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Abstract

A virus ring spot of Odontoglossum Orchid: symptoms, transmission, and electron microscopy.

Abstract

From the University of California, Berkeley, the authors describe a virus disease of Odontoglossum grande differing markedly in symptomatology from other orchid viruses. Minute, necrotic spots or rings appear on the surface of one or more of the older inoculated leaves, while the younger ones show light green to pale yellow, circular, oval, or diamond-shaped areas, 0-5 to 2 cm. in diameter, which may be sharply delimited or present only a mild halo effect. A few weeks later the outline of some rings may be partially or wholly necrotic, while the central portion remains pale green or yellow. The upper surface of the entire zone may become necrotic and the lower appear watersoaked. The necrotic rings are usually single, but there may be several concentric, brown to black circles with green to yellow tissue between and a central necrotic spot. The ring spots may be separated from each other by normal tissue or coalesce to form larger compound patterns. In the case of three plants inoculated on only half of each leaf, the first symptoms were observed on the inoculated halves after about a fortnight and on the unnoculated two to four weeks later. Transient symptoms in the form of irregular, chlorotic, ovoid areas may develop on fully expanded leaves produced by the youngest bulbs, but no sign of infection was discernible on the flowers of two diseased plants inoculated seven months earlier.
The available evidence indicates that the ring spot virus exerts an initial shock effect on O. grande, expressed by the production of conspicuous foliar symptoms, whereas later growth seems to harbour the infective principle without noticeable external symptoms. Juice from a symptomless new bulb of the original diseased plant and from its apparently unaffected foliage proved to be infectious on inoculation into healthy plants. Electron micrographs of the orchid ring spot virus revealed rod-shaped particles, with a tendency to slight curvature, about 24 m/i in width and commonly 280 mµ long.
None of the 20 species representing 18 genera in 11 families contracted the ring spot in mechanical inoculation experiments with expressed sap from diseased plants.