The United States is the third-largest producer of raspberries in the world. Washington State leads the nation in red raspberry (Rubus idaeus L.) production. 'Meeker', the most grown red raspberry cultivar in the Pacific Northwest (Washington, Oregon and British Columbia, Canada) is highly susceptible to Raspberry crumbly fruit, a virusinduced disease that produces drupelet abortion and reduces fruit quality and yield. The disease has long been attributed to Raspberry bushy dwarf virus (RBDV), a pollen-and-seed transmitted virus found in most commercial raspberry fields around the world. In recent years, an increased severity of crumbly fruit was observed in areas where two additional viruses were common. One of these viruses, Raspberry leaf mottle virus (RLMV), was characterized recently and shown to be a novel closterovirus transmitted by the large raspberry aphid Amphorophora agathonica Hottes. The second virus, Raspberry latent virus (RpLV) was a tentative member of the family Reoviridae whose characterization remained to be completed. To investigate the role of these two new viruses in the crumbly fruit disorder, 'Meeker' raspberry infected with single or mixtures of the three viruses, in all possible combinations, were generated by graft inoculation. Eight treatments, including a virus free control, were planted in the field at the Northwestern Research and Extension Center in Mt. Vernon, WA. Plant growth and fruit crumbliness were evaluated during establishment and the second year. Simultaneously, the characterization of RpLV, at the genetic and biological level, was completed. RpLV is a novel member of the plant Reoviridae composed of 26,128 nucleotides divided into 10 genomic dsRNA segments. Analysis of the RNA dependent RNA polymerase (RdRP) indicated that RpLV was related most closely to members of the genus Oryzavirus. However, the genomic terminal regions, conserved at the genus level in reoviruses, did not show homology to those of oryzaviruses, suggesting that RpLV may be a member of a new genus. It was found later, that RpLV was transmitted by A. agathonica. As all other plant reoviruses are transmitted by leaf- and planthoppers, these findings strongly support the creation of a new genus in the plant Reoviridae for the classification of RpLV. Real-time quantification of viral titers in single or mixed infections revealed that the titer of RBDV was enhanced ~400-fold when the virus was found in coinfections with RLMV. Interestingly, plants co-infected with these two viruses showed significant reduction in plant growth during the establishment and second year. Crumbly fruit was more severe in plants infected with the three viruses compared to the virus-free control. Taken together, this work presents valuable information about the interactions between three important raspberry viruses and their effect on plant growth and fruit crumbliness in 'Meeker', the most important red raspberry cultivar in the Pacific Northwest. / Graduation date: 2012