About 10% of world crop production is lost annually because of plant diseases caused by bacteria, fungi and viruses (Fraser, 1985). Crop losses as a result of virus diseases can be especially serious in developing countries in tropical and subtropical regions. For example, in Africa losses of cassava crops caused by infection with African cassava mosaic virus are valued at more than two hundred million pounds per annum (Harrison, 1987). There are three main types of control measure for preventing virus infections in crops. The first of these involves removal of virus reservoirs, such as infected plants or propagules remaining from previous crops, and producing virus-free planting stocks. The second method aims to prevent plant-to-plant spread of virus, usually by killing vectors that transmit the virus. The third control measure is to produce and grow virus-resistant cultivars of crops.