S.M. Garnsey - Horticultural Research Laboratory, United States Department of Agriculture, Orlando, Florida

D. Gonsalves - Department of Plant Pathology, Cornell University, Geneva, New York

This chapter discusses various closteroviruses. Closteroviruses not only share similarities in particle morphology and biochemistry, but several members of the group seem to have common biological properties such as semipersistent mode of transmission by their aphid vectors and induce similar ultrastructural changes in their respective hosts. Members of this group have a moderate host range, but some of them cause extremely important economic diseases. Of the 10 typical closteroviruses, six are described, and purification methods for three are presented that enabled their partial characterization. With certain similar sized viruses such as beet yellow virus (BYV), carnation necrotic fleck virus (CNFV), and probably beet yellow stunt virus (BYSV), cytopathology, serology, and amino acid composition indicate close relationships, with a variation nearing that assumed among different strains of some other elongated plant viruses. The RNAs of BYV and CNFV are about twice the size of chlorotic leaf spot virus (CLSV) and heracleum latent virus (HLV), and it is possible to visualize the origin of the heavier genomes via dimerization of the half-size units. Infectivity is associated with normal sized BYV particles and centrifugation of formaldehyde-treated RNAs indicated that the heavy sized RNAs are intact and resist treatment believed to dissociate noncovalent links among segments of RNA.