POWDERY SCAB OF POTATOES. 99 



whole plants are dug up and examined the differences can 

 usually be detected. Spongo'spora warts are usually smootri 

 and rounded, while those produced by Chrysophlyctis are gen- 

 erally irregular and broken, soon taking on a coral-like structure 

 and always begin around the eyes of the tuber. 



Economic Importance oe Powdery Scab. 



While experience in this State is yet too limited to be made 

 the basis of any definite conclusions the opinions expressed by 

 those who have studied the disease most carefully in Great 

 Britain and Ireland seem to warrant the conclusion that pow- 

 dery scab is without doubt the most serious disease with which the 

 Maine potato growers have ever had to contend. It has already 

 been pointed out that observations made in this State indicate 

 that potatoes badly affected by the ordinary type of powdery 

 scab wither very rapidly and frequently show a discoloration of 

 the surface resembling a dry rot. Also in case of severe infec- 

 tion large warty outgrowths or cankerous areas may be pro- 

 duced. 



One of the most specific statements recently made regarding 

 the economic importance of the disease appeared in an article 

 by Giissow* in which 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 affected by canker (wart disease). This fact is worthy 

 of notice, especially since, as in the case of canker, no preven- 

 tive measures have proved of much value." Pethybridge in 

 1912 called attention ito the fact that the organism causing pow- 

 dery scab not only may attack the tuber, but is also capable of 

 producing galls on the roots as well as on the underground 

 tuber bearing stems of the potato, and the spore-balls produced 

 on these root-galls are for the most part liberated in the soil. 

 Perhaps the most important fact from the economic standpoint 

 is that the disease in addition to being carried by the seed 

 tubers may persist in infected soil for some time, thus mak- 

 ing it worthless for potato culture. Pethybridge has also stated 



