The witchâ€™s spell on millions of hectares of cereal crops in sub-Saharan Africa (SSA) will soon be broken. A deadly â€œpotionâ€ 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 plantâ€™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.
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 BÃ©nin, Burkina Faso, Ghana, Mali, Niger, and Nigeria, scientists discovered isolates of Fusarium oxysporum f. sp. striga that controlled the weed.
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 stagesâ€”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 BÃ©nin in partnership with various NARS and universities. The efficacy of the three isolates selected from laboratory studies were compared with other isolates originating from BÃ©nin 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.
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 soilâ€”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 profilesâ€”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 lâ€™Education et la SantÃ©.
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.
â€œThere 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,â€ 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 â€œwitchâ€.