BROWN streak virus disease of cassava (Manihot utilissima Pohl.) is important in all the cassava-growing areas on the coasts of Kenya and Tanganyika, because necrosis of the starch storage tissues of the roots and stems of infected plants results in serious losses in yield1,2. Transmission of the virus by a white fly (Bemisia sp.) is suspected but has not been confirmed3 and the only reliable means of transmitting it experimentally has been by grafting. The symptoms of the disease vary greatly with variety and environmental conditions, making diagnosis difficult, particularly when plants are infected both with brown streak and cassava mosaic, another virus disease. Thus a major difficulty in studying brown streak has so far been the lack of a means of transmitting the causal virus readily and also uncontaminated with cassava mosaic virus.