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This is the VOA Special English Development Report.

Farmers in Africa call the striga plant by the name witchweed. It grows a pleasant looking purple flower. But it attacks many crops in the grass family. Witchweed is among the few flowering weeds that act as parasites on other plants.

Every year it causes six to seven billion dollars in damage to African grain sorghum. Sorghum is important to the local food supply. The witchweed steals water and nutrients from the roots and attacks the sorghum with a poisonous substance.

Now, scientists say they have produced seeds that can protect sorghum crops from witchweed.

Kassim Al-Katib is a weed expert at Kansas State University in the United States. Mitch Tuinstra is a genetics and plant-breeding expert formerly at Kansas State but now at Purdue University in Indiana.

To deal with witchweed, they developed special sorghum-seed genes. These genes can accept carefully chosen chemical herbicides without being harmed. The researchers placed the herbicides on the sorghum seeds. Kassim al-Katib said the seeds kill the witchweed as the sorghum grows.

Mitch Tuinstra directed greenhouse tests of the seeds in the Netherlands in two thousand five and two thousand six. Similar studies took place in field trials in Mali and in Niger. These studies showed the treatment to be highly successful.

Mitch Tuinstra recently returned from Africa, where he met with agriculture experts. They are working to develop local kinds of sorghum that employ the genes. The project is a collective research program under the United States Agency for International Development.

Witchweed has another unusual quality. It requires chemical signals from sorghum seeds to grow. If no sorghum seeds are present, the witchweed seeds can lie under the soil for years. The seeds begin to grow only when they receive the needed chemical signals.

Witchweed seed capsules can hold four hundred to five hundred seeds. Winds and rain spread the parasite. The plant can reduce a farmer's crop. Or it can completely destroy many hectares of grain. Witchweed is very difficult to remove after it invades an area.

Damage from the parasite is worst in dry soil with low fertility. It often strikes farmers who work the poorest land. This can mean disaster for people who already do not have enough to eat.

And that's the VOA Special English Development Report, written by Jerilyn Watson.

(來源：VOA 英語點津 Annabel 編輯)