Controlling witchweed through biocontrol

IITA and its partners have found a way to control the scourge of witchweed (Striga hermonthica) in Sub-Saharan Africa through a biocontrol agent. Striga infests some 50 million hectares of cereal crops, specifically maize, sorghum and millet, causing farmers an estimated US$ 7 billion in annual losses and affecting over 300 million people in the region.

Developed by a team led by IITA plant pathologist Dr. Fen Beed with partners from the University of McGill (Canada) and University of Hohenheim (Germany), the technology utilizes certain strains of Fusarium oxysporum (F. oxysporum) to fight the parasitic weed. The technology is cheap, environment-friendly and safe as the fungus specifically targets witchweed.

The fungal strains tested originated from Ghana, Mali and Nigeria but, like witchweed, they are common throughout semi-arid Africa. The fungus can be easily grown in sterile water containing sorghum waste. The hard part was finding a way to coat seeds with it.

Through experimentation, the team found that spores of the fungus can be mixed with liquefied Arabic gum - an organic adhesive extracted from trees and commonly found in many SSA countries - without harming the fungus. The mixture is coated onto the seeds, dried then planted. The fungus remains viable for long periods, making the seeds amenable to storage. The fungus could also be directly dispersed into soil holes where the seeds are to be planted. The treated seeds produce crops that are free of the parasitic weed.

Witchweed, which Beed considers to be one of most significant constraints to African agriculture, routinely causes crop losses of 40 to 100 percent in infected farmers’ fields. As deadly as it is pretty (the weed produces beautiful violet petals), witchweed does most of its damage underground, prior to being visible to farmers above the soil surface. By the time the weed appears, it’s already too late - the victim plant is already very sick.

Like all parasites, witchweed seriously weakens its host, draining it of water and vital nutrients. The use of post-emergence herbicides, aside from being too costly for most farmers, are largely ineffective in protecting the crop as the damage has already been done by the parasite. Additionally, witchweed produces hundreds of thousands of seeds per square meter, causing massive build up in soil that can remain active for several years.

"We cannot say that the witch is dead or soon will be," Beed says, "but we definitely have found an extremely effective component of an Integrated Pest Management strategy to kill her – and one that is safe, practical, affordable and sustainable for farmers."

However, he cautions that the technology is not a one-off and stand-alone solution to the witchweed problem. He says that the technology "has a greater chance of success if combined with other approaches such as the use of resistant varieties, pre-emergence herbicides and adding organic matter to the soil, thereby improving its richness and providing an environment that is conducive to beneficial microorganisms such as the biocontrol fungus."

"Now that we have a cost-effective method to control witchweed, the next step is to scale out its use and to get it into the hands of farmers at the soonest possible time", he ends.


May 2008