Ecofriendly bioherbicide approach for Striga control

Abuelgasim Elzein, a.elzein@cgiar.org, and Fen Beed

Root parasitic weeds of the genus Striga are a significant constraint to cereal and cowpea production in sub-Saharan Africa. They can cause total crop losses particularly during drought, in infertile soils and cereal monocropping. Striga causes annual losses of US$7 billion and affects incomes, food security, and nourishment of over 100 million people mostly in sub-Saharan Africa.

Each Striga plant can produce thousands of seeds, viable for over 10 years. Their intimate interaction with different host plants prevents the development of a silver bullet control technology that subsistence farmers can adopt. Hence, it is widely accepted that an integrated approach to Striga management is required for which biocontrol represents a crucial component.

Bioherbicide innovation
A bioherbicide is a plant pathogen used as a weed biocontrol agent (BCA), which is applied at sufficient rates to rapidly cause a disease epidemic that kills or severely suppresses the target weed. The use of biocontrol technology to manage Striga is a desirable control method as it is environmentally friendly, safe to farmers and crop consumers, specific to the target host, and has the potential to be economically viable. In addition, biological control also assists in the development of a balance of nature, the creation of more biodiversity, and sustaining of complex ecological interactions.

Since the early 1990s, a series of intensive disease surveys in many countries of sub-Saharan Africa has evaluated hundreds of microorganisms for their pathogenicity and virulence against Striga. Fusarium oxysporum Schlecht isolates have been the most promising. However, the discovery of a highly effective pathogen is only one step in the process of developing bioherbicides, for which the inoculum mass production, formulation, delivery, and storage ability must be optimized, and the mode of action, host specificity, and biosafety evaluated and fully understood.

The most widely studied and used fungal isolate that met all requirements for a potential bioherbicide for Striga is F. oxysporum Schlecht f. sp. strigae Elzein et Thines (isolates Foxy2 and PSM197). These are highly virulent, attack Striga in all growth stages—from seed to germination, from seedling to flowering shoot; protect the current crop yield; and prevent seed formation and dispersal.

F. oxysporum f. sp. strigae is highly host-specific to the genus Striga, and does not produce any known mycotoxic compounds. Thus, its use does not pose health risks to farmers, input suppliers, traders or consumers or threaten crops or the environment. Its unique DNA constitution differs from other forms of F. oxysporum deposited in GenBank, known to cause crop diseases. Indeed, this ensures its biosafety and greatly facilitates its wider application and use as a bioherbicide.

In addition techniques for massive production of inoculum of F. oxysporum f. sp. strigae was optimized based on simple and low-cost methods and using inexpensive agricultural by-products available in sub-Saharan Africa. The chlamydospores produced by this fungus have the advantage of being able to survive extreme environmental events while still remaining viable. This is an important feature required for a BCA suited to hot and dry climatic conditions of cereal production in sub-Saharan Africa, and to produce stable, durable, and pathogenic propagules.

Extensive research by the University of Hohenheim (UH, Germany), IITA (Benin), McGill University (Canada), and Institute for Agricultural Research – Ahmadu Bello University (Nigeria), has enhanced application of F. oxysporum f. sp. strigae, its formulation into bioherbicidal products, and its delivery for practical field application. The Striga bioherbicide contains the Striga host-specific F. oxysporum f. sp. strigae, applied in massive doses to create a high infection and disease level to kill or severely suppress Striga.

Promotion in West Africa
The bioherbicide is a component of the IITA-led project, Achieving sustainable Striga control for poor farmers in Africa, funded by the Bill & Melinda Gates Foundation to intensively promote technologies to combat Striga in sub-Saharan Africa. The project will validate the potential of the bioherbicide seed treatment technology across major Striga-infested agroecological zones and maize-based farming systems, while also confirming the biosafety and developing molecular detection tools. Here are the highlights of the results:

Technology validation: Several multilocation trials were conducted under natural and artificial Striga infestation across two agroecological zones in northern Nigeria to evaluate the efficacy of Striga bioherbicide (F. oxysporum f. sp. strigae). The inoculum produced by UH and SUET seed company was delivered as a film-coat on maize seeds (see below).The application of the bioherbicide technology in combination with Striga resistant maize reduced Striga emergence by 73% and 39%, compared to the susceptible and resistant controls, respectively, and prevented 81% and 58% of emerged Striga plants from reaching flowering and 56% and 42% of the maize plants from attack by Striga (see next page). The combination of bioherbicide with Striga susceptible variety significantly reduced Striga emergence by 53%, resulting in 42% reduction in number of flowering plants and in 21% increase in grain yield compared to the susceptible control.

