Pferdmenges, F: Occurrence, spread and pathogenicity

pathogens (Rhizoctonia solani Keskin, Fusarium sp., Pythium sp.). Synergism between BNYVV, Rhizoctonia solani Keskin and Pythium sp. could lead to severe virus symptoms and weight reductions particularly in the Spanish soil. To determine if resistance breaks are correlated with the BNYVV inoculum concentration a "Most Probable Number"(MPN) - tests was conducted where same soils as in the resistance tests were examined. Thereby, D soil revealed the highest BNYVV density, the GG soil on the other hand displayed 520 times lower MPN. In order to obtain information on the aggressiveness of particular virus isolates an additional MPN with Rz1+Rz2 genotypes was performed. Within this test D, IV, MN and P resulted again in high BNYVV densities even able to infect Rz1+Rz2 plants after 4 weeks cultivation. These results give strong evidence that high inoculum doses are not responsible for the observed resistance breaks. To prove this conclusion another experiment with normalise inoculum added to sterile soil was carried out. Within this test three time harvests were conducted after 4, 8 and 12 weeks. Obviously, a significant differentiation of virus isolate vs. genotype correlating to tap root weight was only observed after 12 weeks. Consistently, applying adjusted inoculum density, D, IV, MN and P produced the highest virus contents at 12 weeks. Thus, resistance breaks must be connected to high BNYVV pathogenicity and not to inoculum density. Additional, experiments were conducted to test the influence of viruliferous P. betae zoospore concentrations from various origins, carrying different BNYVV-types. But due to uncertainty how many of the zoospores are actually viruliferous, the data resulted in highly different outcomes, not correlating to the results from tests in naturally infected soil. Moreover, efforts were undertaken to shorten resistance tests and replace them with time saving artificial sugar beet leaf inoculation via co-infiltration of a BNYVV RNA3 encoding P25 infectious cDNA clone and a red fluorescing marker gene (mRFP). Although, the method itself worked very well in young sugar beet leaves, no differences concerning the sugar beet genotype could be detected. The expected variability of fluorescence intensity comparing susceptible and resistant sugar beet cultivars was not given.