Studies on the Biology of Polymyxa betae , the Vector of Beet Necrotic Yellow Vein Virus

The development of Polymyxa betae within the roots of a sugar beet variety susceptible to Rhizomania was observed in hydroponic culture over a period of 10 days. Light microscope studies showed that at an average temperature of 20 °C the life cycle of the fungus, containing beet necrotic yellow vein virus (BNYW), was completed within 10 days. A change from the multiplication phase to the survival phase of P. betae became evident. At the beginning of the life cycle the fungus produced mainly zoospores whereas at a later stage plasmodia developed into resting spores. Zoospore density in the nutrient solution reached a maximum between the 5th and 7th day after inoculation and then declined 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 BNYW‐content of the roots. Light‐ and fluorescence microscopy revealed that zoospores of P. betae often attach near the point of release and do not move very long distances. In addition it became evident that zoospores may attach to the thallus wall inside the zoosporangium that they have developed in.