Date of Graduation

5-2021

Document Type

Thesis

Degree Name

Master of Science in Plant Pathology (MS)

Degree Level

Graduate

Department

Plant Pathology

Advisor/Mentor

Ioannis E. Tzanetakis

Committee Member

Amanda McWhirt

Second Committee Member

Neelendra Joshi

Keywords

Aphids, Field Samples, Rhabdovirus, Strawberry, Viruses

Abstract

Strawberry is the most important berry crop, grown in most temperate regions around the world. In the early 2000s and in 2012-2013, viral disease devastated strawberry production in California and Florida respectively; causing multimillion dollar losses. This occurred because viruses were moving in the field undetected. The modern strawberry (Fragaria x ananassa Duch.) is tolerant to single and double infections; yet when several viruses infect plants, symptoms as severe as plant death can occur. To prevent future epidemics, the use of virus-tested planting materials is essential. In 2019, rhabdovirus-like sequences were detected in strawberry using a combination of high-throughput sequencing and bioinformatics. This study focuses on several epidemiological aspects of the two viruses including molecular characterization, phylogenetics, diagnostics, transmission and incidence in major strawberry-producing areas of the United States. The genomes of the two viruses have been obtained and phylogenetic analyses of the most conserved protein, the virus replicase, indicate that the closest relative to the first virus is Lettuce necrotic yellows virus (67% amino acid identities), the type member of the genus Cytorhabdovirus. The second virus is distantly related to plant-infecting rhabdoviruses and therefore is proposed to be the inaugural member of a new genus (Fragarhavirus) in the Rhabdoviridae. This virus shows some similarity to nucleorhadoviruses, which are unknown to infect strawberry except Strawberry latent C (SLCV), which was first described in the 1940s, but there are no molecular or serological data available to date. There has been no original work published on SLCV in the last 30 years and there are no detection methods other than graft indexing. The novel rhabdovirus discovered here could be involved in latent C disease and requires further investigation to determine if a relationship exists between the two.

A survey in four states, including California and Florida, suggest that the two viruses are not widespread in production areas. Diagnostics, targeting multiple genes, were optimized based on available sequences to ensure accurate and reliable detection. The strawberry (Chaetosiphon fragaefolii) and small bramble aphids (Aphis ruborum) were evaluated as likely vectors of the two viruses. As a result of this study, transmissibility by two aphid species will be evaluated, and the genomic data are added to the knowledge of rhabdoviruses and strawberry viruses. Diagnostic tests are now available for clean plant programs and certification schemes. By introducing plants tested free of the two viruses to the field, strawberry growers are better positioned to prevent virus-induced losses.

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