THE SPREAD OF AFRICAN CASSAVA MOSAIC-VIRUS INTO AND WITHIN CASSAVA FIELDS

Only a small proportion of Bemisia tabaci collected in totally infected cassava fields at a site in Cote d'Ivoire transmitted African cassava mosaic virus (ACMV) to test plants. Nevertheless, the monthly increase in disease incidence in an experimental planting was directly related to numbers of adult whiteflies counted on plants 6 weeks earlier. In plots at different spacings, the greatest incidence of ACMV expressed as a percentage of the total stand occurred at the lowest plant density. Much spread into the spacing trial and into two other experimental plantings occurred from outside sources and followed downwind gradients. By contrast, spread from ACMV-infected sources within plantings was limited. It occurred in all directions but over distances of only a few metres. These contrasting patterns of spread are attributed to the different behaviour of B. tabaci above and within the crop canopy. It is concluded that contamination of cassava fields in the coastal forest area of Cote d'Ivoire is due mainly to rapid spread from outside sources which leads to internal foci that contribute to some further, although limited, spread. These findings are discussed in relation to possible control strategies based on the release of healthy cuttings, dense planting and subsequent roguing. Such measures are unlikely to be effective in the coastal forest region of Cote d'Ivoire and adjacent countries unless varieties are grown with greater resistance to infection than those currently used.