INVESTIGATIONS ON THE SUSCEPTIBILITY OF OILSEED RAPE (BRASSICA-NAPUS L, SSP NAPUS) TO DIFFERENT VIRUS DISEASES

Natural occurrence, tendency to spread and damaging effects of different virus diseases on oilseed rape were studied under glasshouse or outdoor conditions. The infestation incidence, assessed as severity of symptom development 6 weeks after artificial inoculation, was found to range from mild to moderate with the lowest incidence for BWYV followed by TYMV, CaMV and TuMV. Despite the weak symptom development induced by BWYV, a significant yield response was recorded. Average yield of single plants of the tested breeding line and cultivar showed considerable decline in the seed production averaging roughly 40 and 50%, respectively. However, the only spring rape cultivar tested failed to respond to BWYV infestation. Field studies with BWYV, CaMV and TYMV, testing the natural spread from inoculated rape plants, showed by far the highest spreading rate for BWYV. Transmission of an isolate of BWYV from infested rape plants to different cultivars of sugar beet did not succeed. Field samples of winter oilseed rape from different locations in Germany revealed the presence of BWYV almost everywhere. Infection rates were found to be up to 100%. Therefore, incidence with BWYV may reach an epidemic level under suitable conditions of infection. On the contrary to this virus, CaMV, CMV, TuMV and TYMV were found to be essentially less frequent. They do not seem to be of a significant value for rape production at the present time.