02:07am Thursday 28 May 2020

Helping to feed the world

By Brian Clark, College of Agricultural, Human and Natural Resource Sciences

Rayapati, right foreground, conducts a workshop for African scientists on diagnosis
of cassava mosaic disease at the International Institute of Tropical Agriculture, Nigeria,
to transfer technologies generated in Rayapati’s lab at WSU.
PROSSER, Wash. – The Nigerian poet and novelist Flora Nwapa calls it “Mother Cassava.”
A fundamental staple in the diet of nearly a billion people, the cassava plant produces a root that is processed in many ways to produce foods and beverages. After rice and beans, cassava is the most important subsistence crop in the tropical regions of South America, Asia and Africa.
In many parts of Africa, cassava’s hardiness allows it to be grown on land unsuitable for the cultivation of cereals or other staple crops. African farmers like cassava because, on a per acre basis, it produces higher yields than other crops, thus assuring that the family has food and income.
Detecting disease in cuttings
Unfortunately, cassava mosaic disease, or CMD, threatens the crop. The disease has been the cause of at least one serious famine in Africa.
CMD causes leaves to become twisted, misshapen or not to develop at all. Reduced leaf area in turn reduces the size of tubers.
In Africa, seven distinct viruses spread by whiteflies and via vegetative cuttings cause CMD. Cassava is propagated via cuttings from existing plants, so if the parent plant has CMD, so too will the daughter plants.
“Part of the problem with managing CMD has been in accurately detecting these viruses,” said Naidu Rayapati, a plant virologist based at WSU’s Irrigated Agriculture Research and Extension Center in Prosser, Wash. “If these viruses can be detected in vegetative cuttings, it should be possible to quarantine contaminated plant material and supply farmers with clean cuttings for new plantings.”
Grad student helps apply WSU work for Africa
Rayapati has long been involved in the business of insuring that growers use clean, healthy planting stock. In Washington, he leads a project working to contain the spread of grapevine leafroll, a complex virus disease that damages valuable wine grapevines.
Rayapati and his Nigerian graduate student, Olufemi J. Alabi, collaborated with Lava Kumar, a virologist at the International Institute of Tropical Agriculture in Nigeria, to address the problem of CMD detection in cassava. A U.S. Agency for International Developmet (USAID) Linkage Grant funded the project.
Safer, cheaper, localized testing
“Previous methods of virus detection in plant tissue required commercial kits that were expensive and that involved handling toxic or carcinogenic materials,” Rayapati explained. The special training and facilities required to handle the materials, coupled with the expense, made it extremely impractical to use on a wide enough basis to effectively help manage CMD in African countries.
“What we did is replace the dangerous materials used in the extraction of plant tissue with safe ones which are also cheaper,” Rayapati said. “When you don’t have facilities or protocols for handling the dangerous stuff, you really need this sort of alternative.”
Rayapati had previously helped develop a method whereby plant samples could be easily and inexpensively transported from remote agricultural areas in Asia and Africa to laboratories capable of conducting analysis. Now, though, the testing can be done at regional hubs, thus speeding up the process of detecting the presence of disease-causing viruses.
“What we essentially did was transfer the technology to the people who really need it,” Rayapati said.
No need to purify plant material first
The modified testing technique not only replaced the expensive and toxic ones with materials that are safe and cheap. It also combined separate tests for individual viruses into one test, further reducing the cost of detecting CMD-causing viruses.
“We essentially adapted a protocol we developed here in Prosser that we were using to detect the presence of viruses in grapes,” Rayapati said.
Typically, plant material must be purified in particular ways in order to reduce the volume of material that has to be screened for the presence of viruses.
“We thought, why eliminate all the ‘junk?’ That’s just another added expense,” Rayapati said. “The test is quite specific in that it looks for particular molecules that signal the presence of disease-causing viruses in cassava. It’s like looking for a needle in a haystack. Even if it’s surrounded by hay, you will know when you sit down if there is a needle in there.”
More work to be done
Rayapati is quick to point out that having an inexpensive means of detecting the viruses that cause CMD is only a first step, albeit a critical one. CMD is transmitted by whitefly, he said, so new plantings need to be done when the whitefly population is low as well as with clean plant material.
“But there is no ‘one size fits all’ solution,” he said. “There has to be an adaptable and flexible strategy that combines the ancient farming practices of the people with modern ones developed by researchers.”
WSU News, Washington State University, Pullman WA 99164-1040 | (509) 335-3581 |

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