Molecular characterization of criniviruses and ilarviruses infecting strawberry

Abstract

Pallidosis is a disease of strawberry identified in 1957 with an unknown etiology. Two previously uncharacterized viruses in strawberry, Strawberry pallidosis associated virus (SPaV) and Beet pseudo-yellows virus (BPYV), have been found associated with disease symptoms. The complete nucleotide sequence of both viruses was determined and molecular detection protocols developed. In addition, an immunological tissue blot assay was developed for SPaV. Phylogenetic analysis of SPaV placed it in the genus Crinivirus, family Closteroviridae, along with BPYV. Transmission studies identified Trialeuroides vaporariorum, the greenhouse whitefly, as a vector of SPaV, while the virus was not transmitted by pollen or seed. The geographic distribution of both SPaY and BPYV in the United States was examined. Two ilarviruses of strawberry were investigated. Tobacco streak virus (TSV) is the type member of the genus Ilarvirus, family Bromoviridae, and can cause severe yield losses in small fruit crops. Sequence and phylogenetic analysis of 15 "TSV" isolates from Fragaria and Rubus revealed that they are homogeneous and represent a new virus species designated as Strawberry necrotic shock virus (SNSV). Nucleic acid-based protocols failed to identify any plants of the study infected with TSV an indication that the virus may not be a pathogen of Fragaria and Rubus. Fragaria chiloensis latent virus (FC1LV) is the second member of the genus Ilarvirus that infects strawberry. Previously, the virus had been found only in Chile and although many plants have been tested in North America using a serological test, none was found infected with the virus. A molecular test has been developed and used to confirm the presence of the virus in strawberries along the west coast of North America. Phylogenetic analysis of the coat protein gene of the virus place it in subgroup 4 of the genus along Prune dwarf virus, while it was previously thought to be related most closely to Lilac ring mottle virus and Asparagus virus-2. The role of the newly identified viruses and other viruses infecting strawberry is discussed in association with the strawberry decline disease found to cause severe symptoms and yield losses in both California and British Columbia, Canada. The significance of the work presented in this publication includes: the identification of two viruses associated with strawberry pallidosis disease; the complete nucleotide sequence of two criniviruses, an emerging group of plant viruses adds to the limited knowledge we have about this diverse virus genus; the identification of FC1LV in North America; the identification of SNSV as a distinct virus species which demonstrates the need of further studies on TSV, a virus species that infect a wide range of plant species and may actually be a cluster of diverse species. The high incidence of the pallidosis associated viruses points to the significance of the disease and their role in strawberry decline. The development of fast, sensitive and reliable tests for four strawberry viruses will have an impact to the strawberry industry, since the need for high quality, virus-free plants is essential for a crop that is asexually propagated.