DISEASES AND INSECTS 279 



Shrinking in Storage. Potatoes always lose weight in storage. 

 The loss is slow at first, usually amounting to 6 or 8 per cent in 

 good storage during the first six months. After that the loss is 

 much faster and will sometimes amount to 4 or 5 per cent a month in 

 the spring under ordinary cellar conditions where the temperature 

 can not be kept down. 



The loss is due to two causes: (1) Loss of water, and (2) loss by 

 respiration. Loss by respiration is due to the slow conversion of 

 starch into sugar and finally breaking up and passing off as carbon 

 dioxide gas. Experiments have shown that about 75 per cent of the 

 loss is due to water loss and about one-fourth due to respiration. 



Cold Storage. The loss by respiration also goes on in cold 

 storage, but at temperatures below 32 F., the sugar developed does 

 not undergo further change but remains, giving the potato a sweetish 

 taste. This is the reason cold storage potatoes or slightly frozen 

 potatoes are sometimes sweet. 



For table stock potatoes should be held at 40 to 50 F., while 

 seed stock is held at from 32 to 40 F. 



Diseases and Insects. The potato plant is subject to a large 

 number of diseases. Many of these do little injury, but at least a 

 dozen are severe in some part of the world where potatoes are grown. 



No potato disease is universally injurious, but is limited to 

 certain regions. All of the diseases are influenced by climate or 

 soil, and hence vary in their injury from year to year. 



For example, the late blight is perhaps the most injurious disease, 

 yet there are large regions, as the Southwest, where it is seldom seen. 

 Even in the eastern States, where it is very common, there are 

 considerable areas in which late blight is seldom found. Other dis- 

 eases are not considered as serious in northern climates, as the State 

 of Maine, but southward are very injurious. 



A brief summary of the principal diseases is given below : 



A. Affecting the foliage only: 



1. Early blight, caused by AHernaria solani. 



2. Leaf blotch, caused by Cercospora concors. 



B. Affecting chiefly the stems : 



3. Brown rot, caused by Bacillus solanacearum. 



4. Black leg, caused by Bacillus phytophthorus. 



