Seed transmission of Alfalfa Mosaic Virus in basil

Infection caused by Alfalfa mosaic virus (AMV) is one of the most important viral diseases of basil (Ocimum basilicum L.) worldwide. Symptoms elicited by AMV in basil are very characteristics consisting in a bright yellow-white mosaic of “calico-type”. For this reason, basil is routinary used as differential host of AMV. Recently, in some basil cultivations of Southern Italy, realized with seed reproduced by farmers, a recrudescence of the disease caused by AMV was observed. In 2010, a survey was conducted in an insect-proof greenhouse at 20-24 °C to verify seed transmission of AMV in basil. Plants of basil “Gigante Genovese” were sap-inoculated with the subgroup I AMV-Lst isolate and bright yellow mosaic appeared after about 3 weeks on inoculated plants. AMV infection on symptomatic plants was confirmed by PAS-ELISA test with specific AMV antisera. About 2100 mature seeds, harvested from a single AMV-infected basil plant, were sown in steam sterilized soil. Germination rate obtained from the infected plant was much lower (26.42%) than from healthy basil “Gigante Genovese” (66.99%). Only 3 plantlets, obtained from seeds harvested from the infected plant, showed typical bright yellow mosaic, whereas AMV infection was tested, both in symptomatic and asymptomatic plantlets, by back inoculations on local lesion hosts (Vigna unguiculata L. “Black eye”), DAS-ELISA and RT-PCR by using primer pairs specific for AMV coat protein gene. All these diagnostic methods confirmed AMV infection only in the three symptomatic plantlets, thus the percentage of seed transmission was of 0.54%. Moreover, the identity of AMV-Lst was confirmed by sequencing the amplicon obtained by RT-PCR on the RNAs extracted from symptomatic plantlets. AMV has a wide host range including ornamental and vegetable species. Consequently, seed-borne AMV infection may have important practical implications since the virus source can be reintroduced every year with basil seed, leading to spread of virus by aphid to other susceptible crops. This study represents the first evidence of AMV as seed-born disease in basil