Occurrence of Alfalfa Mosaic Virus in Potato (Solanum tuberosum) in Middle Volga Region of Russia

Alfalfa mosaic virus (AMV) is transmitted by aphids, has a wide host range, and is common worldwide (Jaspars and Bos 1980). In potato (Solanum tuberosum L.), AMV is the cause of the “calico” disease visible as bright chlorotic spots in the foliage and is often associated with tuber necrosis in susceptible cultivars (Nie et al. 2015). In Russia, AMV was reported to occur in potato in the Russian Far East near Vladivostok (Gnutova 2014) and in the northern Caucasuses near Maykop city (Gorkovenko 2007), but biological and molecular characterization of the potato AMV isolates was not conducted. Here, we report on the occurrence of AMV in potato in Tatarstan, Middle Volga region of European Russia. In July 2017, during the inspection of seed potato crops, two cultivars, Skazka and Impala, were found expressing calico foliar symptoms near Kazan (GPS coordinates N55.680266, E49.363741). These seed potato lots represented the first field generation, grown from virus-free planting material certified as prebasic seed material according to UNECE standard S-1 (United Nations 2016) and contained approximately one calico-symptom-expressing plant per 300 to 350 healthy-looking plants. Foliar samples collected from eight symptomatic plants of both cultivars were subjected to ELISA testing for potato virus Y, potato virus А, potato virus X, potato virus S, potato virus M, and potato leafroll virus (all kits from Neogen, Ayr, Scotland) and were found negative for all tested viruses. Two representative symptomatic samples were selected, one from each of the two cultivars, and subjected to further studies. Mechanical inoculation of the two samples into indicator plants produced local lesions in inoculated leaves of Vigna unguiculata and Phaseolus vulgaris, chlorotic local lesions in Chenopodium quinoa, and mosaic and systemic necrotic lesions in Pisum sativum. Mechanically inoculated Nicotiana tabacum ‘Samsun NN’ and Datura stramonium remained asymptomatic. Inoculation of the sprouting tissue of two potato cultivars, Desiree and Maris Bard, using a needle led to the expression of calico symptoms in the emerging potato foliage and suggested AMV infection. To confirm AMV infection, AMV-specific primers P3 and P4 described by Navarre et al. (2009) were used to successfully amplify the 702-bp band by reverse-transcription PCR from the original symptomatic Skazka and Impala, and also from the inoculated Desiree and Maris Bard plants, as described in Crosslin and Hamlin (2011). The two nucleotide sequences coming from Skazka and Impala were determined for two PCR products through direct PCR sequencing, designated Sk-Tat and Im-Tat, respectively, and deposited in the GenBank database under accession numbers MK695696 and MK695697. BLASTn analysis identified the AMV isolate PL-2-10 (accession KC182569) from pepper in Serbia as the closest match. The two seed potato lots screened did not have any Fabaceae crops nearby, and various weeds or wild legumes could be the most likely source of AMV infection. Because AMV can be associated with tuber necrosis (Nie et al. 2015), in addition to calico foliar symptoms, to prevent infection of seed potato and subsequent spread of AMV to commercial potato production in Tatarstan, surveys of weeds and wild legumes in the vicinity of seed potato production areas should be recommended.