Researchers in Nigeria are combining genomics and conventional breeding to improve the starchy staple crop.

IKENNE, Nigeria

“I like this one,” says Ismail Rabbi, placing his palm on a cassava plant and smiling coyly, like a parent picking favourites. “It doesn’t look impressive — it’s not tall,” he says, “but it beats all the obstacles we throw at it.”

Rabbi, a geneticist at the International Institute of Tropical Agriculture (IITA) in Ibadan, Nigeria, and his colleagues are on a mission to improve cassava (Manihot esculenta). Also known as yuca or manioc, its starchy roots provide food and income to more than 800 million people worldwide. In Africa, where consumption is highest, cassava plants bear smaller yields than their cousins in Asia and South America. But African varieties tend to be more tolerant of blights, such as the deadly cassava mosaic disease now spreading across Asia.

In November, Rabbi shipped five varieties of African cassava that resist the disease to Thailand, the world’s largest cassava exporter. He and his colleagues created the plants under the auspices of the US$62-million Next Generation Cassava Breeding project, which the UK government 

and the Bill & Melinda Gates Foundation launched in 2011. Project scientists are using genomic data to identify useful traits for breeding cassava varieties that will suit the world’s needs — safeguarding against starvation as the climate warms, populations grow and viruses spread.

When the African plants reach Thailand, scientists there will cross them with cassava varieties adapted to Asia. Then they will screen the resulting offspring for genetic markers that Rabbi and his colleagues use to predict a plant’s resistance to mosaic viruses, along with 12 other traits — such as leaf colour and the amount of edible starch in each root.

These genetic markers have helped the researchers in Nigeria to breed eight types of cassava that are now growing at test plots across the country. Scientists and farmers will compare them with the best existing cassava varieties in wide use.

On a blazing afternoon in November, Rabbi meanders through one such plot in Ikenne with Chiedozie Egesi, the leader of the Next Generation Cassava Breeding project and a geneticist at the IITA. Their conversations drift to agronomic and economic strategies that would help farmers to adopt the new cassava varieties, such as creating a market for the plants. “We aren’t just sitting in a room and making sure a plant works perfectly,” says Egesi. “We are bringing it to the places where problems arise, and always asking how we can make our science helpful.”

Root of the problem

Cassava is a mainstay for subsistence farmers on three continents because it survives in shoddy soil and weathers droughts, and its roots can be harvested at any time of the year. But there are wide geographical disparities in cassava yields. Varieties grown in Africa average 8.8 tonnes of usable root mass per hectare, compared with 13 tonnes per hectare in the Americas and 22 tonnes per hectare in Asia (see ‘A starchy staple’).

Researchers have long attempted to improve the situation for African farmers by importing Asian and South American varieties. But these foreign plants, which lack defences against African pathogens, have fared poorly. And breeding hardier hybrid varieties has proved challenging. It takes about five years of breeding to produce a worthy hybrid.

After each cross, breeders must grow the offspring for about a year and then assess the quality of the roots — which requires them to harvest, soak, dice and dry the experimental plants before weighing the starch that remains. Even then, a high-yielding plant might founder when exposed to harsh environments.

See: https://www.nature.com/articles/d41586-019-00014-2

Figure: Chiedozie Egesi, the director of the Next-Generation Cassava Breeding project, surveys a field of plants in Nigeria.Credit: Amy Maxmen