African cassava threatened
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CTA. 1986. African cassava threatened. Spore 4. CTA, Wageningen, The Netherlands.
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The memory of the cassava failure of 1983 is still very much alive.
In Congolese villages tucked away in the tall grasses of the savanna in the Kukuya region north of Brazzavillo, the memory of the cassava failure of 1983 is still very much alive. The spectre of famine hung over these villages that year because the entire crop was lost The peasant farmers stood by helplessly as their subsistence crop was devastated. They watched as the young leaves withered and the branches died. Insidiously, brown spots grow on the forgo leaves which become limp and started to droop. The contamination of the cossovo fields by vascular bacteria hod been overwhelming. Confronted with this plague, which ten years earlier was totally unknown, the powerlessness of the local people was all the more desperate as they had become the main agents in the spread of the disease. Unable to tell the difference, they planted cuttings from sick plants in a hopeless effort to save their crops. These Congolese farmers are not the only ones facing this problem. In most African countries their counterparts fear for the future of this crop which, for many, is the principal source of food for their families. Vascular bacterium is only one of the many diseases that threaten cassava. Others include the African Mosaic Disease (AMD) which has been endemic for a long time; only recently, however, has the full extent of its destructive impact been recognized. Production losses in certain cases can be as high as 90 %. Some consider this disease to be the most important and do not hesitate to compare it to the infamous potato blight which, over a century ago, caused widespread famine throughout Europe. Two other pests made their appearance about 15 years ago: the green spider mite and the cassava mealybug which was first observed between 1970 and 1973 in Zaire and the Congo. It was subsequently identified in Angola, Gabon, Nigeria, Benin and Senegal. The green spider mite represents one of the more dangerous threats to cassava in Africa, spreading out from East Africa where it was first noticed. Both of these insects were accidentally introduced from Latin America and in the absence of native predators, they spread rapidly. By 1985, over 17 countries had been infested covering an area larger than that of the United States. To date, the damage to cassava crops caused by these pests is estimated at 1.8 billion dollars. The symptoms observed on vegetatively propagated plants infected by Cassava Bacterial Blight (CBB) are characterized by the withering of young leaves followed by branch dieback. Secondary infections occur when the bacteria on the foliage penetrate the plant through the lamina and stomata, resulting in angular spots on the leaves. At first quite small, the spots grow and turn brown and are surrounded by a burned band and the leaves become limp and then droop. The vascular infection can be detected by gum excretions on the leaf stalks and new stems. It then spreads to other stems and sometimes to the entire plant including the roots. The symptoms are more noticeable during the rainy than the dry season when the disease generally causes only infected cankers on the stems and some dieback. The causal agent has been identified as Xanthomonas campestris, which is spread primarily by people and the use of cuttings from diseased plants as well as by the rain, tools and insects. These microbes survive for some time in the soil but they can also live in epiphytes on the leaves of otherwise healthy cassava plants. Native to Latin America, where there are about a hundred species, cassava cultivation has been widely adopted in Africa and Asia. Under ideal conditions, it can yield up to 50 tonnes per ha of tubers, branches and foliage. It is one of the easiest plants to grow and, thanks to its hardiness, is ideally adapted to conditions in rural Africa. The tubers keep for months in the soil transforming the fields into root-cellars. Its vital importance is reflected by the fact that almost 400 million people, mostly in ACP countries, depend on it as their subsistence crop. Cassava is also increasingly used for fodder production and also as an input for certain industries, such as pulp and paper, brewing and petroleum-derived fuel substitutes. African Mosaic Disease (AMD) is characterized by the appearance of yellow spots on the leaves which, in severe cases, become deformed (shrivelling, rolling up, stunting). The plant is often weak and deformed with-short inter-nodal growth and sometimes cankerous growths. Recent research has shown that this disease is caused by a virus belonging to the Gemini group and that it is transmitted by an insect virus, notably Bemisia tabaci. In order to spread the virus, the insect must eat for at least four hours on infected foliage, incubate another four hours and then be in contact with a healthy plant for at least 15 minutes. It is the speed with which the disease spreads that makes it so serious. It should be noted that it is also spread by grafting. The cassava mealybug (Phenacoccus manihoti) primarily attacks the undersides of leaves and the branch ends which lose their chlorophyl, deform and dry up. The plant reacts by sending out new shoots which are also attacked, thus further weakening the plant. The insects are spread by cuttings or by their own movements before infecting a host plant. The damage is particularly spectacular during the dry season. The green cassava mite (Mononychellus tanagoa) is a small, green insect that attacks the undersides of young leaves. Their tips turn yellow, they lose their colour, become deformed and ressemble a mosaic. Growth is also stunted. In serious cases, notably during dry seasons, their outer twigs turn brown and the plant progressively dries up before dvino. What should be done First of all, one must recognize that above all, cassava is a subsistence crop grown throughout Africa on a very