An analysis of some necrotic virus diseases of the potato

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Abstract
A summary of the literature on streak is given, from which it appears that 2 distinct clinical states can be isolated. One of these (described by Orton and commonly known as stipple-streak or leaf-drop streak) was later designated as acropetal necrosis. The other is known as top-necrosis, or as described by Quanjer, acronecrosis. Acropetal necrosis is the distinctive reaction in certain vars. of the Y virus of Kenneth Smith. Top necroses are divisible into at least 4 distinct groups based on their varietal reaction, and here designated as top-necrosis X, top-necrosis A, top-necrosis B, and top-necrosis C. The first 3 are alike in that when they produce a top-necrosis in any given var. it is unaccompanied by any mosaic symptom. Top-necrosis C, on the other hand, is distinguished by the fact that necrotic and mosaic symptoms occur together. Top-necrosis X is due to the action of the X virus acting alone. Top-necrosis A is due to a complex containing both X and Y, possibly associated with the virus Z. Top-necrosis B is due to a complex containing both Z and Y. Top-necrosis C is likewise due to the presence of both viruses X and Y. The X and C complexes are capable of transmission by needle inoculation to other potato vars., though the resultant lesion may not be a top-necrosis. Top-necrosis B is not inoculable. Top-necrosis A can be conveyed to the vars. Arran Crest and Epicure by the needle. Carriers of top-necrosis A are found among numerous vars. such as Arran Banner, Majestic, and Up-to-Date; the latter is rarely to be found without this latent infection. A carrier of top-necrosis B has only been found in the field in the var. Di Vernon. A clinical disease, it is held, cannot be defined by the syndrome of its reaction in a particular potato var., but is to be identified rather by the complete course of its reactions in a number of vars., as well as in other spp. of non-related Solanaceae. The goal in the classification of plant virus diseases should be to find a correct formula in terms of the virus entities concerned for each clinical disease.

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