Global Emergence and Spread of Whitefly (Bemisia tabaci) Transmitted Geminiviruses

Abstract

Whitefly-transmitted geminiviruses (WTGs) have emerged as the most destructive pathogens, particularly in the tropics and subtropics. The epidemics caused by the emerging WTGs have spread even in the regions that were free from these viruses earlier. The most seriously affected crops include cassava, cotton, grain legumes, and cucurbitaceous, malvaceous and solanaceous vegetables. Cassava mosaic disease pandemic in East Africa in the early 1990s was caused by the emergence of a highly virulent recombinant WTG, having sequences from African and East African cassava mosaic viruses. Since then, the recombinant WTGs, aided by sharp increase in whitefly population, have spread over large cassava growing areas of Africa, leading to acute food shortages in the affected region. Cotton leaf curl disease has been endemic in the Sudan for a long time. In the 1990s it caused severe epidemics in the north-western region of the Indian subcontinent resulting in enormous economic losses. The expanding host range of WTGs infecting legumes, along with their spread to new geographical regions, limits production of this important group of crops. The most dramatic emergence of WTGs affecting tomato has been in Asia and the Americas during the last two decades. The major contributory factors for the emergence and spread of new WTGs are (a) evolution of variants of WTGs through mutations, recombination and pseudo-recombination, (b) acquisition of satellite-like DNA molecules, (c) appearance of aggressive biotypes and increase in populations of Bemisia tabaci, (d) changes in the cropping systems, (e) introduction of new crops, (f) introduction of host susceptibility genes, and (g) the movement of infected planting materials. In addition, favourable climatic changes and human activity have also played an important role in the emergence of serious WTG associated diseases across the globe. This chapter highlights the emergence and spread of WTGs in selected crops threatened by this large group of plant viruses.