In addition, disease symptoms were recorded on emerged Striga plants parasitizing maize plants coated by the bioherbicide. The reduction in Striga emergence across maize varieties indicates the effectiveness of the bioherbicide to attack seeds under the soil surface. The synergistic effect of the bioherbicide technology combined with the Striga resistant maize is expected to reduce the Striga seedbank and thus the impact of Striga on subsequent maize crops.

Biosafety: To further ensure the safety of Striga BCA and to demonstrate and increase awareness among farmers, regulatory authorities, and stakeholders, a wide host range study was carried out using 25 crops in collaboration with IAR-ABU and the Nigerian Plant Quarantine Service (NPQS)  under field and screenhouse conditions in Nigeria. Results revealed that none of the test plants showed any infection by the biocontrol agent both in the field and screenhouse, and no detrimental growth effects were measured or visual losses to plant health recorded in any of the inoculated crops tested, i.e., inoculation with the Striga BCA did not cause any delay in emergence, and a decrease in plant height, plant vigor, chlorophyll content per leaf, shoot fresh and dry weight. Hence, the Nigerian regulatory authorities (NPQS, NAFDAC) and other stakeholders were satisfied and confident that no disease was produced on plants other than Striga by the BCAs and that it is safe to use. In addition, a mycotoxin produced by Striga bioherbicide  F. oxysporum f. sp. strigae was analyzed and evaluated by our project partner, the University of Stellenbosch in South Africa. An evaluation of existing isolates of F. oxysporum f. sp. strigae does not produce well-known mycotoxins (e.g., Fumonisin and Moniliformin) that pose a threat to animal or human health. This finding further confirms the safety of this bioherbicide.

Molecular detection tools: Development of a monitoring tool specific to the Striga bioherbicide is important to certify inoculum quality, monitor the presence and persistence of the BCA in soils, and validate its environmental biosafety. UH is developing a monitoring tool.

The AFLP fingerprinting technique was successfully used in developing a primer pair capable of differentiating the F. oxysporum f. sp. strigae group from other Fusarium species. In addition, the monitoring tool has shown a high specificity for isolate Foxy2 and was used to monitor its spread and persistence in rhizobox experiments under different management practices using Kenyan soils. This promising result provides a proper baseline to further the existing primer set.

Bioherbicide + pesticide technology: The novel combination and integration of the bioherbicide technology plus imazapyr herbicide for Striga control with pesticides in a single-dose seed treatment to control fungal pests offers farmers with maize seed that is able to achieve its yield potential. The use of each technology (BCA or imazapyr) has been shown to be effective when applied independently using seed coating techniques, but have not been integrated.

The compatibility of Striga BCAs with different pesticides (herbicides and fungicides with insecticide components) was studied in vitro in the laboratory. Striga BCAs showed excellent compatibility with imazapyr (a herbicide seed coating used in combination with IR maize to control Striga), Metsulfuron Methyl (MSM) (a herbicide seed coating developed by DuPont to control Striga in sorghum), and glyphosate (an intensively used herbicide). A similar result was also achieved with the commonly used seed treatment fungicides at the recommended application doses.

Accordingly, doses and complementary seed coating protocols for the three compatible technologies (BCA, herbicide, and fungicides) have been developed and IR maize seeds were successfully coated with a single-dose seed treatment of BCA inoculums and imazapyr. The results showed that imazapyr did not interfere with the BCA during seed coating, with BCA growth and sporulation after coating, and with IR maize seed germination. Seeds of IR maize varieties can thus be coated with the herbicide and the BCA and then fungicide and delivered to farmers using the same input pathway. Screenhouse and field trials are being carried out to generate data on the combined efficacy of the applied technologies. The demonstrated compatibility of Striga BCA with the different pesticides that contain a wide range of active ingredients indicate that the combination and delivery of the Striga bioherbicide technology with a large number of pesticide products is possible. These findings are expected to provide a triple action seed coating package for direct control of Striga and fungal diseases of maize in sub-Saharan Africa.

