The same healing response that protects living cassava roots from injury by sealing wounds is to blame for the rapid deterioration of cassava tubers after harvest. This response is not switched off after the roots are separated from the main plant, so it takes just two days for raw tubers to completely perish and become unedible.
- Cassava (Manihot esculenta) is a hugely important crop in Africa: 200 million people in the continent rely on this crop. In Sub-Sahara Africa, cassava can represent up to 60% of the daily calorie intake. Cassava production in Africa was worth US$ 12.6 billion in 2010 (FAO), and is largely consumed locally.
- Cassava grows in relatively poor soils with limited labour requirements and it tolerates drought. The root system can penetrate below two meters of soil enabling cassava plants to obtain water if it is available deeply. Cassava requires relatively little farming and inputs, and therefore cassava plants often survive where other crops would fail. Cassava is mostly propagated by cuttings.
- The roots can be stored in the ground for several months before harvest although once dug up they need to be used since they perish very rapidly.
- In addition to being a very important food crop, because of its high starch content cassava is also highly versatile: industrial applications include processed foods, confectionery, sweeteners, glues, plywood, textiles, paper, biodegradable products, monosodium glutamate, beer and drugs.
Threats and challenges
The main diseases affecting cassava in Africa are cassava mosaic virus, cassava brown streak virus, cassava bacterial blight, cassava anthracnose, and root rot. In Africa, mealybugs (Phenacoccus manihoti) and cassava green mites (Mononychellus tanajoa) are important cassava pests. Pests, diseases and poor agricultural practices cause losses up to 50% in all of Africa. Average cassava yields in the tropics reach barely 20% of the yields obtained under optimal crop management conditions.
Combating the foes
A dual viral epidemic threatens cassava in Africa
Cassava mosaic disease, caused by a group of viruses, affects all cassava growing regions of Africa. It accounted for losses estimated at 47% of East and Central Africa’s cassava production during a serious outbreak in the 1990′s. Cassava brown streak disease, also caused by viruses, was initially confined to coastal, low altitude areas in East Africa, but since the mid-2000s the disease has spread rapidly, affecting Tanzania, Uganda, Kenya, Rwanda and Burundi. The disease has already infected around 80% of crops in Uganda and around 20% of crops in Rwanda and Burundi. The leaf symptoms of cassava brown streak disease are relatively mild but the disease is devastating because it affects the tubers: brown dead areas make the roots unsuitable for consumption. In addition to the devastating economic losses (the entire crop may be wiped out), the disease also has a huge social impact since farmers who are mainly women have to spend a long time cleaning cassava roots in the effort to save parts for eating.
The root of the problem
Determining the exact extent of the problem is the first step: which areas are affected, and how badly? Scientists working at the International Institute for Tropical Agriculture (IITA) have partnered with Catholic Relief Services (CRS), national research systems and local NGOs, the UK’s Natural Resources Institute and Food and Environment Research Agency, in the Great Lakes Cassava Initiative (GLCI). This was a multi-million $US dollar programme to tackle cassava virus diseases in East and Central Africa (Burundi, DR Congo, Kenya, Rwanda, Tanzania and Uganda) supported by the Bill and Melinda Gates Foundation. One of its most important components has been the regional surveillance of the cassava mosaic and cassava brown streak pandemics. This has helped to show exactly where these two diseases are spreading and where the damage is greatest which is vital in prioritizing improvement efforts.
A clean start
Since cassava plants are mostly propagated by cuttings, diseases affecting the parent plant will also be present in any derived planting material. Hence, efforts have been critical to ensure that new cassava planting material supplied to more than 1.2 million farmers by the GLCI is free of virus. Since it is crucial to determine whether planting material is infected or not, the Mikocheni Agricultural Research Institute (MARI), Tanzania, is one of the institutions that have developed diagnostic kits for selecting healthy material for propagation by tissue culture and for monitoring the spread of the disease.
