Turnip yellows virus (TuYV) is the most common and important virus infecting oilseed rape (Brassica napus) in the UK. It causes reductions in growth and seed yield in oilseed rape. Between 2007 and 2010, the prevalence of TuYV in oilseed rape crops in Lincolnshire, Warwickshire and Yorkshire was determined; incidences of infection ranged from 0 and 100%. The highest levels of infection were detected in Lincolnshire and the lowest in Yorkshire. Highest incidences were recorded during 2009-10 and the lowest in 2008-9. Incidences of TuYV were closely related to the flight activities Myzus persicae vector. Most fields showed slightly aggregated pattern of infection during autumn but spring sampling revealed more random patterns. Phylogenetic analysis of both nucleotide and amino acid sequences of the P0 and P3 genes of TuYY revealed three and two genetic groups of TuYV respectively, infecting oilseed rape in Lincolnshire, Warwickshire and Yorkshire. The P0 gene was more variable than the P3 gene and both were under purifying selection. TuYV populations in the three regions were highly structured with limited gene flow between them. Analysis of molecular variance (AMOVA) indicated 96- 97% of the observed variation was due to the variation between isolates within fields. Three RT-PCR assays were developed to differentiate the three genotypes. They successfully detected and discriminated isolates of the two major genotypes from oilseed rape in Lincolnshire. Twenty seven accessions of a B. napus Diversity Fixed Foundation Set (DFFS) screened for resistance against TuYV infections varied in their susceptibility to the virus. An accession Yudal had partial resistance to some but not all the isolates of the two major genetic groups tested. TuYV caused yield losses of up to 44.7% in a glasshouse experiment. A major QTL for the partial TuYV resistance was detected on chromosome C4 (N14), explaining up to 50.5% of the observed resistance.

The aphid transmitted Polerovirus Turnip yellows virus (TuYV) was found to be widespread with high incidences in oilseed rape (OSR) across Europe. UK, France, Germany and Poland all having >90% TuYV incidence in some OSR crops. From the 179 whole TuYV genomes sequenced in this study the phylogenetic analyses indicated three distinct genetic groups in the UK, two of which were also detected in Europe. These three genotypes were also distinct from the original sequenced TuYV-FL. These groups are proposed to be distinct species due to their genetic distance based on the most variable gene ORF5 and phylogenetic analyses of ORF1, ORF3, ORF4 and ORF5. Mixed TuYV infection was uncommon and only two plant samples had genetically distinct isolates. Whole genome analysis also provided valuable information on two recombination hotspots located within TuYV genes ORF3 and ORF5. Investigation into the epidemiology of TuYV revealed many weed and crop species as hosts, including sugar beet, which it was previously thought not to infect. TuYV isolates detected infecting weed plants in the UK were successfully transmitted to OSR. Previously undescribed hosts, verbascum, geranium, teasel, spear thistle, dock and previously described hosts in the Brassicaceae, Compositae and Lepidium families were found in the UK. A full-length infectious clone of a UK isolate of TuYV has been produced, this will allow further assessment of TuYV in the future. The infectious clone was able to cause systemic infection of TuYV and was aphid transmissible. The Arabidopsis thaliana gene knock-out study did not reveal a single eIF gene or gene linked to virus movement or silencing that could provide extreme broad-spectrum resistance. The gene eIF(iso)4G.1 was able to give a broad-spectrum quantitative resistance, and the potential of eIF3D.2 as well as sucrose symporters SUC1 and SUC2 as candidates for extreme TuYV resistance were discovered. This understanding of the epidemiology and diversity of TuYV is being used to develop strategies for control.