TRANSMISSION, HOST RANGE, AND SEROLOGICAL PROPERTIES OF THE VIRUSES THAT CAUSE LETTUCE SPECKLES DISEASE

The speckles disease is caused by a virus complex that affects lettuce [Lactuca sativa], sugar beets [Beta vulgaris] and spinach [Spinacia oleracea] in the Salinas and Pajaro Valleys of California [USA]. The complex consists of 2 viruses, beet western yellows virus (BWYV) and lettuce speckles mottle virus (LSMV). Both viruses are transmitted in a persistent manner by Myzus persicae, the green peach aphid; however, LSMV is aphid transmissible only when in a mixed infection with BWYV. LSMV is mechanically transmissible and when separated from BWYV by mechanical transmission its aphid transmissibility is lost. Aphid transmission from mixed infections extends the host range of LSMV compared to species infected following mechanical inoculation. Lettuce speckles mottle virus in mixed infections exhibits a serological relationship to BWYV and the in vitro stability of LSMV is greater in sap from plants infected with the speckles virus complex than from those infected with LSMV alone. These data suggest genomic masking of LSMV by BWYV coat protein in the mixed infection.