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It has been known for a long time that the majority of plant viruses contain RNA and in the past decade and a half it has been realized that many have genomes consisting of three molecules of single-stranded RNA with positive polarity. Among these are viruses belonging to four groups recognized by the International Committee for Virus Taxonomy: the Bromovirus and Cucumovirus groups whose genomes are encapsi­ dated in small icosahedral particles or the Ilarvirus and alfalfa mosaic virus groups with spheroidal or bacilliform particles. In addition to their tripartite genomes, these viruses share a number of other properties and it has been proposed that they should perhaps be grouped in a single virus family for which the name Tricornaviridae has been suggested, the tri indicating the tripartite nature of the genome, the co emphasizing the cooperation of the three genome parts required to initiate infection, and the rna indicating that the genome is composed of RNA. Viruses of this'family'are less uniform in their biological proper­ ties. A number of them are widespread, causing very destructive plant diseases. Viruses such as those of cucumber mosaic and alfalfa mosaic have very extensive host ranges and are responsible for serious crop losses in many parts of the world. Others such as prunus necrotic ringspot or prune dwarf viruses are more restricted in their host ranges but never­ theless infect important perennial hosts such as stone fruits and reduce productivity considerably.

The original aim of this book was to cover different aspects of the tradi­ tionally'filamentous'potex-, carla-, poty-, clostero-, and capilloviruses. The title The Filamentous Plant Viruses seemed the only suitable one, but it has led us to discuss also the quite different filamentous viruses of the rice stripe group-recently officially named the tenuivirus group­ which otherwise, indeed, might not have been conveniently covered in any volume of this series. The question must be asked: What is there new that justifies the presentation of a book of this kind? An outline of the answer may be Among the traditional filamentous viruses, much pro­ given as follows. gress has been made in elucidating the physical structure of potexvirus particles, and this work serves as an excellent model for discussion of and future experiments on the poty-, carla-, clostero-, and capilloviruses, which have comparable structures, although they are more difficult to manipulate. Work on the structure and strategy of the genomes of poty­ viruses is, however, relatively advanced and at a very interesting stage. The helper component that assists the aphid transmission of potyviruses has also recently received considerable attention, although the more we know about that, the less seems clear about the aphid transmission of the carlaviruses and closteroviruses, which apparently neither possess nor require a helper component.

The evolution of viruses has been a topic of intense investigation and theoretical development over the past several decades. Numerous workshops, review articles, and books have been devoted to the subject. Medical practitioners have recognized the importance of viral evolution when treating patients with viral diseases. Farmers have recognized the importance of understanding virus evolution in combating emerging viral diseases in their crop plants. As with any field where knowledge is rapidly expanding, many controversies have also arisen about the nature of virus evolution, how to describe virus populations, how to analyze sequence data and estimate phylogenies, etc. Differing points of view will also be found in the various chapters of this book, and I leave it to the readers to decide for themselves which side they find most helpful. In some cases it seems to me that all sides are correct. In other cases, future historians will decide. This book focuses on the evolution of plant viruses, although some chapters also draw on the more extensive knowledge of animal viruses. It covers topics on evo- tionary mechanisms, viral ecology and emergence, appropriate methods for analysis, and the role of evolution in taxonomy. It includes RNA viruses, DNA viruses, integrated viruses and viroids.

It has been ten years since the publication of the third edition of this seminal text on plant virology, during which there has been an explosion of conceptual and factual advances. The fourth edition updates and revises many details of the previous editon, while retaining the important older results that constitute the field's conceptual foundation.Key features of the fourth edition include:• Thumbnail sketches of each genera and family groups• Genome maps of all genera for which they are known• Genetic engineered resistance strategies for virus disease control• Latest understanding of virus interactions with plants, including gene silencing• Interactions between viruses and insect, fungal, and nematode vectors• New plate section containing over 50 full-color illustrations