large number of small plots. It is thus out of the question to resort to pesticides except in the case of the relatively few industrial plantations. The only way to limit the damage is through integrated pest management which, rather than trying to wipe out the pests, seeks instead to reduce their effects to an acceptable level. Researchers are thus concentrating on the use of resistant plants, biological control and special pest control and cultivation techniques. Plant breeders are striving to develop varieties that are more resistant as complete immunity remains elusive. Such plants limit the spread of the pathogens and reduce their damaging effects. The losses could be reduced by combining the use of resistant varieties with preventative measures such as the use of healthy plantation stock. This approach is ideal for small farmers because the major. expenses are absorbed by research institutions and development agencies. The only cost to the farmers is the purchase of a small quantity of healthy cuttings. As for agricultural extension services, they must concentrate on the multiplication and distribution of new varieties. The International Institute for Tropical Agriculture (IITA), based in Nigeria, has succeeded in developing a variety of cassava that is resistant to African Mosaic Disease and vascular bacteria by crossing two types of cassava: Manihot esculenta and M. glaziovii. The laboratory work took 18 months followed by five years of experiments to produce a plant that combined resistance to pests with high yields. ORSTOM, the French agency for development research undertook an epidemiological study of vascular bacteria in collaboration with researchers at the Congolese agency for scientific research and a second study on African Mosaic Disease in collaboration with the University of Abidjan. Genetic manipulation should enable the isolation of genes which resist viral or bacteriological attack and their introduction into the genetic stock of new varieties. It is for this reason that the protection of the world's phytogenetic resources many of which are being threatened with extinction is essential. CTA is planning to organize a seminar on cassava diseases, in collaboration with IITA, ORSTOM and the FAO, sometime during the middle of 1987. It will deal with the improvement of cassava yields through pest control methods. CTA will subsequently publish the proceedings. In Brazil, for example, where there are more than a hundred species of cassava, deforestation and the development of vast, industrial ranching operations threaten the genetic stock of cassava which is considered poisonous for cattle. Finally, one should note the possibilities offered by biotechnology which enables the cloning of cassava in order to develop healthy plants which can, in turn, help to re-establish this crop in contaminated regions. In South America, where cassava is native, there is an equilibrium between the plant and its predators. In Africa, however, such insects and diseases caused much more damage because of the lack of natural predators and because the farmers could not select resistant strains. IITA researchers, in collaboration with other international organisations such as the International Centre for Tropical Agriculture (CIAT) in Colombia, have developed an impressive biological control programme whose 34 million dollar budget is financed primarily by European countries. It consists of flooding infested regions with natural predators taken from Latin America. A parasitic wasp (Epidino carsis lopezi) from Paraguay and a mite from Colombia (Phytoseiidees) have been identified as having the potential to combat the green spider mite. They are now being mass-reared in a fully automated plant in Nigeria. Over 15 million insects per day are being reared for release by aeroplane over infected areas. A one-spot trial release in Nigeria of 3,000 predators resulted in them spreading over 3 million ha. in five months with a significant reduction in crop damage. Researchers at ORSTOM suggest widening the variety of natural predators used in biological control programmes. Such research is now underway in Latin America to find other parasites or insect-eaters like the mite- and mealybug-eating ladybug. Growing techniques are also an important tool in the fight against such problems. By planting healthy cuttings, infections can be slowed down and sometimes stopped entirely. In Kenya, healthy plants have been grown from specimens that presented no symptoms, particularly from terminal shoots. Concerning bacteria control, it is relatively easy to obtain healthy stock. Cuttings showing no symptoms are placed in water-filled bottles where they form roots. All of the infected specimens rapidly develop symptoms and can be destroyed. The healthy ones can be planted in isolated fields where they form the basis of a healthy stock. This technique has been used in Colombia, Brazil, Cuba and Malaysia. The involvement of research institutes is important in controlling such diseases but is insufficient in itself if the appropriate authorities are not aware of the problem. Each African country concerned must develop a programme to multiply and distribute healthy stock and such programmes should be coordinated at the regional level. In fact, if cassava is to be saved in Africa, a concerted effort is needed which must transcend frontiers. Research is essential in order to breed disease resistant crop. BIBLIOGRAPHY << Les principaux ravageurs et maladies d'Afrique a, Robert L. Thebarge, IITA. Ibadan, Niaeria << Root Crops in Eastern Africa', workshop held in Rwanda, November 1980, IDRC, P.O. Box 8500. Ottawa, Canada KIA 3H9 << Tropical Root Crops >>, Pans Manual N° 4, Tropical Development and Research Institute, College House, Wrights Lane, London W8 55J, United Kingdom Etudes comparatives de bio-economie des cocci nel les et aca ri ens predateurs du manioc a, G. Fabres and A. Kiyindou, 1984, ORSTOM. B. P. Brazzaville, Congo Abstracts on cassava (Manihot esculenta crantz). Volume XI, No 3, December 1985, CTA
SubjectsCROP PRODUCTION AND PROTECTION;
- CTA Spore (English)