Cover photograph (Copyright © 1999, American Society for Microbiology. All rights reserved.): Hypovirus infection of the chestnut blight fungus results in reduced virulence (hypovirulence) and the formation of superficial, nonlethal cankers on American chestnut trees. Natural hypovirus isolates can vary widely in their influence on canker development, despite extensive conservation of genomic RNA sequence. The development of infectious cDNA clones for mild and severe hypovirus strains provides new comparative approaches to elucidate the mechanisms underlying hypovirus-mediated attenuation of fungal virulence. The construction of synthetic infectious chimeric hypovirus RNAs demonstrates the feasibility of engineering a virus to fine tune the interactions between a fungal pathogen and its host. (See related article in the February 1999 issue: vol. 73, p. 985-992.)