Breeding for double virus tolerance
Since brown streak and cassava mosaic occur in the same localities in East Africa, cassava varieties with double resistance are required. Scientists are screening different cassava varieties to select new sources of resistance to viral diseases that could be used in conventional breeding crosses. Plant breeders at IITA and its research partners have developed and distributed virus resistant cassava varieties in response to the outbreak, and these varieties are now used in most cassava-growing countries in Sub-Saharan Africa. However, all cassava varieties are susceptible to cassava brown streak disease and some of the most susceptible varieties are those that have been bred for resistance to the other major viral disease: cassava mosaic disease. Scientists cannot therefore rely on conventional plant breeding alone to combat cassava brown streak disease.
A transgenic approach to lend a hand
Scientists are also exploring genetic modification as another line of defence. The Virus Resistant Cassava for Africa (VIRCA) project is a collaboration between the Donald Danforth Plant Science Center, United States, the Kenya Agricultural Research Institute (KARI), and the National Agricultural Research Organisation (NARO), Uganda. The first product of this initiative will incorporate resistance to cassava brown streak disease in the popular cultivar TME204 which has natural resistance to cassava mosaic disease and is favoured by farmers in Uganda and the Lake Victoria region. A second product will be developed in the highly popular, traditional Ebwanateraka cultivar with resistance to both viral diseases. Ebwanateraka has been virtually wiped out in Uganda since the double disease outbreak.
Transgenic cassava varieties resistant to both cassava mosaic virus and cassava brown streak virus are also being developed by scientists working in ETH Zurich, Switzerland, as part of an international consortium for cassava improvement.
The resistance engineered into the plants is based on a broad natural defence mechanism that protects them against viruses. When a plant cell is infected by a virus it produces short molecules of RNA (known as small interfering RNA – siRNAs) which can act as triggers for virus resistance. Specific siRNAs introduced into transgenic plants hence work effectively as a vaccine.
African laboratories develop own transgenic cassava varieties
Several African research institutions have developed cassava genetic transformation platforms using transformation technologies developed at ETH Zurich: Biosciences eastern and central Africa (BecA, Nairobi), University of Witwatersrand (Johannesburg) and Mikocheni Agricultural Research Institute (MARI, Dar es Salaam).
A joint effort between scientists from the University of the Witwatersrand (South Africa) and ETH Zurich resulted in the first production of a transgenic industry-preferred cassava cultivar by an African laboratory.
Scientists at MARI, Tanzania in collaboration with the Danforth Center have also started their own project to develop transgenic cassava plants resistant against both viruses using siRNA sequences specific to virus isolates (genetic variants) found in Tanzania.
Biocontrol methods: your enemy’s enemy is your friend
Both cassava mosaic virus and cassava brown streak viruses are transmitted by whiteflies (Bemisia tabaci) which pick up viruses when feeding on plants and then propagate the infection as they feed on healthy plants. Hence another line of defence against the viral pandemic is to reduce the numbers of whiteflies and limit the damage they do to cassava plants. Scientists at IITA are characterizing the natural enemies of whiteflies on cassava to investigate the potential for increasing their population numbers and use them as a biological control method. The identification of cassava varieties that are less susceptible to whitefly attack is also critical. These varieties would limit the spread of the virus and also reduce the physical damage whiteflies inflict on the cassava plant during feeding which both reduces yield and increases susceptibility to other pests and diseases.
Improving cassava for nutrition and health
Cassava’s main limitations include the low nutritional value of tubers which provide mostly carbohydrates but very little protein and vitamins (although cassava leaves are high in protein and vitamin A content, and are eaten as leafy vegetables in West and Central Africa). A typical diet based on cassava provides less than 30 % of the minimum daily requirement for protein and only 10-20% of the required amounts of iron, zinc, vitamin A, and vitamin E. Bitter cassava varieties also contain cyanogenic glucosides and are poisonous unless the tubers are properly prepared for eating. A multidisciplinary team of scientists has joined forces in the BioCassava Plus consortium to develop more nutritious cassava varieties (with increased zinc, iron, protein, and vitamin A levels), reduced levels of cyanogenic glucosides and improved storage qualities of tubers after harvesting. This “super cassava” is being tested in Nigeria by the National Root Crop Research Institute, Umudike.
We thank James Legg for information and comments.