Technical Abstract: Fusarium oxysporum f. sp. betae, which causes Fusarium yellows in sugar beet, can be highly variable in virulence and morphology, with further diversity derived due to the wide geographic distribution of sugar beet production. Little is known about factors that determine pathogenicity to sugar beet and how these may differ between non-pathogenic F. oxysporum. Sugar beet is often grown in rotation with other crops including dry edible bean and onion, with F. oxysporum able to cause disease on all three crops. Fifty one F. oxysporum isolates were collected from symptomatic sugar beet throughout the United States production region. To investigate diversity of the F. oxysporum population and how crop rotation may influence pathogenic variation, these isolates were characterized for pathogenicity to sugar beet, dry edible bean and onion, as well as vegetative compatibility. Pathogenicity testing indicated that sugar beet may harbor pathogens that cause disease on hosts grown in rotation with sugar beet, particularly onion. None of the isolates tested were pathogenic to dry edible bean. Furthermore, vegetative compatibility testing supported previous reports that F. oxysporum f. sp. betae is polyphyletic and that pathogenic isolates cannot be identified from non-pathogenic F. oxysporum using vegetative compatibility.