The analysis and synthesis of some disease of the mosaic type. The problem of carriers and auto-infection in the potato

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Abstract
Following the isolation of the X and Y viruses by Kenneth Smith, the writer has discovered a 3rd virus element which he has named the Z virus. A table is given of the reaction on different potato vars. of each of these viruses acting alone under greenhouse conditions. The Z virus alone has very limited pathogenicity, but in unison with either X or Y, or both, it brings about the diseases known as Crinkle "A" and Para-crinkle. It is also present in a streak-carrying Di Vernon and probably in many other infected plants. As a result of both analytical and synthetic methods it has been possible to ascribe compositions in terms of X, Y and Z to certain definite clinical virus diseases. When certain of the virus elements X, Y or Z are brought together experimentally in appropriate grouping, it has been possible to build up such clinical pictures as Interveinal Mosaic, Crinkle "A," and Para-crinkle. It appears that the Y'' virus of Para-crinkle differs somewhat from the common Y virus of the other potato diseases, but there is not yet evidence enough to demonstrate it to be a distinct virus element. The reaction of a virus complex is not the mere summation of that of its constituents. The view is put forward that the grouping of the virus elements in a complex is not to be regarded as comparable to a chemical valency, but rather to a linkage whose strength varies with the nature of the viruses concerned and in particular with the internal environment of the plant. Evidence is given showing that the addition of a further virus element, or of a complex of such, to a pre-existing virus complex may, in the 1st season at least, bring about a condition in which no clinical reaction occurs and the plant behaves as a carrier. A virus complex when present in a potato plant and exerting on it a definite reaction, may at some active growing point spontaneously break up and allow one or another of its constituent elements to pass alone into a growing bud and there express itself. This phenomenon has been observed on 3 occasions in Para-crinkle in Arran Victory. If a similar spontaneous splitting takes place in a carrier plant, one of the free elements may exert a pathological effect in its own capacity and convert the symptomless carrier into an openly sick plant. Such a phenomenon is described as one of auto-infection. It has been definitely observed in one var. and is suspected in 2 others. X-bodies were found in infections produced by the X virus, or where the X virus is present, but are absent in those consequent on the Y virus alone and, with a doubtful exception, absent in that due to the Z virus alone.

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