145 Ann. appl. Biol. (2003), 142:145-164 Printed in UK *Corresponding Author E-mail: anupamvarma@vsnl.net ' 2003 Association of Applied Biologists Emerging geminivirus problems: A serious threat to crop production By ANUPAM VARMA* and V G MALATHI Advanced Centre for Plant Virology, Indian Agricultural Research Institute, New Delhi 110 012, India (Accepted 28 February 2003; Revised version received 17 January 2003) Summary Geminiviruses form the second largest family of plant viruses, the Geminiviridae, represented by four genera: Mastrevirus, Curtovirus, Topocuvirus and Begomovirus. During the last two decades these viruses have emerged as devastating pathogens, particularly in the tropics and subtropics, causing huge economic losses and threatening crop production. Epidemics caused by re-emerging and newly emerging geminiviruses are becoming frequent even in regions that were earlier free from these viruses. Compared to mastreviruses and curtoviruses, begomoviruses have emerged as more serious problems in a variety of crops, for example, cassava, cotton, grain legumes and vegetables. Major contributory factors for the emergence and spread of new geminivirus diseases are the evolution of variants of the viruses, the appearance of the whitefly B biotype and the increase in the vector population. Variability in geminiviruses has arisen through mutations, recombination and pseudorecombination. Genomic recombination in geminiviruses, not only between the variants of the same virus but also between species and even between genera, has resulted in rapid diversification. From the disease point of view, most virulent variants have developed through recombination of viral genomes such as those associated with cassava mosaic, cotton leaf curl, and tomato leaf curl diseases. Heterologous recombinants containing parts of the host genome and/or sequences from satellite-like molecules associated with monopartite begomoviruses provide unlimited evolutionary opportunities. Human activity has also played an important role in the emergence of serious geminivirus diseases across the globe, like the changes in cropping systems, the introduction of new crops, the movement of infected planting materials and the introduction of host susceptibility genes through the exchange of germplasm. Key words: Geminivirus, mastrevirus, curtovirus, begomovirus, Bemisia tabaci, whitefly, emerging viruses Introduction Geminiviruses are the cause of devastating plant diseases, particularly in the tropics and sub-tropics. Beet curly top, cassava mosaic and maize streak diseases, recognised as serious threats to crop production about a century ago, continue to be serious problems of world agriculture. In addition, the present century faces severe disease epidemics caused by newly emerging geminiviruses affecting crops such as cotton, grain legumes and tomato. Economic losses due to geminivirus infections in cassava are estimated to be US $1300-2300 million in Africa (Thresh et al., 1998), US $5 billion for cotton in Pakistan between 1992-97 (Briddon & Markham, 2000), US $300 million for grain legumes in India (Varma et al., 1992) and US $140 million in Florida for tomato alone (Moffat, 1999). Despite concerted efforts to contain geminiviruses and their vectors, menacing disease epidemics caused by newly emerging or re-emerging geminiviruses are becoming frequent and appearing even in new regions, previously free from such diseases. Geminiviruses have a unique twin particle morphology and either monopartite or bipartite ssDNA genomes and they form the second largest family, Geminiviridae, of plant viruses. Geminiviruses have been grouped into four genera: Mastrevirus , Curtovirus , Topocuvirus and Begomovirus, depending on their vector, host range and genomic characteristics (van Regenmortel et al., 2000; Fauquet & Stanley, 2003; Fauquet et al., 2003). The leafhopper-transmitted mastreviruses primarily infect monocotyledonous hosts; their monopartite genome of 2.5-3.0 kb has two virion-sense and two complementary-sense open reading frames (ORFs), with one long and one short intergenic region (Fig. 1). The leafhopper transmitted curtoviruses infect dicotyledonous plants; their monopartite genomes have three ORFs in the virion-sense and four in the complementary-sense with one intergenic region (Fig.1). The treehopper-transmitted topocuvirus has only one member, Tomato pseudo curly top virus which has a monopartite genome with two ORFs in the virion-sense and four in the complementary sense (Fig.1). More than 80% of the known geminiviruses are transmitted by whiteflies and belong to the genus Begomovirus, they mostly have bipartite genomes