416 



FIELD CROPF^ 



spreads rapidly upward; the diseased parts quickly turn 

 black and v\^ilt. The spores which fall from the leaves to 

 the ground may infect the tubers and cause them to rot, thus 

 completely destroying the crop. 



Blight is controlled by spraying the potato vines thor- 

 oughly with Bordeaux mixture several times during their 



Figure 140. — Sprasnng to prevent disease and insect injury is an important part 

 of modern potato growing. 



growth. Bordeaux mixture is made by dissolving 6 

 pounds of high-grade stone lime in 25 gallons of water and 5 

 pounds of blue vitriol (copper sulphate) in 25 gallons of water 

 in another receptacle. Allien the blue vitriol is completely 

 dissolved, the contents of the two barrels are poured together 

 and the mixture is complete. Care must be taken that suf- 

 ficient lime is used, or the mixture may injure the plants. 

 The treatment has no effect after the disease has started, 

 but is a preventive which must be used often enough to pro 



