Disease Note

Freeway Daisy (Osteospermum fruticosum) as Host for Lettuce Mosaic Virus. D. C. Opgenorth, California Department of Food and Agriculture, Sacramento. J. B. White, B. Oliver, and A. S. Greathead. California Department of Food and Agriculture, Sacramento; Monterey County Agricultural Commissioner’s Office, Salinas; and University of California Cooperative Extension, Monterey County. Plant Dis. 75:751. Accepted for publication 8 March 1991. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/PD-75-0751E.

In September 1990, mild mosaic symptoms were observed on trailing African, or freeway, daisy (Osteospermum fruticosum (L) Norl.) obtained from a nursery and from plantings of the ground cover in Monterey County, California. Examination of plant samples by electron microscopy revealed a flexuous, rod·shaped virus approximately 13 X 740 nm long. The virus was easily transmilled mechanically to Chenopodium quinoa Willd, and to lettuce, producing symptoms similar to lettuce mosaic. Original O. fruticosum samples, as well as C. quinoa and lettuce indicator plants tested strongly positive for lettuce mosaic virus (LMV) by an ELISA method. The virus was transmitted from infected Osteospermum plants by placing virus-free, healthy container-grown lettuce in areas with LMV-infected Osteospermum ground cover for 7 days. After a greenhouse incubation for 7 days, the lettuce plants showed symptoms of lettuce mosaic, and ELISA tests were positive for the virus. Observations and extensive mapping of patterns of LMV incidence in a fie1d adjacent to an infected Osteospermum planting strongly suggest that this plant can pose a significant threat to commercial lettuce plantings. In addition, O. fruticosum may serve as an alternate host that allows LMV to persist through the lettuce-free period, now used as a control measure.