POTATO DISEASES 



1721 



However, growers sliould be constantly 

 on the lookout for it. 



New .Ipi'sey Agricultural Experiment Stntimi 

 Circular :{:i. 



Powdery Dry Rot. See Dry Rot, this 



section. 



Powdery Scab 



Spongospora subterraiica Wallr. 



This serious disease of the potato has 

 recently been introduced into Canada 

 from Europe and has spread to one of 

 the seed-producing sections of Maine. In 

 1914 there was a quarantine established 

 by the federal authorities against ex- 

 portations of seed potatoes from that 

 state. 



The Government also prohibits im- 

 portations of potatoes from countries 

 where the disease is known to exist. 



Prof. J. W. Morse, of the Maine Experi- 

 ment Station, describes the disease as 

 follows: 



"The early stages of powdery scab on 

 young tubers appear in the form of small 

 pimples, with a slight discoloration of 

 the surface. When cut open the infected 

 areas appear purplish. These minute 

 pustules may occur in patches or scat- 

 tered over the surface of the tubers. As 

 they enlarge they become raised and 

 break through the epidermis, which 

 stands up and curls back around the spots 

 in a characteristic manner. 



"If the surfaces of the diseased pota- 

 toes have not been exposed to too much 

 friction specimens may be collected fre- 

 quently in the storehouses in the winter 

 which show distinct raised, blister-like, 

 dark-colored spots, usually not more than 

 one-sixteenth to one-eighth of an inch 

 in diameter, in no way resembling the 

 common type of scab. These are shown 

 indistinctly on the surface of the appar- 

 ently decayed tuber. 



"If the top of these blisters or pustules 

 is carefully removed by means of a 

 needle or penknife the interior will be 

 found to be filled witli dead tissue and a 

 dark-colored or brownish powder, fre- 

 quently having a slight olive tinge. 

 Usually, however, when affected tubers 

 come under observation they have been 

 subjected to more or less friction, which 



has not only removed the top of the pus- 

 tules but most of the powder as well. 

 After one acquires a slight familiarity 

 with the disease one can usually recog- 

 nize it without trouble, even in this stage, 

 on account of the size, grouping and ap- 

 pearance of the spots. 



"Unless several spots have run together 

 and coalesced, they are usually oval to 

 circular in shape, small, seldom more 

 than one-sixteenth to one-eiglith of an 

 inch in diameter; frequently several oc- 

 cur together in clumps. Occasionally 

 they may extend diagonally across the 

 surface of the potato in more or less 

 irregular, parallel lines. The ragged, 

 turned-back margins of the rutpured epi- 

 dermis already mentioned are usually 

 still in evidence after the tops of the 

 pustules have been rubbed off. 



"This is by no means the only form of 

 alteration that the parasite may produce 

 in the normal appearance of the host. In 

 severe attacks, especially In moist soil, 

 a distinctly warty appearance may de- 

 velop, quite different from the ordinary 

 type of powdery scab. These warts are 

 several times larger than the scab pus- 

 tules and are usually smooth and round- 

 ish, although somewhat irregular, vary- 

 ing in color from a light to dark brown 

 and more frequently occurring at the 

 terminal or seed end of the tuber. Still 

 another stage is recognized, where con- 

 siderable portions of the surface of the 

 tuber become destroyed and eroded, leav- 

 ing a distinct, hollowed-out, cankerous 

 area. 



"A few other characteristics of the dis- 

 ease as observed in Maine should be 

 noted. The most important is, perhaps, 

 that potatoes affected by powdery scab, 

 especially in severe or moderately severe 

 cases, show a tendency to wither rapidly. 

 This takes place even under good storage 

 conditions, and it becomes very evident 

 it the tubers are removed from the cellar 

 and kept at the temperature of the aver- 

 age living room for a few days. 



"Very frequently the skin of the potato 

 in a circle immediately surrounding the 

 individual scab spots becomes depressed 

 and browned, taking on a color very sim- 



