DISEASES AND IN SEC T8 415 



Grain crops are slightly more likely to lodge following a pota- 

 to crop, probably due to the fact that potatoes draw rather 

 heavily on the potash supply and leave the soil more mellow 

 and loose than corn. 



A very common rotation in the general potato-growing 

 sections in the North is: First year, clover; second year, 

 potatoes; and third year, grain. In this rotation the pota- 

 toes are planted on clover sod, and if available, a good appli- 

 cation of manure or commercial fertilizer is added. On light 

 or worn soils, such a rotation is desirable, at least until the 

 condition of the soil is greatly improved. In many cases, 

 soils thus cropped have become so enriched as to cause suc- 

 ceeding grain crops to lodge. The rotation may then well 

 be changed to a 4-year system, introducing a crop of corn 

 following the potato crop. Such a rotation would then be: 

 First year, grain; second year, clover; third year, potatoes; 

 fourth year, corn. The two cultivated crops, corn and pota- 

 toes, usually draw heavily enough on the fertility so that the 

 succeeding grain crop will not grow too rank and lodge. 



In the South, the supply of vegetable matter is main- 

 tained usually by growing cowpeas or some other green 

 manure crop, preceding and following the potato crop. A 

 2-year rotation commonly followed is: First year, corn 

 and cowpeas followed by r>^e; second year, Irish potatoes 

 followed by winter vetch or crimson clover. 



DISEASES AND INSECT ENEMIES 



566. Blight. The more prevalent diseases are blight, 

 scab, and internal brown rot. Blight appears in two forms, 

 the early and the late. Early bhght attacks the leaves of 

 the potato plants early in the season, and gradually spreads 

 until the entire plant is killed. It is usually first seen as 

 dark spots on the leaves. Late bhght attacks the plants in 

 a somewhat similar manner, but considerably later in the 

 season. It first affects the plants near the ground and 



