Technical Abstract: Beet Necrotic Yellow Vein Virus (BNYVV) causes one of the most economically destructive sugar beet diseases, rhizomania, which may reduce sugar yield by 100%. The virus has rod shaped particles containing four to five single stranded RNAs and is transmitted by the root-infecting parasite Polymyxa betae. This field investigation was conducted to evaluate the interactions between spinach genotypes and different BNYVV strains, and to determine whether BNYVV is transmitted through spinach seeds. Eight commercial spinach cultivars were planted in two BNYVV infested fields and two control fields in Salinas, CA in 2009. Spinach plants in the BNYVV infested fields showed disease symptoms of yellow-green or light-green vein clearing, mottling or yellow-green chlorotic lesions on younger leaves as early as 28 days after planting (4 - 6 true leaf stage). Leaves may also become stiff, more crinkled, and necrotic. There was an increase of lateral roots and leaf number but a decrease of leaf weight compared to healthy plants. Infected plants often became stunted, deformed, wilted, and dead. Symptomatic leaves and roots from plants with or without leaf symptoms in BNYVV infested fields all tested positive for BNYVV by ELISA. There were significant differences in disease development among cultivars, with disease incidence ranging from 8% (‘Unipack 277’) to 44% (‘Polar Bear’), suggesting that genetic improvement of BNYVV-resistance through spinach breeding should be feasible. A more aggressive (resistance-breaking) strain of BNYVV led to higher disease incidence in spinach than the wild type. BNYVV was not transmitted through spinach seeds. Although sugar beet acreage in California has declined in the past decade, BNYVV and its vector can persist in soil for at least 15 years. The rising acreage of spinach host may increase BNYVV in soil. Diseased spinach plants were found in a grower’s field in Ventura County, CA recently and were tested positive for the aggressive strain of BNYVV. Therefore, BNYVV is a new threat to spinach production in California.