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 l arge number of soil samples collected in eastern Germany. Additional P . betae isolates were received from several institutions in western Ge rmany and abroad. Isolates were grown on sugarbeet seedlings and teste d for the presence of beet necrotic yellow vein virus (BNYVV) and beet soilborne virus (BSBV). BNYVV was only present in isolates from weste rn Germany and abroad but absent in all isolates from eastern Germany. In contrast, BSBV was detected in more uniform geographic distributio n 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 for mation in the root system of sugarbeet seedlings. Differences in virul ence were found among virus-free as well as virus-carrying P. betae is olates. 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.