14Evolutionary Constraints on Emergence of Plant RNA Viruses

Over the recent years, agricultural activity in many regions has been compromised by a suc-cession of devastating epidemics caused by new viruses that switched host species, or by new variants of classic viruses that acquired new viru-lence factors or changed their epidemiological patterns. Although viral emergence has been clas-sically associated with ecological change or with agronomical practices that brought in contact reservoirs and crop species, it has become obvi-ous that the picture is much more complex, and results from an evolutionary process in which the main players are the changes in ecological factors, the tremendous genetic plasticity of viruses, th