184 The Potato 



When the leaves of the plant become crinkled and 

 curled, mottled, rolled, spotted, blighted or dead at the 

 edge, when the vines show a yellowing and wilting, or 

 become stunted, or the stem shows discolorations on the 

 outside or within, or when the tubers have rough scabs 

 or pustules or show rotted areas, or the tissue within is 

 streaked with brown or is decayed, we say the plant is 

 diseased. Any one of these appearances, called symp- 

 toms, may indicate the presence of disease, and its appear- 

 ance means that the plant is unable to produce its maxi- 

 mum yield. It is to the interest of the farmer to know 

 what these diseases are and especially to know how to 

 prevent them. 



A plant, like a person, may be small and also be healthy. 

 Poor soil often produces small plants, and late planted 

 tubers may produce vines which may be killed by frost 

 before they have attained their maximum growth. Such 

 small plants are not diseased. Other things being equal, 

 however, small vines will not produce as large yields as 

 large vines. But plants may be small because they are 

 affected by some disease, so that it is often difficult to 

 tell whether small plants growing in a field are affected 

 by disease or are small for other reasons. Some plants 

 grow "all to vines," that is, the vines continue to grow 

 vigorously all the season and by so doing use up a large 

 amount of the food made by the leaves so that there is 

 but little left for storage in the tubers. Such plants 

 usually have small or unmarketable tubers. This condi- 

 tion may be brought about by too much shade or by a soil 

 too rich in nitrogen. On the other hand, such a condition 

 may be the result of a disease known, for the want of a 

 better name, as "large vines and many small tubers," 



A disease of a plant is an abnormal condition which 



