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.[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]

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]

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 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. Unfortunately, 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]

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] In fact, it is so prolific that in 1957 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 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] In fact, 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] The witchweed infestation is so bad in parts of Africa, 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]

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  Striga densiflora in Hyderabad, India
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  Striga densiflora in Hyderabad
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  Striga densiflora in Hyderabad

• Push–pull technology

• Clark, Lawrence J.; Shawe, Keith G.; Hoffmann, Gŕrard; Stewart, George R. (1994). "The effect of Striga hermonthica (Del.) Benth. Infection on gas-exchange characteristics and yield of a sorghum host, measured in the field in Mali". Journal of Experimental Botany. 45 (2): 281–3. doi:10.1093/jxb/45.2.281.
• Gérard, Hoffmann; Loisel, Roger (1994). Contribution à l'étude des Phanérogames parasites du Burkina Faso et du Mali: quelques aspects de leur écologie, biologie et techniques de lutte [Contribution to the study of parasitic Phanerogams of Burkina Faso and Mali: some aspects of their ecology, biology and control technics] (PhD Thesis) (in French). OCLC 489977820. INIST:163863.
• Gérard, Hoffmann; Diarra, C; Dembele, D (1994). "Outbreaks and new records: Striga asiatica, new pest of maize in Mali". FAO (Food and Agriculture Organization of the United Nations) Plant Protection Bulletin. 42 (42): 214–5.
• Gérard, Hoffmann; Marnotte, P.; Dembélé, D (1997). "Emploi d'herbicides pour lutter contre Striga hermonthica: Striga" [The use of herbicides to control Striga hermonthica]. Agriculture et Développement (in French). 13: 58–62. INIST:2781044.
• Khan, Zeyaur R.; Hassanali, Ahmed; Overholt, William; Khamis, Tsanuo M.; Hooper, Antony M.; Pickett, John A.; Wadhams, Lester J.; Woodcock, Christine M. (2002). "Control of witchweed Striga hermonthica by intercropping with Desmodium spp., and the mechanism defined as allelopathic". Journal of Chemical Ecology. 28 (9): 1871–85. doi:10.1023/A:1020525521180. PMID 12449513. S2CID 21834435.
• Khan, Zeyaur R.; Midega, Charles A. O.; Hassanali, Ahmed; Pickett, John A.; Wadhams, Lester J. (2007). "Assessment of Different Legumes for the Control of in Maize and Sorghum". Crop Science. 47 (2): 730–4. doi:10.2135/cropsci2006.07.0487.
• Moore, T. H. M.; Lane, J. A.; Child, D. V.; Arnold, G. M.; Bailey, J. A.; Hoffmann, G. (1995). "New sources of resistance of cowpea (Vigna unguiculata) to Striga gesnerioides, a parasitic angiosperm". Euphytica. 84 (3): 165–74. doi:10.1007/BF01681808. S2CID 30202739.

• The Parasitic Plant Connection: Striga Photo Gallery
• The Parasitic Plant Connection: Striga asiatica in the USA
• Witchweed
• UN Development Programme
• Striga research at the International Institute of Tropical Agriculture (IITA)
• Parasitic Plants as Weeds
• Striga weed control with herbicide-coated maize seed, CIMMYT
• A recipe for Striga control in sub-saharan Africa
• Moore, T. H. M.; Lane, J. A.; Child, D. V.; Arnold, G. M.; Bailey, J. A.; Hoffmann, G. (1995). "New sources of resistance of cowpea (Vigna unguiculata) to Striga gesnerioides, a parasitic angiosperm". Euphytica. 84 (3): 165–74. doi:10.1007/BF01681808. S2CID 30202739.
• Facebook community page "Striga Research and Control"
• "DP 30: Striga spp". International Plant Protection Convention. Retrieved 2021-10-25.