root hemiparasites native to tropical and subtropical regions of the . These species exhibit diverse morphological traits, but taxonomic consensus is provided by authoritative databases like (POWO). Taxonomic revisions have refined species boundaries over time. For instance, Striga lutea Lour. is frequently treated as a of S. asiatica (L.) Kuntze in broader classifications due to overlapping morphology and distribution, though POWO recognizes it as distinct. Recent splits, such as S. barthii described in 2010 on the basis of distinct morphological features like corolla shape and host specificity, highlight ongoing refinements in the genus. (for analogous recent description in S. magnibracteata, illustrating split trends) Conservation assessments for Striga are limited, as most are weedy hemiparasites adapted to disturbed habitats and thus not typically evaluated under IUCN criteria. The majority are considered of least concern or not assessed, reflecting their resilience and wide distributions. However, some rare endemics face threats from habitat loss and fragmentation; for example, facultative or non-parasitic relics, such as S. angustifolia, may be vulnerable to in their native grasslands. (for S. angustifolia status implications) Approximately 80% of Striga species are endemic to , particularly sub-Saharan savannas and grasslands, with others distributed across , the , and ; all thrive in the seasonally dry tropical . For a complete of accepted species, refer to POWO. The following table provides a partial enumeration of accepted species, with representative habitat notes and threat levels based on available data (primarily IUCN not assessed, as few have formal evaluations).
SpeciesHabitat SummaryThreat Level
S. africanus (Boerlage) SchnellTropical African savannasNot Assessed
S. alba PennellEast African grasslandsNot Assessed
S. angustifolia (D.Don) DubySouthern African to Asian dry tropicsLeast Concern
S. asiatica (L.) KuntzeAfrican and Asian seasonal drylandsNot Assessed
S. aspera VatkeEast African highlandsNot Assessed
S. barthii Acheampong, G.K.Agama & R.B. HallWest African wetlandsNot Assessed
S. bellidifolia Engl. & DielsCentral African forestsNot Assessed
S. brachycarpa PetterSouthern African grasslandsNot Assessed
S. breviflora Benth.East African dry bushlandsNot Assessed
S. bulbifera K.I.Mohamed, Musselman & L.J.MusselmanEast African coastal regionsNot Assessed
S. chrysantha Schweinf.East African savannasNot Assessed
S. coffea K.I.Mohamed & MusselmanCentral African montane forestsNot Assessed
S. columnea E.PhillipsSouthern African shrublandsNot Assessed
S. coruscans Benth.East African grasslandsNot Assessed
S. curviflora Benth.Southern African dry areasNot Assessed
S. daltonii Hook.f.East African highlandsNot Assessed
S. densiflora (Benth.) Benth.Asian and Arabian dry tropicsNot Assessed
S. discolor K.I.Mohamed, Musselman & L.J.MusselmanEast African coastal dunesNot Assessed
S. egena Schweinf.East African savannasNot Assessed
S. euphrasioides Benth.West African grasslandsNot Assessed
S. fasciculata ThwaitesAsian seasonal dry forestsNot Assessed
S. ficifolia HiernCentral African woodlandsNot Assessed
S. forbesii Benth.Southern African grasslandsNot Assessed
S. gemussarensis SchinzSouthern African arid zonesNot Assessed
S. gesnerioides (Willd.) VatkePan-African savannasNot Assessed
S. glabrescens Hochst. ex Benth.East African drylandsNot Assessed
S. gracilis R.E.Fr.East African highlandsNot Assessed
S. hallidayi S.MooreSouthern African bushlandsNot Assessed
S. hermonthica (Delile) Benth.Sub-Saharan African savannasNot Assessed
S. hirsuta Wall. ex Benth.Asian tropical grasslandsNot Assessed
S. japonica (Thunb.) KuntzeEast Asian temperate edgesNot Assessed
S. latifolia Benth.West African wetlandsNot Assessed
S. linearifolia Welw. ex Engl.Southern African dry areasNot Assessed
S. loranthioides Engl. & DielsCentral African forestsNot Assessed
S. micrantha BrandegeeEast African coastal regionsNot Assessed
S. pubiflora Engl.East African grasslandsNot Assessed
S. puberula K.I.Mohamed, Musselman & L.J.MusselmanEast African savannasNot Assessed

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