How Similar are Plant and Insect Viruses? Strawberry latent virus: A Study Case

A major outbreak of virus-induced strawberry decline in western North America from 2001 onwards, led us to investigate whether the graft-transmissible agents reported in the crop are also involved in this recently observed symptomatology. Strawberries infected with virus-like agents maintained at the National Clonal Germplasm Repository in Corvallis, OR were studied for the presence of viruses. One of those agents was discovered in the 1980s, in the course of identification of the causal agent of strawberry mild yellow edge disease. Martin and Converse (1985) identified and purified an icosahedral virus from strawberry mild yellow edge diseased plants. Later, it was discovered that the virus was also present in strawberry seedlings, derived from apparently healthy plants. This unknown virus was not studied further and its importance remained unknown, particularly when found in mixed infections with other viruses. Sequence data were obtained from dsRNA extracted from a plant of the original study. The data indicated that the virus, currently designated as Strawberry latent virus (StLV), is most closely related to the insect-infecting viruses of the genus Cripavirus. This may have implications in our understanding of the evolution of picorna-like viruses. Although there are some plant viruses that have distant relationships with insect-infecting viruses, StLV is linked more closely with members of the Cripavirus genus than any genera of plant viruses. The completion of the genome may reveal the presence of a movement protein or movement mode and transmission similar to that of cryptic viruses. Future plans include the study of the potential synergism of StLV with other major strawberry viruses and its role in the aphid transmissibility of Strawberry mild yellow edge virus. INTRODUCTION Strawberry (Fragaria spp.) is infected by more than 25 virus and virus-like agents (Martin and Tzanetakis, 2006). Until recently viruses were not a major problem in strawberry production in the United States, partially due to cultivar tolerance and the annual production system employed in many areas. The situation has changed dramatically in the last six years when a disease referred to as strawberry decline has emerged as a severe problem across the western coast of North America (Tzanetakis, 2004). Leaves of affected plants turn red and growth ceases. In severe cases, plant death follows. The symptoms correlated with the presence of three or more viruses (Tzanetakis, 2004). The virus species present in affected plants were not consistent in all areas. The aphid-borne viruses were prevalent in the northern latitudes and the whitefly-transmitted viruses were found in the vast majority of diseased plants in southern latitudes. Since almost all strawberry cultivars grown today do not show symptoms when infected with one or two viruses, we decided to investigate whether any of the known but poorly characterized viruses and virus-like agents contributed to the development of the strawberry decline disease. One of the poorly characterized viruses was discovered in the early 1980’s by Martin and Converse (1985). The icosahedral virus was first identified in plants infected Proc. XI IS on Small Fruit Virus Diseases