SIMULATION OF THE EFFECTS OF HOST-RESISTANCE, REVERSION, AND CUTTING SELECTION ON INCIDENCE OF AFRICAN CASSAVA MOSAIC-VIRUS AND YIELD LOSSES IN CASSAVA

A simulation model, developed earlier to describe epidemics of African cassava mosaic, was used to investigate the effects of resistance and sanitation on epidemic severity and cassava productivity in successive annual cropping cycles. Parameters characterizing host resistance, secondary spread within plantings, latent period, and yield losses were incorporated into the model. Resistance and sanitation were modeled in two ways: reversion (the percentage of healthy cuttings derived from infected plants) and preferential cutting selection (the ratio of the number of cuttings from a healthy plant to the number from an infected one). When reversion or cutting selection occurred for several successive crop cycles in highly resistant cultivars, disease incidence increased during the first few annual crop cycles but ultimately reached an equilibrium considerably below 100%. At this equilibrium stage, new infections caused by transmission of the virus by insect vectors balanced ''escapes'' through reversion or cutting selection, and yield losses were limited. Respective and combined effects of host resistance, reversion, and cutting selection on disease incidence and yield losses are assessed.