New natural weed host Raphanus raphanistrum L. (Brassicaceae) for Beet necrotic yellow vein virus and its vector Polymyxa betae Keskin

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Rhizomania is an important virus disease of sugar beet (Beta vulgaris) caused by Beet necrotic yellow vein virus (BNYVV). The virus is transmitted to the roots of host plants by the plasmodiophorid Polymyxa betae. During survey studies in September and October 2009, yellow vein banding symptoms were observed on wild radish (Raphanus raphanistrum) plants growing in spinach (Spinacia oleracea) fields with a history of rhizomania in Samsun Province in the Black Sea Region of Turkey. To verify possible alternative hosts for BNYVV and P. betae, R. raphanistrum and spinach plants and soil samples were collected. BNYVV was detected in the leaf samples of field-collected R. raphanistrum using the DAS-ELISA test. This result was confirmed by RT-PCR and the Raphanus isolate was determined to be from the type A strain based on restriction fragment length polymorphism analysis. The presence of P. betae cystosori was not detected in the roots of R. raphanistrum plants using a light microscopy technique, but some objects resembling sporogenic plasmodia of P. betae were observed in the infested root cells. To confirm this result, total RNA was extracted from the roots of these samples and tested by RT-PCR using Polymyxa-specific primers. In contrast to the microscopy method, all samples tested positive for P. betae using RT-PCR. Healthy seeds of R. raphanistrum were planted in plastic pots containing soil from the same fields infested with BNYVV and the seedlings showed similar symptoms to those growing in natural conditions. To our knowledge this is the first report of a BNYVV infection of R. raphanistrum under natural conditions.