Combating the threat of CBSD

Cassava brown streak disease (CBSD) is a virus disease that has emerged as a serious threat to production in Eastern and Southern Africa.

Brown streak-affected cassava. Photos by L. Kumar, IITA.
Brown streak-affected cassava. Photos by L. Kumar, IITA.

Two virus species, Cassava brown streak virus and Cassava brown streak Uganda virus, the cassava brown streak viruses or CBSVs, have been recognized to cause CBSD. The infection results in mosaic symptoms on leaves, brown streaks on stems, and a corky necrosis in tuberous roots.

Root necrosis has the most damaging effects on the use and marketability of the tubers and thus affects the livelihoods of cassava farmers. It can make susceptible varieties unusable if the roots are left in the ground for over 9 months.

CBSVs are spread through the planting of infected stem cuttings and also by a vector, a whitefly, Bemisia tabaci. The foliar symptoms of CBSD are less conspicuous and farmers are often unaware of the problem until they harvest the roots and the corky, yellow-brown necrotic rot becomes evident.
There is no cure for the disease. Once plants become infected, the only option for growers is to uproot and destroy them. The use of virus-free planting material and the cultivation of resistant varieties are the only options for the control of CBSD.

Where is it and where is it heading?
CBSD is endemic in Kenya, Malawi, Mozambique, Tanzania, and Uganda and its occurrence is suspected in Burundi, Gabon, Madagascar, DRC, and Rwanda. Available evidence suggests a westward spread of the disease.

What is IITA doing about it?
IITA has adopted a multipronged strategy to tackle CBSD, to reduce the effects on cassava in epidemic areas, and prevent a further spread of the disease. Its efforts begin with informing governments about the threat. The four technical pillars of this strategy are as follows.

-Monitor disease spread and assess its impact: Key outputs include (a) the development of disease distribution maps, (b) estimates of yield loss, and (c) identification of targets for development.

-Understand disease etiology and epidemiology; develop tools for monitoring and forewarning: Key outputs include (a) understanding the effects of the viruses in cassava, (b) examining the characteristics of virus spread, (c) creating diagnostic tools for CBSVs, and (d) using digital-enabled field surveillance tools for real time reporting and a monitoring network.

-Develop and disseminate durable CBSD-resistant cassava cultivars: Key outputs include (a) screening and selecting over 40 elite cassava cultivars with dual resistance/tolerance to CBSD and cassava mosaic disease (CMD) appropriate for various countries, (b) deploying tolerant varieties for farmers to cultivate in East Africa, (c) developing molecular markers and modern molecular breeding tools for the accelerated development of CBSD-resistant varieties, (d) pre-breeding in areas currently not affected by the viruses, and (e) developing clean seed systems for the multiplication and dissemination of virus-free planting material.

-Capacity building through the transfer of knowledge, technology, and products to stakeholders: IITA has (a) built a coalition of international teams to combat CBSD, (b) trained scientists, extension workers, and plant quarantine officials in disease recognition, monitoring, and diagnostics, (c) established regional diagnostic labs, (d) created awareness through the use of the mass media, and (e) provided technical backstopping to national efforts in combating CBSD.

A suite of knowledge, technologies, and products derived so far from IITA’s R4D efforts is playing a vital role in checking the spread of the disease and has contributed to reviving cassava production in areas affected by the epidemic. However, complete recovery and the prevention of any further spread of CBSD are still a long way off. They require a strong commitment from national and international communities to sustain the ongoing and emerging research and development efforts that are devising effective and eco-friendly technologies for sub-Saharan Africa.

Prof Mike Thresh, Scientist Emeritus, Natural Resources Institute, UK (right), during his visit to IITA-Ibadan. Photo by L. Kumar, IITA.
Prof Mike Thresh, Scientist Emeritus, Natural Resources Institute, UK (right), during his visit to IITA-Ibadan. Photo by L. Kumar, IITA.

Advice to stakeholders
In countries where CBSD is already established, IITA recommends that governments require the use of available CBSD control programs, including the adoption of promising CBSD-resistant cultivars, and the production and distribution of clean cassava planting material.

Countries not yet affected need to increase their vigilance and develop the capacity to recognize CBSD and deploy eradication programs; establish plans for preemptive action to reduce the risk of CBSD spreading from affected regions; and put in place programs to produce and distribute clean planting material.

All the cassava-producing countries in Africa should:

-Organize large-scale awareness creation programs to inform farmers, extension workers, CSOs, and national research entities about CBSD, the eradication of infected plants, and the steps for disease control.
-Strengthen the monitoring capacity of the national quarantine authorities and other relevant bodies including the establishment of communication systems for a rapid response to prevent disease and eradicate infections where they are identified.
-Develop resistant varieties most urgently, through breeding, using both conventional and transgenic approaches.
-Put in place a strategy for the production and distribution of clean cassava planting material, and adopt improved varieties with resistance to CBSD and CMD.
-Affirm financial and political support for collaboration, cooperation, and coordination to prevent the further spread of CBSD in tropical Africa.