July 29, 2002

A Weizmann-developed method imbeds herbicide in resistant corn seed to kill witchweed before it can sprout above ground. A scientist from Israel’s Weizmann Institute has zeroed in on a novel way of applying herbicide to kill witchweed, a scourge that devastates corn crops and contributes significantly to the total of 24,000 people the U.N. estimates die of starvation each day worldwide.

Striga hermonthica, or witchweed, infests approximately 50 to 100 million acres of farmland cultivated by poor farmers in Sub-Saharan Africa and is responsible for lost crop yields valued at about $1 billion annually. An estimated 100 million farmers lose from 20 to 80 percent of their crops to the parasite. Witchweed infests 76 percent of the farmland in Western Kenya, causing an estimated annual crop loss of $38 million in Kenya alone.

Witchweed thrives by attaching itself to the roots of a suitable host crop. It 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.

African farmers commonly remove the witchweed by hand, but by the time it emerges above ground, it has already destroyed the crop. Herbicides, applied during that same post-emergence period, are also ineffective for the same reason.

The solution proposed by Jonathan Gressel of the Weizmann Department of Plant Sciences relies on a new use for a certain type of corn developed in the United States through biotechnology. The corn carries a mutant gene that makes it resistant to a specific herbicide and leaves the corn plant unharmed when treated with this herbicide.

As an alternative to spraying entire fields, Gressel coated the herbicide-resistant seeds with the herbicide before planting. Once the crop’s plants sprout from the seeds, the parasites unwittingly devour the weed-killing chemical from the crop roots or surrounding soil and die. By the time the crop ripens, the herbicide, applied in this way at less than one-tenth the normal rate, has disappeared, leaving the food product unaffected.

The Weizmann-produced method has already produced striking results on experimental plots and in farmers’ fields in four East and Southern African countries in long-term trials of the innovative witchweed-fighting technology.

Dr. Fred Kanampiu, a Kenyan scientist, has tested this approach for more than ten crop seasons while breeders crossed the gene into African corn to produce high-yielding varieties that are resistant to major African diseases, as well as to the herbicide.

Witchweed was virtually eliminated in plots planted with herbicide-coated seeds. The experiments indicate that a low-dose herbicide seed coating on resistant corn can increase yields up to four times in highly infested fields. The herbicide is coated on the seed together with a fungicide-insecticide mix that is normally used in Africa. With this technology the farmer does not have to purchase spray equipment and can continue planting legumes between the corn plants – a common practice on small farms in Africa.

Gressel’s research was supported in part by the Canadian International Development Agency through the CIMMYT East Africa Cereals Program and by the Rockefeller Foundation. Initial herbicide-resistant corn seeds were provided by Pioneer International, USA.

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Jason Harris

Jason Harris

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