Some Experiments On Inoculating Methods With Plant Viruses, And On Local Lesions

Abstract

S ummary . It is shown by means of dilution tests that a light rubbing with virus, in which no visible wound is produced on the leaf, is a more effective method of mechanical inoculation than scratching with a needle in the case of the five viruses: cucumber mosaic, spot necrosis, tobacco ring spot, petunia mosaic, and yellow tobacco mosaic, all inoculated on to tobacco. The contentions of Holmes with regard to inoculating methods with tobacco mosaic are thus extended to these other viruses. Some of these viruses form local lesions of a definite type which may prove to be of value in quantitative work. It is shown that temperature, age of the leaf inoculated, and other factors influence the character and the number of the local lesions formed. The development of the virus in the inoculated leaf (the “primary phase” before systemic symptoms become visible), as well as certain later stages, can be followed particularly well in the case of yellow tobacco mosaic, using the iodine method for demonstrating the areas of normal starch formation (Holmes, 1931). In an area closely corresponding to where the virus has spread, starch formation is inhibited, so that the path followed by the virus shows up as a light‐coloured area in the treated leaves. An intimate relation between the vascular system and the path of travel of the virus is made strikingly evident by this treatment.