STUDIES ON THE BIOLOGY OF POLYMYXA-BETAE, THE VECTOR OF BEET NECROTICYELLOW VEIN VIRUS

The development of Polymyxa betae within the roots of a sugar beet var iety susceptible to Rhizomania was observed in hydroponic culture over a period of 10 days. Light microscope studies showed that at an avera ge temperature of 20-degrees-C the life cycle of the fungus, containin g beet necrotic yellow vein virus (BNYVV), was completed within 10 day s. A change from the multiplication phase to the survival phase of P. betae became evident. At the beginning of the life cycle the fungus pr oduced mainly zoospores whereas at a later stage plasmodia developed i nto resting spores. Zoospore density in the nutrient solution reached a maximum between the 5th and 7th day after inoculation and then decli ned to the initial concentration. The number of zoospores attached to the root surface increased progressively at 48 h intervals, correlated with a parallel increase in BNYVV-content of the roots. Light- and fl uorescence microscopy revealed that zoospores of P. betae often attach near the point of release and do not move very long distances. In add ition it became evident that zoospores may attach to the thallus wall inside the zoosporangium that they have developed in.