THE EFFECTS OF CASSAVA MOSAIC-VIRUS DISEASE ON YIELD AND COMPENSATIONIN MIXED STANDS OF HEALTHY AND INFECTED CASSAVA

The effects of cassava mosaic virus disease (CMD) on yield in fully an d partly infected stands of cassava were investigated in field trials in Uganda in 1990-91 and 1991-92. Three cultivars (Ebwanateraka, Bao a nd Bukalasa 11), each at three levels of cutting infection (0%, 50% an d 100%) and harvested 5, 10 and 15 months after planting (MAP) were us ed in a randomised block design with split-split plots and four replic ates. Moreover, yield and growth data for individual infected and unin fected plants were considered in relation to the health status of thei r nearest neighbours. In each experiment, fresh tuberous root yields o f plants from 100% infected plots gave significantly lower yields than those from 0% or 50% infected plots at each harvest date and the loss es were greatest in cv. Bao. Yields of plants from 0% and 50% plots fo r each of the three cultivars were not significantly different, 10 and 15 MAP. The loss in yield differed between cultivars and harvest date s. Fresh stem, leaf and root yields and the number of tuberous roots w ere influenced by the health status of the plants harvested and that o f their nearest neighbours. Uninfected plants surrounded by infected o nes had more roots and heavier total fresh root, stem and leaf weights than those surrounded by uninfected ones. Overall, 26% and 42% compen sation was recorded in 1990-91 and 1991-92, respectively. The effects of CMD on cassava production and of compensation in mixed stands of in fected and uninfected plants are discussed, especially in relation to control strategies such as roguing.