POTATO SCAB. RY JAS. ELLIS HUMPHREY, PROFESSOR OF VEGETABLE PHYSIOLOGY. The value of the potato crop in Massachusetts exceeds that of any other planted crop; consequently, the loss by any widespread and serious disease of this crop must be an important item. The commonest and most constant disease which attacks the potato in the field is that commonly known as the "scab." It is well known in both Europe and America, and attacks the tubers, giving little or no evidence of its presence in those parts of the plant above ground. The cause of this trouble is not at all understood, though various theories are held as to its nature. It is proposed in the present paper to discuss briefly the present state of our knowledge of the potato scab, by way of introduction to a series of investigations of the disease which the writer expects to carry on during the coming year. The disease first manifests itself in the form of small corroded spots or pustules on the surface of the potato. Writers on the subject generally agree that these spots replace the "lenticels" of the tubers. If a smooth potato tuber be closely examined, there will be seen spots of the size of a pin's head or smaller, of a slightly different shade, and somewhat roughened or granular in appearance. These breaks in the continuity of the tissue of tabular cork-cells which form the so-called "skin" of the potato, are filled with loose, globular cork-cells, through whose intercellular spaces an interchange of gases can take place between the interior of the potato and the outer air. They are then, so to speak, the ventilators of the tuber, and are known as "lenticels." (The normal structure of the potato tuber is shown in the accompanying Fig. 2.) It is in these lenticels that the scab originates or first shows itself. From these spots the disease rapidly spreads, until sometimes almost the whole exterior of the tuber becomes involved in the decay and breaking down of the surface tissue. In many cases, at least, there are developed over these patches, rough, brittle scales or crusts of corky tissue, which poel readily from the surface, and which render the name |