NSP - Cassava-related-diseases

Cassava-related diseases

Cassava production in the Great Lakes region is being seriously threatened by insect pests and diseases. The two major diseases are the Uganda variant of the East African cassava mosaic virus (EACMV-Ug) and cassava brown streak virus (CBSV). These are just the latest in a long line of disease strains affecting cassava (1983, 1993, 1997, 2004). Although timing is not regular, depending on biological events and conditions such as area under cultivation and climatic factors, major new diseases or strains of cassava disease tend to appear every 7–10 years.

These two diseases, spread by a whitefly vector (Bemisia tabaci) and the movement of planting materials, pose a severe threat to cassava production in many areas in the region. According to researchers, there has been a significant increase in the density of whitefly populations in recent years, to the extent that the whitefly itself has become a pest to the crop (causing damage to cassava leaves) as well as being a disease vector.

Maps showing the extent of spread of the various strains of cassava mosaic virus (CMV) and CBSV among the 15 targeted countries (see below) show that the epicentre for the newest strain of CMV has reached the southern parts of Burundi. The spread pattern is not regular and is not predictable, which suggests that movement of planting material is an important factor in the spread. This is particularly problematic in areas where stems are not of the highest quality and health status.

Leaves of a healthy cassava plant. Credit: FAO/G. Napolitano
The shrunken and discoloured leaves of a plant infected with CMV. Credit: FAO/G. Napolitano

Responding to cassava-related diseases

In the first half of 2009, FAO conducted a regional review of current efforts to combat cassava diseases. The review identified a number of underlying causes in the spread of diseases in the region, including: weakness in farmer education and information transfer; lack of institutional capacity on the part of government, specifically plant health and extension services; and technical challenges in disease identification and the development, release and multiplication of new improved varieties. More on FAO's review of current efforts to combat cassava diseases

The review suggested that the large scale replacement of susceptible varieties with tolerant or resistant varieties – the principal response to disease – may reduce the diversity of varieties grown over large areas, and render the cassava production system more susceptible to potential or emerging pest or disease outbreaks. It stressed the need for a combination of short- and long-term options to address the immediate needs of rural populations as well as to ensure long-term sustainability of the crop and preserve biodiversity. More on what FAO is doing to help strengthen the resilience of vulnerable cassava smallholders in the region

Harvesting healthy cassava. Credit: FAO/G. Napolitano
Root yield is dramatically reduced by CMV infection. Credit: FAO/C. Ferrand

Current cassava disease programmes

Given the severity of cassava diseases and the threat they pose to the food security of millions of people, a number of donor-funded cassava disease programmes have been operational in the region. In particular:

a) The FAO Regional Cassava Initiative, sponsored by the Humanitarian Aid Department of the European Commission, involves: (i) the multiplication and distribution of CMV‑resistant planting material in five worst-affected countries; (ii) support to country and regional data and information management; and (iii) the basis for improved consensus and coordination mechanisms. The project, implemented in two phases between 2006–2009, works by multiplying and distributing clean material to primary beneficiaries and then relies on subsequent farmer-to-farmer re-distribution. It is estimated that a minimum of 500,000 households have received improved vegetative material from nurseries in this programme. Furthermore, it is estimated that some 2,000 farmers, individually contracted or engaged in some form of Farmer Field School activity, now have the skills and knowledge to generateplanting material.

b) The Great Lakes Cassava Initiative (GLCI), funded by the Bill and Melinda Gates Foundation and implemented by the Catholic Relief Services addresses cassava disease in six countries. This programme (2008–2012) covers cooperative research into new varieties, surveillance in partnership with the IITA and also involves multiplication and distribution of materials, together with communication and farmer awareness activities. By 2012, the programme aims to have provided resistant planting material to over 1.1 million households and transferred skills on disease identification.

c) The United States Agency for International Development is sponsoring a cassava project, implemented by the Association for Strengthening Agricultural Research in Eastern and Central Africa (ASARECA). This project (2008–2012) focuses on: (i) developing knowledge on CMV and CBSV diseases; (ii) developing multiplication system for generating quality material of improved varieties; (iii) building capacity on cassava production; (iv) supporting the development of policies and standards for cassava; and (v) scaling-up processing technologies.

d) An International Institute of Tropical Agriculture (IITA) project entitled “Integrated protection of cassava from emerging insect pests and diseases that threaten rural livelihoods”, is meant to increase and sustain cassava productivity and improve livelihoods of farmers through the reduction of crop losses due to insect pests and diseases in sub-Saharan Africa (2007–2010). This project is sponsored by the International Fund for Agricultural Development.

e) The Common Market for Eastern and Southern Africa (COMESA) is also implementing a programme entitled Cassava Transformation in Southern Africa (CATISA). It aims to analyse and help accelerate cassava commercialization in southern Africa in order to help improve food security in the region. CATISA focuses on the rapidly growing commercialization of cassava – an integrated food staple market-shed in which cassava commercialization offers significant potential for improving food security in drought prone areas in five countries, the Democratic Republic of the Congo, Malawi, Mozambique, Tanzania and Zambia. The programme budget of USD 2 million was complemented with start-up funds provided by the Swedish International Development Cooperation Agency.

CBSV disease symptoms on cassava plants - leaves and stem show limited symptoms. Credit: IITA
CBSV disease symptoms on tuberous roots - damaged and unusable. Credit: IITA

Maps showing spread and distribution of cassava-related diseases

The map shows (purple) that the most severe variant (to date) of CMV, the EACMV-Ug, has spread from Uganda to devastate cassava production throughout the Great Lakes region, reducing cassava yields of affected farms by up to 80 percent, and the disease appears to be moving southwards. The spread of the disease has been accelerated by the transport of vegetative material by population displaced as a result of conflicts. Another effect of the conflicts in the Democratic Republic of the Congo and Burundi is that communities cut off from government services have been unable to organise large-scale, coordinated responses to the disease. The map (red) also shows the broader area which is affected by the previous strains of CMV, namely EACMV and the Africa cassava mosaic virus (ACMV).
CMV incidence (various strains) in eastern, central and southern Africa in 2008. Click to enlarge or scroll over the map for further information. Source: Adapted by FAO from various IITA reports.
The map illustrates the distribution of CBSV, a viral disease spread mainly through infected planting materials. Symptoms can affect leaves, stems and roots. CVSV is more difficult to diagnose. Definitive signs of root damage appear late, making early positive identification of the disease difficult. Again losses can be close to total in affected fields. Previously (from the 1930s), CBSV was known only in lowland and coastal east Africa (below 800 metres above sea level) and along the shores of Lake Malawi. Since 2004, there have been worrying reports of CBSV inland and at higher altitudes in Uganda, western Kenya and north-western Tanzania.
Distribution of CBSV in eastern, central and southern Africa in 2008. Click to enlarge or scroll over the map for further information.