DI8EA8E8 AND INSECTS 417 



vent the gennination of the bhght spores. The common 

 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. 



567. 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 

 five 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 cutting 

 seed to let nothing get into the field that shows any indications 

 of the disease. A rotation of crops which provides for the 

 growing of potatoes but once in several seasons on the same 

 soil is also effective. 



568. Scab. Scab, which attacks the outside of the 

 tubers, causing rough, unsightly blotches, also does immense 

 damage throughout the countiy. The disease may be car- 

 ried over in the soil or on the seed. As it works on the out- 

 side of the tubers, it may be controlled by treating the seed. 

 Potatoes that show any indications of scab should be treated 

 before they are cut. Mix 1 pound of 40 per cent formalde- 

 hyde in 30 or 35 gallons of water and soak the seed potatoes 

 in this solution for two hours. If the soil is infected with 

 scab, potatoes should not be planted on it for several years. 

 Plowing under green manure crops which will develop acid in 

 the soil is somewhat effective in destroying the spores. 



569. Insects Injurious to Potatoes. There are numerous 

 insects which affect the potato crop. Cutworms, wire- 

 worms, and grubs often attack the crop on sod land. 



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