Suitability to African farming systems
Our strategy for scaling-up the bioherbicide innovation is based on using technology appropriate to Africa to ensure that sustained production of the bioherbicide is feasible at a cost affordable to African small-scale farmers. The seed-coating treatment requires significantly less inoculums, establishes the BCA in the cereal rhizosphere, i.e., the infection site of Striga, and provides a simple, practical, cost-effective delivery system for adoption by input suppliers to subsistence farmers. Arabic gum as a coating material has been shown to increase the rate of mycelia development and enhance BCA sporulation. Its availability in sub-Saharan Africa at a low price is an additional economic advantage. A commercial seed coating process, developed and optimized at UH with SUET Seed Company in Germany, is being transferred and adapted at IITA, Ibadan, to be used as an experimental production unit for capacity building and as a model for eventual transfer of seed treatment technology to the private sector after validation.

Outlook
One unique advantage of this bioherbicide is that the ability of Striga to become resistant to it is virtually unknown as a consequence of the suite of enzymes and secondary metabolites that the BCA produce to become pathogenic and virulent against the target (Striga). Hence after validation, delivering the bioherbicide technology in combination with resistant maize or with the herbicide imazapyr is expected to increase efficacy in controlling Striga. Bioherbicide and other compatible technologies have different modes and sites of action against Striga, and in a combination they will have a much greater chance of reducing the potential risk of development of resistance to a single technology (resistant varieties or herbicides) used separately and repeatedly.
The potential delivery of coated seeds of resistant maize with bioherbicide in one package to farmers using the same input pathway will reduce transaction and application costs and enhances the economic feasibility and adoptability of the technologies. Similarly, compatibility of BCA with imazapyr and fungicides allow seed coating of IR-maize with bioherbicide, imazapyr, and fungicides with a single-dose seed coating application.

Future plans
Currently, large-scale field testing is ongoing and is being implemented to further validate bioherbicidal efficacy across two agroecological zones where the common scenarios for maize infestation by Striga in northern Nigeria are represented. For understanding of farmers’ preferences and perceptions, socioeconomic analysis and cost-benefit analysis of bioherbicidal technology based on field data/surveys and interviews, current market information, and links with other Striga control strategies will be undertaken. After validation, dissemination and commercialization will be promoted through private sector partnerships and integrated with other control options such as resistant varieties, IR varieties combined with seed treatment with imazapayr, crop rotation with legumes, and soil fertility management practices, to achieve sustainable management of Striga.

Partners
IITA (Dr F. Beed, Dr A. Elzein & Dr A. Menkir), Institute for Agricultural Research – Ahmadu Bello University (Dr A. Zarafi), Nigeria; University of Hohenheim (Prof G. Cadisch, Dr F. Rasche & Prof J. Kroschel), Germany; The Real-IPM Company Ltd (Dr H. Wainwright), Kenya; University of Stellenbosch (Prof A. Vilioen), South Africa; and McGill University (Prof A. Watson), Canada.

References
Beed F.D., S.G. Hallet, J. Venne, and A. Watson. 2007. Biocontrol using Fusarium oxysporum; a Critical Component of Integrated Striga Management. Chapter 21 in Integrating New Technologies for Striga control: Towards ending the Witch-hunt (Ejeta, G. and J. Gressel, eds). World Scientific Publishing Co. Pte. Ltd. pp 283-301.

Ciotola, M., A. DiTommaso, and A. Watson. 2000. Chlamydospore production, inoculation methods and pathogenicity of Fusarium oxysporum M12-4A, a biocontrol for Striga hermonthica. Biocontrol Science and Technology 10: 129-145.

Ejeta, G. 2007. The Striga scourge in Africa: A growing pandemic In: Ejeta, G. and J. Gressel, eds. Integrating New Technology for Striga Control: Towards Ending the Witchhunt. World Scientific Publishing Co. Pte. Ltd., UK. pp. 3-16.

Elzein, A.E.M. 2003. Development of a granular mycoherbicidal formulation of Fusarium oxysporum “Foxy 2” for the biological control of Striga hermonthica. In: “Tropical Agriculture 12– Advances in Crop Research (2)” (J. Kroschel, ed.). Margraf Verlag, Weikersheim, Germany, 190 pp, ISBN 3-8236-1405-3.

