TROUBLES OF -POTATO GROWERS 247 



potatoes after becoming abnormally checked in growth to break 

 out with secondary tubers. 



Aerial Tubers. Sometimes tubers appear upon the stems 

 above ground and none are found below ground as they should be. 

 They come about in this way; normally the potato tuber is an en- 

 largement of an underground stem, formed by the action of the 

 return flow of the sap of the plant. The upward flow of sap is 

 largely through ducts in the central parts of the aerial stems. The 

 downward flow of sap, after its elaboration by the leaf-surfaces, is 

 through the tissue which lies just under the skin or bark of the 

 stem and it deposits its burden in the tuber underground. When 

 this tissue is injured in some way so that the return-flow of sap 

 cannot pass along to making tubers underground it goes to work 

 above the injuries and makes tubers in the air. The tubers are 

 simply modified stems either above or below ground, as conditions 

 may determine. Injury to the stem may be mechanical such as a 

 scrape with the hoe, the work of an insect or a disease. Anything 

 that holds up the sap may do it, if the plant is otherwise vigorous 

 enough for it. Fortunately it is of rare occurrence. 



Leaky Potatoes. Sometimes potatoes go off quickly after 

 sacking and discharge so much liquid that the trouble is called 

 "leak" disease. It is caused by a fungus which exists in the soil 

 and is usually introduced to the potato by a worm in digging. The 

 only remedy so far known for this disease is to prevent the wound- 

 ing of the tuber if they must be harvested during warm weather. 

 After a severe frost occurs very little trouble is experienced from 

 this disease. 



Potato Blights. It is an interesting fact that the great "potato 

 blight," as the world knows that disease, is not a great trouble of 

 the potato in California in fact, this disease is largely conditioned 

 upon excessive moisture in the air and that seldom occurs during 

 sufficient duration even in the rainy season except in the upper coast 

 district. When that district was largely producing potatoes there 

 was trouble with "blight," or more properly "late blight," until 

 resistant varieties were introduced. Another disease known as 

 "early blight," appearing in the spring time, is rather more abun- 

 dant but even that is only active when the air is cool and moist. 

 These blights are manifested by blackening of the top growth of 

 leaf and stem. Wherever these blights occur the treatment is 

 prompt spraying with the Bordeaux mixture as described in Chap- 

 ter XXXVIII. It should be applied when the new growth is about 

 six inches high and repeated two or three times at intervals of 

 about ten days, 



Varieties. As with other vegetables, California has tried many 

 kinds of potatoes and grows very few on a commercial scale. The 

 first notable varieties were brought from Chile and Peru in very 

 early days, and are still grown to some extent, though the main 



