Newswise — Witchweed, a parasitic weed that can strangle crops, has been nearly extinguished in the United States. But in Africa and Asia, it still grows rampantly, posing a threat to crops and forests. One estimate places agricultural losses due to a single variety of witchweed at $1 billion per year.
A published symposium in the journal Weed Science discusses several species of parasitic weeds, including witchweeds, broomrapes, dodders, and mistletoes, and the control measures being taken against them. The crops most affected by these weeds include corn, rice, sorghum, millets, and sugar cane.
Although parasitic weeds are problematic worldwide, eastern and southern countries of Africa are seeing major infestations of witchweed affecting crops. The same variety of this plant has been seen in the United States. In India, sorghum and sugar cane crops are being damaged by witchweed, although the impact is not as widespread.
Witchweed species depend on a host plant for their nutrition. They also reduce the process of photosynthesis in the host. Sorghum infested with witchweed may experience a 62 percent reduction in its photosynthetic ability. Plants show symptoms of drought stress—wilting and increased root development—when attacked by parasitic weeds.
In the United States, complicated control methods have helped bring these weeds to heel. These include the use of ethylene gas, methyl bromide, herbicides, and intensive quarantine and monitoring procedures. Partially resistant varieties of corn, rice, and sorghum have been developed, although they are not yet widely available. However, none of these methods can be easily transferred to African fields.
Partial control of these parasitic weeds has been achieved in some locations through cultural practices. The use of green manure—growing a cover crop then turning it under when green to add nutrients to the soil—has successfully suppressed these parasitic weeds. In Tanzania, legume crops turned into green manure have allowed rice fields to thrive.
Despite significant efforts over many years, parasitic weeds have not been reduced on a large scale. Most successful efforts at eradication have taken place on a local level. Current research is now trending toward genetically resistant crops. In Africa, increasing soil fertility, and particularly nitrogen for cereal crops, will build resistance to these weeds. Continued research and efforts are needed to maintain the world’s food and timber supplies.
Full text of the article, “Parasitic Weeds: A World Challenge,” Weed Science, Vol. 60, No. 2, March-April 2012, is available at http://www.wssajournals.org/
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About Weed ScienceWeed Science is a journal of the Weed Science Society of America, a non-profit professional society that promotes research, education, and extension outreach activities related to weeds; provides science-based information to the public and policy makers; and fosters awareness of weeds and their impacts on managed and natural ecosystems. For more information, visit http://www.wssa.net/ .
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