Begomovirus Evolution and Disease Management

Publisher Summary

This chapter considers the knowledge of the roles of vector, viruses, and plant hosts in the emergence of begomovirus epidemics, and outlines areas in which further research is needed. It looks at the vector–virus–host interaction from an epidemiological and evolutionary perspective and considers how the understanding gained can be used to modify cropping practices to achieve sustainable disease management. Consideration has also been given to the implications of international traffic in plants for the continuing spread of begomovirus epidemics. Knowledge of vector and virus diversity and evolution, and the complex interactions between plant host, vector and virus, leads inevitably to the conclusion that no single or sustainable solution is found to control begomovirus disease epidemics. The emergence of future epidemics will only be reduced by limiting some of the selection pressures that have arisen in the past, including some of the negative aspects of agricultural intensification, the introduction of highly susceptible virus or vector hosts, or conversely the wide-scale cultivation of some types of plant host resistance that are vulnerable to the evolution of new virulent strains. The adoption of particular cropping practices will assist controlling the spread of begomovirus diseases, and appearance of diseases caused by variant begomoviruses, if the practices reduce vector transmission rates and virus population densities or both.

Introduction

Harrison (1985) in a prescient review emphasised the need for concerted research efforts to facilitate the control of diseases caused by geminiviruses. Nevertheless, in recent decades, there has been a marked increase in the emergence of whitefly‐transmitted geminiviruses (now ascribed to the Family Geminiviridae, Genus Begomovirus) and of populations of their vector Bemisia tabaci (Hemiptera: Aleyrodidae) (Morales, this volume, pp. 127–162; Morales 2001, Polston 1997, Seal 2006, Varma 2003). Research in recent decades has been intense and many new begomovirus species have been designated and the polyphyletic nature of B. tabaci has been recognised (Brown 1995, De Barro 2005, Fauquet 2003). Considerable progress has been made in understanding geminivirus replication (Gutierrez 1999, Gutierrez 2004), gene function and silencing (Chellappan 2004, Hanley‐Bowdoin 1999, Moissiard 2004), sources of genetic variation (Mansoor et al., 2003b) and the role of DNA satellite molecules (Briddon 2003, Stanley 2004). Similarly, much progress has been made in understanding the phylogenetic relationships between different B. tabaci populations (De Barro et al., 2005). The above references are generally those of recent reviews, where additional details are presented.

Research is now needed that integrates biological and molecular knowledge of viruses, vectors and host plants within an epidemiological framework, and to relate this to new and changing cropping practices in general. Increased knowledge in these areas should lead to a better understanding of the factors that promote the selection of more virulent recombinant or other novel strains, and more fecund or increased vector populations with extended host ranges. This may assist the development of control measures that limit the exposure of the viruses or their vector to selection pressures that are undesirable from the human perspective. In this chapter, we consider first knowledge of the roles of vector, viruses and plant hosts in the emergence of begomovirus epidemics and outline areas in which further research is needed. We then look at the vector–virus–host interaction from an epidemiological and evolutionary perspective and consider how the understanding gained can be used to modify cropping practices (in the widest sense) to achieve sustainable disease management. Finally, consideration is given to the implications of international traffic in plants for the continuing spread of begomovirus epidemics.