Adopt Uganda’s CBSD-resistant cassava to halt spread of the disease in your region, scientists tell West Africans

Richard Wetaya

February 11, 2023

The use of genomic selection has enabled Uganda’s breeding program to make significant progress, particularly in cassava breeding.

Genomic selection is a well-established technology linked to improved genetic gains in plant and livestock breeding and shorter breeding cycles.

Uganda’s National Crops Resources Research Institute (NCRRI) was one of the first agricultural research institutes in sub-Saharan Africa to use genomic selection to develop virus-resistant and high-yielding cassava clones for routine breeding of economically and agronomically important end-user traits.

Cassava is a staple food for 800 million people in Sub-Saharan Africa.

It is the continent’s most produced cash crop, according to Visualcapital.

In 2019, Africa ostensibly produced 192.1 metric tons of the crop, accounting for 63 percent of total global production.

Many of the continent’s subsistence farmers grow cassava for food, as a cash crop for local markets, and on larger farms for animal feeds and starch processing.

Over the years, NCRRI’s promising bean breeding initiatives and cassava mosaic and brown streak disease-resistant varieties have piqued the interest of several plant scientists eager to learn about Uganda’s blueprint.

Scientists from Nigeria’s National Roots Crops Research Institute, the International Institute of Tropical Agriculture, and Makerere University now say Uganda’s blueprint should be benchmarked in West Africa where a brown streak disease outbreak is imminent, according to recent epidemiological studies.

In a recent study conducted in Uganda, the scientists recommended that Uganda shares its cassava brown streak disease (CBSD) resistant cultivars with West Africa for hybridization to enable the development of genomic selection as a CBSD breeding approach for the region.

Recommendations are implemented

Uganda’s cassava-breeding program, part of the Next Generation Cassava Breeding Project which focuses on cassava mosaic and brown streak disease resistance and end-user root quality attributes, has produced three recurrent genomic selection cycles.

By all accounts, Nigeria, the world’s largest producer and consumer of cassava, and other producers of the tropical root crop such as Ghana and Ivory Coast, will benefit if the researchers’ recommendations are implemented.

Scientist Paul Kuria shows a disease-free cassava tuber and one infected with CBSD. [File]

“There is evidence of the brown streak disease spreading towards West Africa, especially Nigeria. Therefore, precautionary measures through sharing resistant or tolerant varieties with West Africa is critical to avert feature disease impacts,” said Alfred Ozimati, a plant breeder at the NCRRI and one of the study’s authors.

“West African cassava-breeding programs need strong partnerships with programs that are actively developing cassava varieties resistant to the brown streak disease.”

Brown streak disease frequently manifests as mottling of cassava leaves, brown streaks on stems, and hard rots on roots.

It usually results in the complete spoilage of the cassava harvest and other significant quality reductions.

Weak quarantine measures

Falidah Kayiira, a smallholder farmer in Buikwe, Central Uganda, with her workers after harvesting cassava. [Richard Wetaya]

Dr Robert Kawuki, a cassava breeder and the principal research officer at the NCRRI agrees that Uganda’s cassava CBSD-resistant cassava cultivars would help West Africa if there is an outbreak.

“Uganda and East Africa in general have a lot of capabilities and research knowledge on the disease that can help West Africa in the event of an outbreak. The genomic selection novel approach (which undertakes predictions) and which we used for the study revealed that all West African cassava germplasm was very susceptible to the disease,” Kawuki said.

“The disease’s spread to West Africa is imminent owing to weak quarantine measures. The projections are that if it gets there, all cassava plants will be wiped out and the cassava value chain will significantly reduce.”

Genetic selection is however not new to West Africa. The first cassava cultivars resistant to cassava mosaic disease, developed using genomic selection technologies under the auspices of the Next Generation cassava project, were released in Nigeria in 2020.

Since its inception in 2001, genomic selection has advanced at a rapid pace.

As a research strategy, it has sparked a revolution in the fields of animal and plant breeding and gained popularity due to its ability to overcome the shortcomings of traditional breeding methods.

The Ugandan study, however, discovered that no cassava varieties with resistance to brown streak disease exist in West African cassava germplasm.

“That worries me because most smallholder farmers lack knowledge on how to combat pests and diseases that attack their cassava crops, yet it is a crop they rely on and which contributes significantly to food security in West African households,” said Frieda Tiako, a smallholder cassava farmer from Cameroon’s Northwest region.

“If plant scientists and researchers from our region collaborate  with Ugandan researchers, I am optimistic the brown streak disease will be kept at bay.”

Some farmers in Nigeria say their crop is already getting affected, and there is a need for disease-resistant cultivars.

“The mosaic disease is already causing yield losses for many cassava farmers in the region. They do not need another disease that will destroy their crops,” said Sarah Adetutu, a farmer from Abia in Nigeria’s southeast.

“I encourage our plant scientists and researchers to work even more closely with East African plant scientists to develop CBSD-resistant varieties.”

Primary biotic stressors

Many plant breeders on the continent agree that more progress is needed in germplasm development for cassava than for any other crop.

“This is because, despite its importance as a food security crop, average yields on the continent remain low in comparison to yield averages in Asian countries such as Thailand, which is the world’s largest exporter of cassava products and the third largest producer of cassava, producing approximately 29 million metric tons per year,” Dida Opio, a Ugandan plant scientist, explained.

According to recent research, cassava mosaic and brown streak diseases are the primary biotic stressors that contribute to the continent’s cassava low yields.

“While the crop is climate-resilient and can withstand high temperatures and long periods without rain, increased public and private investment in crop genomic selection research and biotechnology is required to enable more genetic gains and to develop other plant germplasms that are climate change adaptable,” Opio said.