The African Witchweed menace
December 2009
Africa is the only continent that cannot feed itself: food production per person is less than it was in 1960. One of the reasons is the prevalence of African Witchweed, also known as Striga, the root parasite of cereal crops.
Striga (purple flowers) strangles maize crops. Image: Julie Scholes
Striga is considered the major biological constraint to crop production in sub-Saharan Africa. In Africa, 100M hectares of land are infested with Striga seed and yield losses of 40-100% are common. Each Striga flower spike produces more than 50,000 seeds which remain viable in the soil for up to 20 years.
Striga causes annual losses in excess of $7Bn and adversely affects food security for more than 100M people; in fact there is a near perfect overlap between areas of Striga infestation and subsistence agriculture where hunger prevails.
Julie Scholes, from the University of Sheffield, is leading a project to identify combinations of genes responsible for resistance to the African witchweed (Striga spp.). Working on rice and wild relatives of rice, her team is trying to locate areas on the rice chromosomes where Striga resistance genes lie, with the hope of enabling breeders to transfer resistance to other cultivars. If successful, the information from rice can be used to identify similar combinations of genes in maize and sorghum.
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There are other ways to control Striga. Treating the soil with nitrogen, or planting nitrogen-fixing leguminous crops, can suppress its effects to a limited extent. However, John Pickett, Director of the Centre for Sustainable Pest and Disease Management at Rothamsted Research (RRes, an institute of BBSRC), found that intercropping maize with the legume silverleaf (Desmodium) led to a massive suppression of the parasitic Striga weed. It was an accidental discovery – the Desmodium was planted to repel insect pests of maize.
Striga (green shoots) parasitizes maize roots (white) before flowering. Image: Julie Scholes
Pickett says they had a job selling the idea. “I remember meetings with funding agencies where they would look at you with one eyebrow raised as if to say, “Pull the other one”, or “There are lots of things that control Striga, such as cowpea, or a forest legume called lablab, or sweet potato.”
Pickett and his colleagues were convinced that Desmodium was more effective and published their results in the scientific literature. “We demonstrated a much bigger effect using an intercrop of Desmodium, not just orders of magnitude better than any other intercrop, but having a qualitative difference as well. Desmodium prevents Striga development and also causes suicidal germination, which reduces or even eliminates the seed bank in the soil.”
Increasing nitrogen in the soil can suppress Striga. Image: ICIPE
This technique has been tried successfully on over ten thousand small-scale subsistence farms across Kenya, Tanzania, Ethiopia and Uganda with great success, and sustainable success too.
More research is needed to see if this can be replicated on an industrial scale, where breeding resistant crops may have a larger overall impact.
This technique has been tried successfully on more than tem thousand small-scale, subsistence farms across Kenya, Tanzania, Ethiopia and Uganda with success, and sustainable success, too (see ‘Push-pull’).
However, more research is needed to see if this could be replicated on a larger, industrial scale where breeding resistant crops may have a bigger overall impact.
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