Gazania spp.: A New Host of Lettuce Mosaic Potyvirus, and a Potential Inoculum Source for Recent Lettuce Mosaic Outbreaks in the Salinas Valley of California.

Lettuce mosaic potyvirus (LMV) outbreaks in the Salinas Valley of California have increased in recent years despite a comprehensive lettuce mosaic management program. In a previous study, it was established that new serologically and/or genetically distinct LMV strains were not responsible for these outbreaks. Therefore, the role of alternate hosts was investigated. Here we provide evidence that the ornamental plant, Gazania spp., is an alternate host of LMV. A number of lines of evidence were used to establish that gazanias were infected by LMV, including indirect enzyme-linked immunosorbent assay, dot blot hybridization with an LMV coat protein (CP) probe, and sap- and aphid-transmission of LMV from gazania to lettuce and Nicotiana benthamiana. Additionally, an LMV CP fragment was amplified from an infected gazania plant by reverse transcription-polymerase chain reaction, and the nucleotide sequence of this fragment was 99% identical to the CP sequence of an LMV isolate from the Salinas Valley. We further show that LMV-infected gazanias are widely distributed in the Salinas Valley, and are present near areas where LMV outbreaks have occurred. Field experiments demonstrated that LMV could be transmitted from infected gazania to lettuce. LMV-infected gazanias also were found in local nurseries, suggesting that infection of gazania with LMV had occurred prior to planting in the Salinas Valley (i.e., lettuce was not serving as the source of inoculum for infection of gazania in the Salinas Valley). These results establish that gazania is an alternate host of LMV, and suggest that gazanias may be a primary inoculum source for recent lettuce mosaic outbreaks in the Salinas Valley.