The Need: protection from cassava diseases

 

Plant viral diseases can destroy up to 100 percent of a cassava crop yield, threatening livelihoods and leading to hunger. Cassava varieties that are resistant to both diseases could dramatically improve the crop’s ability to feed and provide income for smallholder farmers in Africa.

Cassava production in Africa is limited by two viral plant diseases: Cassava Brown Streak Disease (CBSD), which destroys the edible roots even when rest of the plant looks healthy, and Cassava Mosaic Disease (CMD), which can stunt plants to varying degrees or kill them completely. These two diseases are often found together in the same field and can devastate entire crops. Both diseases are spread by white-flies (high populations of which cannot be controlled effectively with pesticides) and by infected cassava stem cuttings shared among farmers.

CMD Leaf Symptoms

Mottled, misshapen & twisted leaflets with an overall reduction in size of leaves

CMD Root Symptoms

Severe decrease in root weight resulting in lower yields

CBSD Leaf Symptoms

Severe chlorosis & necrosis on leaves, giving them a yellowish, mottled look

CBSD Root Damage

Severe yellow/brown necrosis with a cork like appearance

CMD has been damaging farmers’ fields across Africa for decades. Over the last 15 years, CBSD has also spread rapidly and now affects cassava production across East and Central Africa. It threatens to move into West Africa where it would have devastating effects on cassava production and food security in Nigeria, Africa’s most populous country and largest cassava producer. As a result, CBSD has been defined as one of the seven most dangerous plant diseases in the world due to its ongoing and potential impact on food and economic security.

Conventional plant breeding has been used successfully to develop cassava varieties that are resistant to CMD. But plant breeders have been less successful with CBSD, thus far developing varieties that are merely tolerant to the disease. CBSD tolerant cassava plants may still show some virus symptoms and suffer damage from the disease – and may cause the disease to spread further.

 

Our work: developing cassava varieties that will be resistant to both Cassava Brown Streak Disease (CBSD) and Cassava Mosaic Disease (CMD)

 

Our researchers are using genetic engineering to introduce a small component of the Cassava Brown Streak Virus into cassava to make the plants resistant to CBSD by a mechanism called RNA interference (RNAi). RNAi has been used to develop a number of disease resistant crops, most notably saving the papaya industry in Hawaii from the devastating Papaya Ringspot Virus Disease.

We have successfully developed cassava with strong and stable resistance to CBSD using genetic modification techniques and conducted several confined field trials in both Uganda and Kenya, with the approval, oversight and guidance of government regulators.

Cassava varieties that are resistant to CMD have already been developed using conventional plant breeding, so the VIRCA Plus project is undertaking additional breeding work to combine those CMD resistant varieties with transgenic CBSD resistance. Field development and testing of these new varieties is ongoing in Kenya and Uganda to ensure that they control both plant diseases while producing good yields and maintain farmer preferences for taste, texture, processing and storage practices.

When both cassava virus disease resistance traits are combined and bred into VIRCA Plus varieties, and fully assessed for safety, farmers will be able to grow cassava with specific resistance to both diseases for the first time. Kenya, Uganda and other East African countries are expected to be the initial markets for disease resistant VIRCA Plus varieties.