A NEW VIRUS-DISEASE INDUCING A MOTTLE AND A YELLOWING OF ORNAMENTAL DANAE-RACEMOSA FOLIAGE CAUSED BY A SOWBANE MOSAIC-VIRUS STRAIN

Since the beginning of the 1980s, a disease has been observed on Danae racemosa Moench (Alexander Laurel), a plant of horticultural interest . In the south of France, as in Italy, D racemosa is commonly cultivat ed for its ornamental foliage, like Asparagus plumosus L. This monocot from the Liliaceae family shows typical damage on its foliage: first a mottle appears on the young phylloclades which turn yellow with time . This disease is rarely widespread in a plantation. Only single plant or small areas are usually affected but the regular extension of the disease is responsible for important economic consequences. In the pre sent paper we show that these symptoms are due to the propagation of a virus in the plant. The use of general virological tests, host range inoculation and retroinoculation, purification and ENT, demonstrated t he systematic presence of isometric virus particles (26 nm) in the dis eased D racemosa collected from various geographic areas. The study of the serological properties of the virus (immunodiffusion, immunoelect rophoresis and DAS-ELISA) with the MW determination of the capsid-subu nit and the RNA, led to its identification as a Sowbane mosaic virus s train. When healthy D racemosa were inoculated with the virus, the typ ical symptoms of the disease could only be identified after a long del ay (9 months after the inoculation) on the young neoformed boughs, fro m which the virus was isolated. We therefore consider the Sowbane mosa ic virus as the single pathogen inducing the mottle and the yellowing of the D racemosa foliage. In the diseased tissues, the virus was pres ent. in very low amounts and could not be defected by classical immuno enzymatic assay (DAS-ELISA). Retoinoculation and rare direct inoculati ons from crude extracts, on a sensitive host (ie Chenopodium quinoa) w ere necessary to propagate the virus. Disease diagnostic is discussed, especially as the virus seemed to be latent with no external symptoms during a certain growing period of the plant under special conditions (shadowing, high nitrogen fertilization). A rapid and efficient metho d for detection of the virus is absolutely necessary to carry out a ph ytosanitary programme and to obtain virus-free plants for vegetative p ropagation. Evidence was found. that the virus could be transmitted by several aphids; this is an important parameter for the control of the disease.