Elzein, A., J. Kroschel, and V. Leth. 2006. Seed treatment technology: an attractive delivery system for controlling root parasitic weed Striga with mycoherbicide. Biocontrol Science and Technology, 16(1) 3-26.

Elzein, A., F. Beed, and J. Kroschel. 2012. Mycoherbicide: innovative approach to Striga management. SP-IPM Technical Innovations Brief, No. 16, March 2012.

Kroschel, J. and D. Müller-Stöver. 2004. Biological control of root parasitic weeds with plant pathogens. In: Inderjit, K. (ed.), Weed biology and management. Kluwer Academic Publishers, Dordrecht, The Netherlands, pp. 423–438.

Kroschel, J., A. Hundt, A.A. Abbasher, J. Sauerborn. 1996. Pathogenicity of fungi collected in northern Ghana to Striga hermonthica. Weed Research 36 (6), 515–520.

Marley, P.S., S.M. Ahmed, J.A.Y. Shebayan, and S.T.O. Lagoke. 1999. Isolation of Fusarium oxysporum with potential for biocontrol of the witchweed Striga hermonthica in the Nigerian Savanna. Biocontrol Science and Technology 9: 159–163.

Venne J., F. Beed, A. Avocanh, and A. Watson. 2009. Integrating Fusarium oxysporum f. sp. strigae into cereal cropping systems in Africa. Pest Management Science 65: 572–580.

Saving maize from parasitic Striga in Kenya and Nigeria

Thousands of farmers in Kenya and Nigeria are successfully battling the invasion in their farms by Striga, a deadly parasitic weed. They are now enjoying higher yields in maize, the number one staple in Kenya and an important cash crop in Nigeria.

The key to managing this weed is to combine sustainable multiple-pronged technology options being advocated by the Integrated Striga Management in Africa (ISMA) project to sustainably eliminate the weed from their fields, says Dr Mel Oluoch, ISMA project manager.

Striga attacks and greatly reduces the production of staple foods and commercial crops such as maize, sorghum, millet, rice, sugarcane, and cowpea. The weed attaches itself to the roots of plants and removes water and nutrients and can cause losses of up to 100% in farmers’ crops. Furthermore, a single flower of the weed can produce up to 50,000 seeds that can lie dormant in the soil for up to 20 years.

The weed is the number one maize production constraint in Western Kenya, and Nigeria, infesting most farmers’ fields.

The management technologies range from simple cultural practices such as intercropping maize with legumes such as groundnuts; crop rotation of maize with soybean which stimulates Striga to germinate but which later dies in the absence of the maize host to latch onto; deploying a “push-pull’ technology that involves intercropping cereals with specific Striga-suppressing desmodium forage legume; using Striga resistant maize varieties; and using CIMMYT-developed maize varieties resistant to Imazapyr—a BASF herbicide (StrigAway®), which kills the Striga seed as it germinates and before it can cause any damage; and adopting Striga biocontrol technologies which uses a naturally occurring host-specific fungal pathogen that kills the Striga at all stages without affecting other crops.

Imazapyr-resistant maize varieties with natural resistance to Striga hermonthica have been developed. The best hybrids produce 19% to 333% more grain yields under Striga infestation, sustain 17% to 57% less Striga damage, and support 63% to 98% less emerged Striga plants compared with the commercial hybrid check. In addition, new Striga resistant hybrids and open-pollinated synthetic varieties (OPVs) that combine Striga resistance with good standability have been developed. The hybrids and OPVs produce 47% to 126% more grain yields under Striga infestation, sustain 17% to 60% less Striga damage, and support 45% to 90% less emerged Striga plants compared with the common farmers’ varieties and commercial hybrids.

ISMA (http://www.iita.org/web/striga/) is funded by the Bill & Melinda Gates Foundation and is being implemented with the International Center of Insect Physiology and Ecology, CIMMYT, African Agricultural Technology Foundation, BASF Crop Protection, and other national agricultural research and extension services and private sector players in Kenya and Nigeria.

