An analysis of some necrotic virus diseases of the potato

The term “streak” has been applied somewhat indiscriminately to a variety of diseases of the potato in which the outstanding lesion is the appearance of necrotic patches in stem or leaf or both, such areas being more or less elongated. The first description of a disease of this type in the potato was made by Orton (1914) in 1912 ; a more detailed account of the same followed in 1920 (1920). Orton’s description brings out certain salient points : in the diseases he describes the lesions are first seen in the adult leaves about one-third of the way down the stem as elongated spots which follow the veins and invade the parenchyma, and when viewed from the under surface of the leaf are seen to follow the veins as discoloured streaks. The disease spreads with great rapidity on the plant, the petioles become involved and collapse and, as a consequence, the leaf withers and hangs by a thread to the stem. The stem itself becomes brittle, turns brown, and dies at a point below the tip. The discolouration is superficial and apparently does not involve the vascular bundles. He observed no lesions in the tuber. Orton further noted the fact that whilst certain varieties such as Factor (Up-to-Date) were highly susceptible, many seedlings appeared to be immune. He could find no pat ho gene for this disease. Murphy (1921) described two conditions which he denoted as streak and leaf-drop ; it would appear that the former is merely the seasonal, the latter the secondary form of one and the same affection. Schultz and Folsom (1925) give a description of an affection in Green Mountain which corresponds closely to that given by Murphy for his streak. Atanasoff (1922,a ) gave a full account of this same disease as exhibited in Duke of York and President. Most of his conclusions are based on the behaviour of the former variety, which is unfortunate, for, as we shall see, the Duke of York or Schottische Muis, as Atanasoff calls it, is a frequent carrier of another type of streak. Later, Atanasoff (1923—1925) mentions that this same streak could be transmitted by sap inoculation and by aphides, a characteristic of the streak under discussion. He (1922,b ) has laid all workers under an obligation by his research into the history of the so-called degeneration diseases of the potato in general, and particularly in his attempt to track down this particular disease as the evil genius of the potato from 1775 onwards. Recent work which has shown this disease to be due to a single aphis-carried virus, adds much strength to this claim.