440 FIELD CROPS 



black and wilt. 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 thoroughly spraying the potato 

 vines with Bordeaux mixture several times during their 

 growth. Bordeaux mixture is made by dissolving 5 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. When the blue vitriol is completely dis- 

 solved, 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 

 prevent the germination of the blight spores. The com- 

 mon practice is to spray the crop first when the vines are 

 from 6 to 8 inches high, and repeat the spraying every ten 

 days or two weeks, or often enough to keep the vines well 

 coated with the mixture. There are some sections where 

 blight is not sufficiently troublesome to warrant spraying, 

 but in most instances it proves profitable. 



576. Internal Brown Rot. In some sections and in some 

 seasons, internal brown rot causes immense losses, while in 

 other sections the disease is hardly known. The disease 

 usually gets into the soil with the seed potatoes, or it may 

 live over in a soil that has produced diseased potatoes. It 

 may be seen when tubers are cut open as a dark brown streak 

 around the potato a short distance from the surface. 



The only remedy for this disease as yet known is to make 

 sure that clean seed is planted. Care must be used in cut- 

 ting seed to let none get into the field that shows any indi- 

 cations of the disease. A rotation of crops which provides 



