Beet necrotic yellow vein virus (BNYVV) is a plant virus and the type member of the Benyvirus genus. BNYVV is responsible for rhizomania, a disease of sugar beet (Rhizo: root; Mania: madness) that causes proliferation of thin rootlets, and leads to a smaller tap root with reduced sugar content. Infected plants are less able to take up water, and wilting can be observed during the warm period of the year. If the infection spreads to the whole plant, vein yellowing, necrosis and yellow spots appear on the leaves, giving the virus its name.

BNYVV is transmitted by the protozoan Polymyxa betae (initially thought to be a fungus). Polymyxa forms highly resistant spores that can rest in soils for more than two decades. The virus can rest in these spores, and when the spores germinate into mobile zoospores, they transmit the virus to the plant.