SCIENCE: Different Visions Compete to Control Africa’s Witchweed

By Stephen Leahy Reprint | | Print |

Stephen Leahy

BROOKLIN, Canada, Jul 30 2002 (IPS) - A biotechnology approach to controlling African farmers’ persistent enemy – witchweed – may be getting the upper hand on more “ecological” solutions to the problem, thanks to funding from Western agencies.

Scientists at the Weizmann Institute in Israel and at the International Maize and Wheat Improvement Center, better known as CIMMYT, are killing the witchweed with herbicide-resistant seeds that are coated with a herbicide before being planted.

With this new technology, maize yields have shot up between 38 and 82 per cent higher in Kenyan field trials

Money for CIMMYT’s herbicide-resistant strategy has come from the Canadian International Development Agency (CIDA) and the Rockefeller Foundation, with seed provided by Pioneer International, a division of the U.S. company, DuPont.

Meanwhile, a strategy that uses locally grown soil fungus to control the witchweed, initially funded by Canada’s International Development Research Centre (IDRC), is languishing because researchers are broke.

Witchweed or Striga is a parasitic weed found in poorer soils of Asia and Africa. In Africa, 40 million hectares in cereal production, including sorghum, wheat, millet and maize, are severely stricken and 70 million are moderately infested by Striga species.

The weed destroys 40 percent of Africa’s total cereals harvest, directly affecting the welfare and livelihoods of over 100 million people, causing a $7 billion loss in yields.

The weed thrives by attaching itself to the roots of a suitable host crop. It then sends up a signal that says “feed me”, and not only sucks up the crop’s energy, but also competes for much of its nutrients and water, while poisoning the crop with toxins and stunting its growth

When a farm is infested with Striga, a maize plant will grow only six inches tall and produce bunches of pink flowers at its base. Each Striga plant produces some 50,000 seeds, fine as sand, which then lie dormant in the soil for over 15 years, until stimulated to germinate by the next crop.

By feeding and growing underground before emerging above the ground, witchweed gets a head start on other plants. African farmers commonly remove it by hand but by the time it emerges above ground it has already damaged the crop. Herbicides applied during that same post-emergence period also fail for the same reason.

The best strategy is to attack the weed underground, said Jonathan Gressel of the Weizmann Institute’s department of plant sciences.

Maize seeds that resist a broad-spectrum herbicide called imidazolinone were coated with the chemical before planting. When the seeds sprout, witchweed unwittingly devours the weed-killing chemical from the crop roots or surrounding soil and dies. Much less herbicide is used with this method, noted Gressel.

It wasn’t difficult to find herbicide-resistant maize – millions of hectares of it have been planted in North America. But it took several years to move the herbicide resistance gene into African varieties that are high-yielding and resistant to major African diseases, said Fred Kanampiu, a CIMMYT scientist based in Kenya.

These herbicide-resistant maizes are not transgenic, Kanampiu was careful to point out. Instead, the imidazolinone-resistant maize was developed using chemicals to create genetic mutations.

Recombinant techniques were not used, so no foreign DNA was introduced to create the herbicide tolerance. Traditional breeding methods were then used to move the mutated gene into desired varieties.

In test plots at a research centre in Kenya, witchweed was virtually eliminated, Kanampiu said. Herbicide costs only four dollars per hectare (only an eight percent increase in cost) and can be easily coated on maize during seed dusting to control fungal and insect.

Dosages are also low enough not to affect the environment, researchers said..

“Use of seed-treated imidazolinone resistance maize is a quick fix to supply farmers with an immediate way to control Striga and deplete its seed,” said Kanampiu. The regulatory aspects and how the technology can reach farmers are currently being debated in Kenya.

But that herbicide-resistant fix is not going to get at the real problem, says Brian Halweil, a research associate with the Worldwatch Institute, who went to Kenya last year to study the issue.

He says continuous planting of maize sucks nitrogen from the soil, creating the conditions under which Striga thrive. One solution is to plant a crop that puts nitrogen back into the soil, such as a legume or nitrogen-fixing trees, during the fallow period of the year, from February to April.

On farms following this regime, maize yields increase two to four times and witchweed is reduced by 90 percent Halweil says. The down side is, farmers must purchase additional seeds, and do not get any rest from planting.

Meanwhile another effective “ecological” solution remains in limbo.

Four years ago, Mali was the site of another set of field trials employing a naturally occurring soil fungus, Fusarium.

“Most of the Striga was wiped out and crop yields doubled in the trials,” says Alan Watson, of the Biopesticide Research Laboratory at McGill University in Montreal, Canada. The Fusarium pathogen attacked the seeds as well as the plant, reducing future outbreaks of the weed, he added.

Funded by Canada’s International Development Research Centre (IDRC), researchers also developed a method for the fungus to be manufactured locally.

Using a starter culture in a capsule, the fungus is grown and dried at a central location. Farmers then have their seed coated first with wet Arabic gum, which acts as an adhesive, and then with the fungus.

Once the seeds are planted, the rain activates the Fusarium and makes it available exactly where it is needed: at the roots of the cereal plant.

The manufacturing uses inexpensive, locally available ingredients like sorghum straw and Arabic gum; the process of preparing the dried fungus involves cooking pots and boiling water. This process could provide a new source of income for women, since it is in their traditional sphere of work, suggested Watson.

A socio-economic survey of 100 farms conducted by the McGill team showed that village women could produce the Fusarium on a cottage industry-type scale, coat batches of seed for individual farmers, and sell the service to the local growers.

How many African farms are using the McGill fungus today? None.

“The funding stopped. We haven’t been able to find any agency or group to help us get this into the hands of African farmers,” Watson said. Manufacturing the fungus could even be a good commercial venture for sale to commercial farmers he suggested.

Watson also pointed out a practical problem to the CIMMYT herbicide-resistant approach. “Subsistence farmers don’t use herbicides because they can’t afford to buy them,” he said.