Fusarium oxysporum is a fungal plant pathogen responsible for causing disease in many economically important crops. There are many 'special forms' (formae speciales) which cause infection in specific plant hosts. F. oxysporum f. sp. pisi (FOP) is the causal agent of wilt in peas and has been reported in every country where peas are grown commercially. Disease is currently controlled using resistant cultivars but due to the single gene resistance mechanism in pea there is a constant risk of resistance breakdown. Therefore, the aim of this work was to understand the molecular and genetic factors affecting pathogenicity of different FOP races. Additionally, the diversity of Fusarium species affecting UK peas was assessed. Isolations from infected peas in the UK indicated that numerous species could potentially be responsible for causing root rot. F. oxysporum isolates from infected peas separated into distinct clades compared to FOP isolates, leading to the conclusion that root rot causing isolates are more prevalent in UK pea fields than wilt causing isolates. Pathogenicity assays were developed for assessing root rot (seed inoculation) and wilt pathogens (root dip), which confirmed the difference between these isolates. Whole genome sequencing of three pathogenic FOP isolates revealed the presence of Secreted In Xylem (SIX) genes, and their confirmation in multiple isolates of races 1, 2 and 5 revealed a potential race specific complement of SIX genes in FOP. Significant upregulation of these genes in planta was observed over the time course of infection. Additional putative effectors predicted by genome analysis were also shown to be upregulated in planta at 96 hpi using RNAseq. Finally, FOP race type was confirmed, with results suggesting that putative race 5 isolates were most likely a subset of race 2, and therefore only clear differences between race 1 and race 2 could be established with effector gene profiles. Overall, this work shows that there are multiple factors influencing pathogenicity of FOP towards pea. The race specific SIX gene complements, and other novel race specific effectors, indicate targets for molecular identification of FOP races.