3«o 



TEXTBOOK OF BOTANY 



Fig. 214. — A potato leaf at- 

 tacked by the late blight. 



This fungus can be fairly well 

 controlled by spraying the grape 

 plants with the well-known Bor- 

 deaux mixture.^ 



384. Late Blight and Rot of 

 Potato. — This, commonly called 

 the " potato disease," is at times 

 very destructive and has been 

 responsible for the " potato 

 famines " of history. Because 

 of its destructiveness it has been 

 extensively studied and was one 

 of the first plant diseases against 

 ,which governments waged a real 



warfare. The fungus that causes the disease is very com- 

 mon in some of the northern United States, and- its spread 



raay at any time when conditions are favorable become a 



serious problem. It seems to prefer the 



colder climates and wet, heavy soil. 



The fungus first attacks the potato 



leaves, causing spots which are purplish 



in color when wet, but brown when dry. 



The disease may be confined to the leaf ; 



but it may spread to the stem and finally 



to the tubers, causing a dry rot of the 



latter if they are growing in dry soil or 



a wet rot if the soil is heavy and wet. 



The diseased tubers are very likely to 



decay in storage because many of their 



' Bordeaux mixture. This is probably the most im- 

 portant and most commonly used of fungicides. The 

 formula usually regarded as standard is the following, 

 known as the "5 — 5 — 50" formula: 



Copper sulphate, 5 pounds. 



Stone lime, 5 pounds. 



Water, 50 gallons. 



Fig. 215. ■ — .\ few 

 cells of a potato leaf on 

 which spores of the late 

 blight fungus have 

 landed; «, a spore; 6, a 

 germinating spore; c, a 

 germinated spore, the 

 germ tube from which 

 is entering an air-pore 

 of the leaf. After Weed. 