Reducing crop loss from Striga

Scientists based in Nigeria and Kenya started an initiative against two parasitic weeds that have spread across much of sub-Saharan Africa. These weeds cause losses of up to US$1.2 billion from damage every year to the maize and cowpea crops of millions of small farmers.

The project, coordinated by IITA, will introduce proven technologies for fighting Striga (witchweed), and Alectra, which attack crops such as maize and cowpea, reducing yields or destroying entire harvests.

Extension workers explains about the Striga problem. Photo from SP-IPM.
Extension workers explains about the Striga problem. Photo from SP-IPM.

Witchweed primarily affects smallholder farmers. The most widespread species is estimated to have infested up to 4 million ha of land under maize production in sub-Saharan Africa, with yield losses of up to 80%. IITA’s researchers estimate that this represents about $1.2 billion in losses for farmers and affects approximately 100 million people in the region.

The Striga project is supported by a grant from the Bill & Melinda Gates Foundation. It aims at helping 200,000 maize farmers and 50,000 cowpea farmers who work in areas with high rates of Striga infestation in Kenya and Nigeria. By the project’s end in 2014, organizers estimate that farmers will see up to 50% higher maize yields and 100% higher cowpea yields.

The 4-year project will focus on improving and expanding access to methods of Striga control, while supporting research to identify the most effective means of control under varying conditions. It will evaluate and implement four approaches: using Striga-resistant crop varieties; using a “push-pull” technology that involves intercropping with specific forage legumes that inhibit the germination of Striga; using herbicide-coated seeds; and deploying biocontrol of Striga. After a 2-year evaluation period, the project will scale up the most effective approaches.

Partners in the project are the International Maize and Wheat Improvement Center, African Agricultural Technology Foundation, International Centre of Insect Physiology and Ecology, and BASF Crop Protection. The poject will work with farmers, seed companies, community-based organizations, extension workers, policymakers, and researchers.

Scientists expect that the interventions will generate annually additional grain with an estimated value of $8.6 million at the project locations. This will result in increased incomes, better nutrition, and reduced poverty, and employment opportunities.

The witch menace

Maize ravaged by Striga
Maize ravaged by Striga. Photo by IITA

The witch’s spell on millions of hectares of cereal crops in sub-Saharan Africa (SSA) will soon be broken. A deadly “potion” using natural enemies is being developed by IITA and its partners to manage the menace.

Striga hermonthica or witchweed, the parasitic weed that attacks cereal crops, such as maize, sorghum, and millet, has caused devastating annual production losses estimated at US$7 billion among small-scale farmers, contributing to hunger, malnutrition, and poverty in SSA.

The sight of the deceptively beautiful pink flowers of Striga spells doom for farmers. The weed grows on the roots of host plants absorbing the plant’s water, photosynthates, and minerals. When the flowers are in bloom, it is already firmly established. Thus, the use of aboveground herbicides is ineffective, since the damage has occurred long before the weed is visible to farmers. Each plant can produce tens of thousands of seeds that are dispersed far and wide by man and nature, and which lie dormant but still potentially active for many years.

Angry farmer with Striga plant
An angry farmer with Striga plant. Photo by IITA

Loss of millions of tons of food
Fen Beed, an IITA plant pathologist, explains that production losses from Striga routinely range from 15 to 90% depending on the crop cultivar, degree of infestation, rainfall pattern, and degree of soil degradation.

Striga infests about 50 million hectares of land in SSA resulting in the loss of over 8 million tons of food annually. The larger areas affected are in Nigeria, Niger, Mali, and Burkina Faso.

Unfortunately, measures developed to control the weed in the developed world, such as soil fumigation, are too costly for the poor subsistence farmers who make up 70 to 80% of farmers in SSA. New management options are thus urgently needed.

One promising, sustainable, and environmentally friendly technology under development is biocontrol using indigenous fungi that are natural enemies of the weed.

Poisoning the witch
A team led by Beed with partners from the University of McGill (Canada) and University of Hohenheim (Germany), and national agricultural research systems (NARS) and universities in West Africa, have identified isolates of a fungus that attacks Striga for use as a bioherbicide.

By studying over hundreds of diseased shoots of Striga in Bénin, Burkina Faso, Ghana, Mali, Niger, and Nigeria, scientists discovered isolates of Fusarium oxysporum f. sp. striga that controlled the weed.

