Great strides are being made in improving this important staple through conventional breeding Prof Peter Saunders
Cassava is a food that most people in a country like the UK never think about. It’s easy enough to find, at least if you live in a city with a large immigrant population, but it has not become part of the British diet in the way that so many other exotic foods have. And it tends to be ignored in discussions about how to feed a world facing increases in both temperature and population.
On a global scale, however, cassava is very important. It is a staple food for more than a tenth of the world’s population, and in tropical countries it is the third source of calories after maize and rice . In 2011, Africa produced about 145 million metric tonnes of cassava , more than double the amount of maize.
Cassava is a perennial that can grow on poor soils. It is tolerant to drought and high temperatures. Once the tubers start to bulk, about eight months after planting, they can be left in the soil for several months before harvesting, which makes it a good hedge for the subsistence farmer against the failure of other crops.
It is often cooked in much the same way as potatoes, but it can be ground into flour for making bread. Some countries make beer from it. Another product is tapioca, probably the only form in which most Europeans have consumed it. Cassava is also used for making industrial starch and to replace cereals as a source of bioethanol.
Cassava has a number of drawbacks. The most obvious is that the roots and leaves contain cyanide and must be properly processed before they are consumed. What sort of processing is required depends on the variety. This is a particular hazard for malnourished people who lack the sulphur-rich amino acids that promote cyanid detoxification in the body, and for poor people in cities who can suffer because producers are tempted to shorten the fermentation from the traditional four days to two, which may not be enough to complete the detoxification. And while cassava keeps well as long as it is the ground, the roots are highly perishable once they are harvested. In addition, while cassava can produce food calories at a rate of 250 000 per hectare per day - the rates for maize, rice and wheat are 200 000, 176 000 and 110 000, respectively  - it is low in protein and in many other nutrients. It is, however, relatively rich in Vitamin C, as are potatoes.
In the past cassava has received far less attention than other staple crops. That’s probably because most scientists do not work in regions where it is as important as crops like maize, rice and wheat. Another factor may be that it reproduces vegetatively, which makes breeding new varieties more difficult.
In 2000 the Global Cassava Development Strategy (GCDS) was agreed at a meeting of the FAO (Food and Agriculture Organization of the United Nations) . This led to the founding in 2003 of the Global Cassava Partnership for the 21st Century (GCP21). The aim is two-fold: to combat pests, especially viruses, and to improve the yield and nutritive value of the crop .
Cassava has always been subject to attack from viruses and pests. The most serious current threat is Cassava Brown Streak Disease (CBSD), which has become much more prevalent over the past ten years . CBSD can destroy an entire crop without giving any warning: the leaves of infected plants can look quite normal. It is only when the tubers are taken from the ground that the damage becomes apparent.
The other major disease is Cassava Mosaic Virus Disease (CMD). This has been a problem for over a century, reducing the total harvest by a minimum of 50 million tons. During the first decade of this century, CMD resistant varieties of cassava were developed and distributed. By the mid-2000s half of all cassava farmers in large parts of Central and East Africa were growing the new varieties.
In the past few years, rising temperatures and shifting rainfall patterns in Africa have led to concern about the viability of maize and wheat, and increased interest in cassava, which is more tolerant of such conditions. But rising temperatures have also contributed to a huge increase in the population of the whitefly, which transmits the viruses that cause CMD and CBSD. The spread of these diseases has been speeded up by farmers planting new fields with infected stem cuttings .
Work is now under way to breed high-yielding, disease resistant plants drawing on resources from South America, where cassava originated. There are also plans to replace infested plants with virus-free planting material of resistant cultivars, progressing village by village.
Vitamin A deficiency is a serious problem in sub-Saharan Africa. In 2011, the Nigerian government announced the release of three new beta-carotene enriched varieties of cassava. These are being multiplied through stem cuttings. It is estimated that by 2014 more than 150 000 farmers in Nigeria will be growing beta-carotene rich cassava and the government’s Agricultural Transformation Agenda includes a plan to disseminate it to millions of farmers over the next several years [7-9]. This important development is never mentioned by the GM supporters who want to rush the controversial Golden Rice (see  The Golden Rice Scandal Unfolds, SiS 42) into production. They also ignore the development and increased use of varieties of sweet potatoes with increased beta-carotene content .
The international Centre for Tropical Agriculture (CIAT) in Colombia is developing “asparagus” cassava, so called because it is slim and stands up to three metres high, whereas ordinary cassava plants are squat and bushy. Asparagus cassava can be planted closer together and so yield more food per hectare .
It has been known for a long time that high protein cassava can be obtained from hybrids with wild species and indigenous cultivars . This work continues in the laboratory of Nagib Nassar in the Universidade Brasilia in Brazil . The varieties tend to have amino acids that are lacking in most cassava and also contain less cyanide.
These improvements, like the development of CMD resistant cassava, have all been accomplished by conventional breeding, though sometimes with the use of modern techniques such as marker assisted selection. Other researchers are, however, working to develop GM varieties, though so far with limited success. In January 2011, researchers at the Donald Danforth Plant Science Center in St Louis in the US and Mayaguez University in Puerto Rico published a paper claiming to have created cassava that expressed a gene called zeolin that increased the protein content by 12.5% . The paper was retracted later in the year ; according to the Danforth president James Carrington “the materials published in the paper were not as described and the materials that were described could not be found.”
Article first published 01/07/13
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Todd Millions Comment left 12th July 2013 13:01:22
I recall reading decades ago that cassava,due to its posionous roots,was well able to protect bacteria,and fungus of the free ranging types,and so-in unharvested multi year stands and like dandilion-could be regarded as legumues,as some of the cultures thriving with the undisturbrd roots,were nitrofen fixing.Last I did a google scan,no references to this pre interweb(bless the sainted Al Gore)work turned up.One wonders if the posionous when un processed feature was a diliberate breeding goal by early agromonists-several other very old staple crops seem to have started as poisionous in raw state,How far back does evidence exist for this roots domestication?
EULU Comment left 5th August 2013 23:11:03
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Ramsey Affifi Comment left 20th November 2013 16:04:25
It should also be mentioned that young cassava leaves and flower buds are very high in vitamin a and contain much more protein than the tubers. These are traditionally eaten in many cassava producing countries, are delicious, and provide a perennial leafy-green vegetable source with very little work. In fact, simply by lopping a few branches, farmers can easily induce a new sprouting of young leaves, even in the dry season.