1246 Skin Spot Disease of Potato Tubers. [mar., 



what different form according to the variety of potato on which 

 it occurs. In the case of the coarser-skinned varieties, such as 

 Arran Chief," a definite pimple is developed, and the skin, 

 which is never ruptured, is stretched tightly over the swelling, 

 giving it a rather shiny appearance, which is most marked 

 when a number of spots have arisen so close to one another 

 that they coalesce. These swelHngs when dry are very similar 

 in colour to the normal skin, but on being wetted stand out 

 as a darker brown. In the case, however, of the thinner- 

 skinned and smooth varieties, such as " King Edward," the 

 diseased areas are rarely if ever raised, but consist of small 

 sunken dark circular spots with a sHghtly raised centre, the 

 area being brown or even black in colour. The latter agrees 

 exactly with the description given by Pethybridge. These 

 two forms have never been noted on one and the same tuber ; 

 intermediates, however, sometimes occur. In the case of both 

 the pimple and the fiat form, each affected area appears to be 

 the result of individual and separate infection, and rarely 

 extends for more than i mm. in diameter, or to a greater 

 depth than 2 mm. It has been stated that the rose-end of the 

 tuber is more liable to infection than the heel-end, but when a 

 large number of specimens are examined this statement is not 

 found to hold good. Very frequently, however, one side 

 ^vill be much more spotted than the other. Often the spots 

 occur so close to one another that many of them merge, and 

 thus extend over a considerable area, rendering the tuber 

 very unsightly and injuring a considerable portion of the 

 surface tissue. 



A noteworthy point in connection with Skin Spot is the ease 

 wdth which an entire spot or scab may be picked out. If 

 removed with the point of a needle a white socket of sound 

 potato tissue is usuahy left behind. The explanation of this 

 is seen by means of a section, when it becomes apparent that 

 the injured area is cuticularised and thus sharply isolated from 

 the cellulose parenchyma of the flesh. 



Microscoplo Characters. — On sectioning one of these spots 

 the hyphal threads of the invading fungus are plainly seen 

 among the cells of the potato tissue which have become brown 

 and dead. The filaments are exceedingly fine, 2-4 ^ in diameter, 

 hyaline or pale brown. The cell-walls appear to be somewhat 

 thickened and cuticularised and to be lined with a brown 

 substance, due probably to the decomposition of the cell- 

 contents. The f fungus penetrates below the periderm and 

 invades the parenchyma, often to a depth of 12-15 cells. 



