Controlling African Cassava Mosaic Disease

Abstract

African cassava mosaic disease (ACMD)is caused by a virus and, as its implies, appears to be confined to Africa.A similar disease caused by a closelyrelated virus occurs in India, but the virus which causes the disease known ascassava mosaic disease, found in South America, belongs to adifferent group. It follows, therefore, that the original cassava introductionsinto Africa were free of the disease and were invaded by a virus present insome other host or hosts whose identity has yet to be established.ACMD was first described in the 19th century (Warburg, 1894)and isnow found wherever cassava is grown in Africa. Ironically, it is because thedisease is widespread that its importance has received little attention-somany plantings few, if any, healthy plants that ACMD infection hasto be regarded as a normal condition of the crop. Consequently, it notgenerally realized that ACMD causes serious yield losses.Plants infected with ACMD are not killed but show pale green or yellow areason the leaves, which are commonly small and distorted. Tubers are reduced insize and number. Stem diameter and overall size are also reduced. Yieldreduction may be severe. of up to 95%have been reported and theoverall reduction in Africa may be as high as 50%.The virus which causesACMD belongs to the gemini virus group, whose paired particles are visibleonly under an electron microscope.A number of strains of the virus have nowbeen recognized (Bock and Harrison, but strain differences are not importantfor practical field control.ACMD is spread in two ways: when the whitefly tabaci) feeds first ondiseased plants and then on healthy plants; or when diseased cuttings areused to establish a crop. The relative importance of the two ways depends onseveral factors, but yield losses are greatest when plants are derived frominfected cuttings (Briant and Johns, 1940).The reduction in yield caused when a previously healthy plant is infected bywhitefly depends on the stage of growth at which this occurs. There nosignificant yield reduction if infection occurs more than 120days afterplanting (Fargette et al., 1986) but of course cuttings taken from such plantswill give reduced yields in the next crop.Cassava has become the most important food crop in Africa because of itshigh yield capacity and its ability to grow in poor soils