Controlling African Cassava Mosaic Disease
African cassava mosaic disease (ACMD) is caused by a virus and, as its name implies, appears to be confined to Africa. A similar disease caused by a closely related virus occurs in India, but the virus which causes the disease known as cassava common mosaic disease, found in South America, belongs to a different group. It follows, therefore, that the original cassava introductions into Africa were free of the disease and were invaded by a virus present in some other host or hosts whose identity has yet to be established.
ACMD was first described in the late 19th century (Warburg, 1894) and is now found wherever cassava is grown in Africa. Ironically, it is because the disease is so widespread that its importance has received little attention-so many plantings contain few, if any, healthy plants that ACMD infection has come to be regarded as a normal condition of the crop. Consequently, it not generally realized that ACMD causes serious yield losses.
Plants infected with ACMD are not killed but show pale green or yellow areas on the leaves, which are commonly small and distorted. Tubers are reduced in size and number. Stem diameter and overall size are also reduced. Yield reduction may be severe. Losses of up to 95% have been reported and the overall reduction in Africa may be as high as 50%. The virus which causes ACMD belongs to the gemini virus group, whose paired particles are visible only under an electron microscope. A number of strains of the virus have now been recognized (Bock and Harrison, 1985), but strain differences are not important for practical field control.
ACMD is spread in two ways: when the whitefly (Bermisia tabaci) feeds first on diseased plants and then on healthy plants; or when diseased cuttings are used to establish a crop. The relative importance of the two ways depends on several factors, but yield losses are greatest when plants are derived from infected cuttings (Briant and Johns, 1940).
The reduction in yield caused when a previously healthy plant is infected by whitefly depends on the stage of growth at which this occurs. There no significant yield reduction if infection occurs more than 120 days after planting (Fargette et al., 1986) but of course cuttings taken from such plants will give reduced yields in the next crop.