THE MORPHOLOGY AND HOST RANGE OF BRASSICA NIGRA VIRUS

Abstract

NUMEROUS VIRUS diseases of cruciferous plants have been described and named. Systems of classification and nomenclature have been proposed by Holmes (1939), Smith (1937), and Pound and Walker (1945). The first two used mainly host range and symptomatology, the others, Pound and Walker, emphasized symptom manifestation at different temperatures and host immunity reactions. A property which might be used to differentiate viruses, the rate of electrophoretic mobility, has been suggested by Frampton and Takahashi (1946). Any system of classification should include, if possible, two important characters, the morphology and the size of the virus. The form and typical size of a mosaic virus of Brassica nigra have been ascertained by means of the electron microscope. A selected host range study has also been conducted in an attempt to identify the virus. METHODS.-Mosaic Brassica nigra leaf tissue (1 g.) was homogenized in a ground glass homogenizer and the slurry diluted with 5 ml. of pH 7.0 phosphate buffer. To identify the virus, eight different species of Brassica, B. nigra Koch, B. oleracea L., B. campestris L., B. carinata Braun, B. juncea Czern. et Coss., B. napus L., B. arvensis L. B. S. P., and B. alba (L.) Boiss, and two species of Nicotiana, N. tabacurn L. and N. glutinosa L., were inoculated with this preparation. Brassica species were inoculated with the aid of carborundum powder (Rawlins and Tompkins, 1936) and the leaves of Nicotiana were inoculated by means of a stiff bristle brush. For isolation of the virus and electron microscopy two hosts B. nigra and B. juncea were selected. Approximately 250 g. each of fresh mosaic and healthy leaf tissue was frozen, then ground in a food chopper. The juice was squeezed out through several thicknesses of cheesecloth and 40 g. of ainmonium sulfate dissolved in each 100 ml. of extract. The resulting heavy green precipitate was centrifuged at 2,000 X gravity for 20 min. The brown supernatant liquid was discarded and the green precipitate was redispersed in pH 7.0 phosphate buffer (1/4 the original volume). The mixture was then centrifuged at 12,000 X gravity for 10 min. The supernatant was retained and centrifuged for 1 hr. at 75,000 X gravity. The pellets were suspended in pH 7.0 phosphate buffer and again centrifuged at 12,000 X gravity for 10 min. The opalescent somewhat viscous supernatant liquids were diluted with distilled water until only a faint Tyndall beam was visible in the path of a narrow intense beam of light. Small drops of this liquid were used to prepare electron microscope mounts. To test its activity, ten young plants of B. juncea were inoculated with an aliquot of the undiluted preparation.