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Agriculture (1917), Skin Spot is listed as a disease prevalent 

 during- the winter of 1916-17, especially in the variety "Aran 

 Chief 1 ' (Bd. Agric. Miscel. Publ. No. 21, 1918, p. 13). Finally 

 it has been briefly referred to by Mr. A. D. Cotton, in his paper 

 on Potato Diseases issued in' the March Supplement of the Board 

 of Agriculture's Journal for March, 1919. it is there stated 

 (p. 47) that the causal fungus is not Spicaria soluni, but probably 



an undescribed species. 



External Appearance op the Disease. 



The external characters of "Skin Spot," although not very 

 striking, are quite distinctive, and there is not much likelihood 

 of confusing it with other " spots." "When a large number of 

 potatoes are examined it becomes apparent that the spot may 

 assume a somewhat 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, whch 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 swellings when dry are very 

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

 out as a darker brown. In the case of the thinner-skinned and 

 smooth varieties, however, such as " King Edward," the diseased 

 areas are rarely if ever raised, but consist of small sunken dark 

 circular spots with a slightly 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 flat 

 form, each affected area appears to be the result of individual and 

 separate infection, and rarely extends for more than 1 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 will 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 



with which an entire spot or scab mav be picked out. If removed 

 with the point of a needle a white socket of sound potato tissue is 

 usually 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 

 parenchvma of the flesh. 



Microscopic 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 



a 2 



