African Cassava Mosaic Disease

Abstract

Cassava mosaic disease (CMD), recognized in Africa for more than a century, is now known to be caused by a group of geminiviruses in the genus Begomovirus from the family Geminiviridae . All are transmitted in a persistent manner by the whitefly vector Bemisia tabaci . Structurally, cassava mosaic geminiviruses (CMGs) are comprised of two small, single-stranded, circular DNA molecules, DNA-A and DNA-B, each of which is encapsidated within a geminate coat. CMG infection results in symptoms comprising misshapen leaves with a mosaic-like chlorosis and general plant stunting, leading to reduced tuberous root production. Through the processes of virus–virus synergism, pseudo-recombination, and true recombination, CMGs have evolved into a diverse and highly successful group of plant pathogens and seven species are currently recognized from Africa. Rapid spread of a recombinant strain, East African cassava mosaic virus-Uganda (EACMV-UG), has been associated with a pandemic of unusually severe CMD, which has affected much of East and Central Africa, leading to production losses of 47%, equivalent to nearly 14 million tonnes. Although the deployment of conventionally bred host plant resistance is the main approach being used to manage CMGs, with considerable success, increased attention is being given to the development of transgenic resistance strategies.