220 



Diseases of Economic Plants 



general heading, p. 236. Though the disease may not develop 

 annually in severe form, spraying is necessary each year as a 

 preventive measure. Since the bhght develops rapidly, it is 

 quite impracticable to wait for signs of the disease before 

 spraying is begun. Moreover, experiments extending over 

 several years have shown that even in years when the blight 

 does not become epidemic, sufficient 

 l)enefit is derived from spraying to 

 warrant its adoption. 



The Geneva Experiment Station 

 says: ''Judging from the experiments 

 thus far made, it appears that spray- 

 ing for l)light is an operation which no 

 potato grower in New York can afford 

 to neglect. Forty-eight farmers' busi- 

 ness experiments show an average 

 net profit of $20.51 per acre due to 

 spraying." 



Commence spraying with Bordeaux 

 mixture when plants are 6 to 8 inches 

 high and repeat at intervals of ten to 

 fourteen days throughout the season, 

 making, in all, five or six applications. 

 When Ijugs or flea beetles are numerous, add Paris green or 

 other poison. In case the tops have blighted, the danger of 

 tuber infection and consequent rot is lessened by delaying 

 the harvest of the tubers until at least a week or ten days 

 after the death of the foliage. 



Common-scab -''2-274 (Actinomycc-! i^cabies (Thaxt.) Giiss.) — 

 Scab prevails in Europe, Africa, New Zealand, in all parts of 

 the United States, and probably wherever the potato is 

 grown. It was first attributed to its proper cause by Thaxter 

 in 1890. 



Common-scali is recognized by all potato growers as a char- 

 acteristic, rough pitting of the tubers. If potatoes are at- 

 tacked when quite young, the scabs are deep; if the attack is 

 later, they are more shallow. In scn^ere cases the tuber is 



Fig. 120. — Common 

 scab produced by 

 inoculation forming 

 monogram R. T. 

 After Thaxter. 



