Virus Content and Virulence of Polymyxa betae Keskin Isolates Obtained from Different Regions in Europe

Polymyxa betae isolates were obtained by means of bait plants from a large number of soil samples collected in eastern Germany. Additional P. betae isolates were received from several institutions in western Germany and abroad. Isolates were grown on sugarbeet seedlings and tested for the presence of beet necrotic yellow vein virus (BNYVV) and beet soilborne virus (BSBV). BNYVV was only present in isolates from western Germany and abroad but absent in all isolates from eastern Germany., In contrast, BSBV was detected in more uniform geographic distribution in 14 out of 33 P. betae isolates tested. The virulence of P. betae isolates was estimated on the basis of the extent of resting spore formation in the root system of sugarbeet seedlings. Differences in virulence were found among virus-free as well as virus-carrying P. betae isolates. The mean value of virulence ratings was distinctly lower with BNYVV-carrying isolates and slightly lower with BSBV-carrying isolates as compared to virus-free isolates.