Container trial, Ibadan, Nigeria
Container trial, Ibadan, Nigeria. Photo by IITA

A series of controlled laboratory studies identified the most effective of these as M12-4A, an isolate from Mali, Foxy 2 from Ghana, and PSM-197 from the Nigerian savanna. The isolates attacked Striga in all its growth stages—from seed to germination, from seedling to flowering shoot. They significantly lessened the number of attachments and flowering Striga plants, thus reducing the number of seeds deposited in the soils and limiting the future reappearance of the weed. Furthermore, the isolates were specific to S. hermonthica, had no impact on cereal hosts or any other plants, and did not produce any toxins that harm man or livestock.

Repeated field trials were performed for the first time under West African conditions using Striga-resistant and Striga-susceptible varieties of sorghum and maize in Nigeria, Burkina Faso, and Bénin in partnership with various NARS and universities. The efficacy of the three isolates selected from laboratory studies were compared with other isolates originating from Bénin and Burkina Faso. Amino acids found to disrupt germination of Striga under laboratory conditions were also included but failed to produce significant improvements in weed control under field conditions.

Results showed that PSM-197 and Foxy 2 were the most effective in repressing witchweed, whereas isolate M12-4A was less effective under the range of field conditions tested. Also, there was a 90% reduction in Striga emergence when the biocontrol technology was used in combination with a Striga-resistant maize line.

Two methods were used to apply the fungi: either directly coating the seed using locally available gum arabic or directly adding the fungus in powder formulations of kaolin-based PESTA granules into planting holes. The granular formulation was found to be more efficient, especially for sorghum which has much smaller seeds than maize, where the larger seeds receive more fungal inoculum when applied as a seed coating. However, it is more costly and difficult to distribute to farmers.

Drying sorghum and maize seed coated with PESTA
Drying sorghum and maize seed coated with PESTA. Photo by IITA

Therefore, the seed-coating method offers the most cost-effective method, especially when combined with Striga-resistant germplasm.

Another important finding is that the biocontrol agent works most efficiently when the soil is rich in beneficial (friendly) and not antagonist (nuisance) microorganisms. Container trials at IITA Ibadan showed that the profile of both bacterial and fungal microorganisms was changed when different species of cereals were grown in the same soil—this is because each plant type produces different exudates that are excreted around roots that promote or inhibit the growth of different microorganisms.

Furthermore the profile was changed when different cultivars of the same species of cereal crop (maize or sorghum) were grown. Different fertilizer combinations had similar impacts on microorganism profiles—all of these changes in profiles affect the success of introduced biocontrol agents. This study was done using state of the art PCR-DGGE technology in collaboration with the University of Purdue.

Making the potion available and affordable
Supplying fungal-coated seeds of improved varieties to farmers requires a delivery pathway. Researchers face the challenge of mass producing the biocontrol agents and encouraging farmers to use them. The models being tested for mass production of the F. oxysporum inoculum include on-farm, cottage-industry, small entrepreneur industry, and government initiatives, such as that in Senegal initiated by Foundation Agir pour l’Education et la Santé.

Under the small entrepreneur industry models, one company in Kenya, Real-IPM, has secured funding to register Foxy 2 before mass production using large-scale commercial tanks for liquid culture of the fungus. Another company, Western Seed Company Ltd., has carried out preliminary field tests with support from the Kenya Plant Health Inspectorate Services.

Finding a way to curtail the negative impact of witchweed has been a long journey, but the biocontrol option can provide an important component in an integrated package of strategies for managing this pest.

PESTA granules
PESTA granules. Photo by IITA

“There will never be a silver bullet solution to alleviate the problems faced by farmers from witchweed. It is important to recognize that efficacy and persistence of the biocontrol agent is improved when steps are taken to prevent the soil from being degraded and to enrich it with organic matter,” says Beed.

New techniques are also needed for measuring the extent of losses caused by witchweed and their economic impact. Likewise, control technologies need to be developed and implemented, and their efficacy assessed across the different environments scourged by the pest, he added.

Biocontrol combined with the use of improved cereal cultivars that have increased tolerance/resistance to Striga, and the use of seed-coated herbicides such as imazapyr, in addition to the regular use of trap crops, at last offers small-scale farmers real hope against the “witch”.