POWDERY SCAB OF POTATOES. 7 



DAMAGE TO THE POTATO CROP. 



The scabby stage of Spongospora, like the common Oospora scab, 

 is a skin disease confined to the tubers, marring their appearance 

 and thereby decreasing their market value. The cankerous stage, as 

 shown, in Plate III, completely destroys the tubers for both food and 

 seed purposes. This observation is confirmed by the following 

 quotation from Pethybridge (1911, p. 442); 



As was pointed out last year, Spongospora scab presents two forms of attack, In the 

 one case that of small spots on the surface of the tubers, and in the other the form of 

 a " canker " or eating away of the tuber. This latter is, of course, the most serious one, 

 but there are all degrees of transition between it and the spot form. 



Pethybridge is inclined to classify the effect on the potato as 

 producing "scab spots" and "cankers," the former doing little harm 

 to the tuber, while the latter, as shown by his illustrations, com- 

 pletely deform and dwarf its growth, so as to make the tubers 

 worthless. 



Osborn (1911) holds that the soil moisture determines to a great 

 extent the damage done by the disease and says — 



Under dry conditions of the soil the external appearance is limited to small circular 

 patches about 5 mm. across. Under wet conditions the damage is more serious and 

 the scabs may be as large as 3-4 cm. in diameter and as much as 2 cm. in depth. This 

 is the only external appearance; there is no sign of hypertrophy or any distortion other 

 than that caused by the pitting. 



The presence of the fungus in the cells stimulates the host to lay 

 down a new layer of cork cells surrounding the sorus, if the soil is 

 not too wet, which checks its growth. 



By Giissow (1913), who has, as already stated, found powdery 

 scab in Canada, the disease is not considered trifling. He says — 



The disease should by no means be regarded lightly. Severe attacks occur when 

 potatoes are planted year after year on infected land. Where this is practiced the 

 result will be potatoes hardly superior in quality to those badly affected with canker. 

 This fact is worthy of notice, especially since, as In the case of canker, no preventive 

 measures have proved of much value. 



In a very recent publication, Pethybridge (19135, p. 459) refers to 

 the damage done by Spongospora in his experimental plats, as 

 follows : 



They were particularly disastrous on those . portions of the land which for special 

 purposes have now been cropped for four successive seasons with potatoes, the cankerous 

 form of the disease being extremely common. In one or two plats nearly two-thirds 

 of the total crop were practically ruined by it, while the general average loss in the 

 plats on the old land due to it would be about one-third of the crop. 



EFFECT ON SEED POTATOES. 



Besides injuring the potato for market purposes and decreasing 

 the yield, as already, noted, powdery scab also depreciates the value 



