The Need: protection from cassava brown streak disease

Cassava growers across Africa face major challenges from pests and plant diseases, especially cassava brown streak disease (CBSD).

Although the rest of the plant may look healthy, storage roots produced by plants infected with CBSD develop brown lesions.  They are inedible as food and feed and valueless in the marketplace.

In severe infections, CBSD can result in up to 100% loss of usable storage roots, threatening livelihoods and leading to hunger.

CBSD is spread between plants and farms by whiteflies which are difficult to control with pesticides.  The disease is also spread unintentionally by farmers who share and plant infected cuttings. Thus far, breeders have not been able to develop varieties with natural resistance to CBSD.

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

Over the last 15 years, CBSD has 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 is considered as one of the most dangerous plant diseases in the world due to its ongoing and potential impact on food and economic security.

Cassava varieties that are resistant to CBSD could dramatically improve the crop’s ability to feed and provide income for smallholder farmers in Africa.

Our work: developing, testing and delivering CBSD resistant cassava to farmers

VIRCA Plus researchers have used conventional plant breeding and modern biotechnology tools to develop CBSD resistant cassava.

A small part of two viruses that cause CBSD has been introduced into the cassava plant using tools of modern biotechnology. This enables the plant to activate RNA interference (RNAi) a naturally occurring defense mechanism prior to exposure (arrival of the pathogen) and resist development of the disease.

CBSD resistant cassava was evaluated over a period of five years in more than 20 confined field trials (CFTs) across six different locations in Kenya and Uganda. It showed high and stable defense against CBSD, a disease that can result in up to 100 percent loss of usable storage roots in severe infection.

At the same time, conventional plant breeding combined CBSD resistance with resistance to another important cassava disease, cassava mosaic disease. Field development and testing of new varieties took place in Kenya and Uganda to ensure control of both plant diseases, good yields and maintain farmer preferences for taste, texture, processing and storage practices. Gender-inclusive farmer participatory evaluation and selection of new varieties continues.

In June 2021, the Kenya Agricultural and Livestock Research Organization (KALRO) received approval from the National Biosafety Authority (NBA) for environmental release and placing on the market of cassava that is resistant to CBSD. The approval was based on a rigorous and thorough review of food, feed and environmental safety assessment data, a range of socio-economic issues, and public participation and comment activities.

Current status of our work

In Kenya, KALRO will conduct national performance trials (NPTs) of varieties. National Performance Trials are a routine requirement for all new crop varieties and an essential step in demonstrating value for cultivation and use prior to registration and release to farmers.

At the same time, VIRCA Plus partner MEDA – Mennonite Economic Development Associates is developing a system to multiply, deliver and steward the new disease-resistant cassava varieties to Kenyan farmers once they are registered and fully approved by regulatory authorities.

CBSD resistant cassava is also under testing in Rwanda.