CHANGES IN THE INCIDENCE OF AFRICAN CASSAVA MOSAIC-VIRUS DISEASE AND THE ABUNDANCE OF ITS WHITEFLY VECTOR ALONG SOUTH-NORTH TRANSECTS IN UGANDA

Surveys of the incidence of African cassava mosaic disease (ACMD) and its whitefly vector, Bemisia tabaci, were made at three-month interval s in 1992 and 1993 along two south-north transects. One was in central and the other in eastern Uganda. ACMD incidence was high within the n orthern localities of each transect and low towards the south. Inciden ce increased over the period at localities on the ACMD epidemic 'front ' which was the interface between high incidence areas to the north an d low incidence areas to the south. Combining data for both transects, the relationship between 'infective potential' (the product of whitef ly number and the incidence of ACMD attributed to the use of infected cuttings,'x') and the increase in proportion of infected plants caused by whiteflies ('y', as transformed to account for multiple infection) was described by the straight line regression: y = 47.8x + 9.2 (F rat io = 63.0, r(2) = 0.78). This emphasises the importance for control of reducing ACMD incidence by phytosanitation and/or use of resistant va rieties since reduction of whitefly numbers is not easily achieved. Po ssible mechanisms for the expansion of the ACMD epidemic and implicati ons of the study findings for control of ACMD are discussed.