440 



ELEMENTARY STUDIES IN BOTANY 



Potato disease. This is a notable and dangerous disease. 

 Famines in Ireland have been brought about by its ravages, 

 because in destroying the potato crop there were no other 

 crops to replace it as a food supply. The spores of the 



parasite enter the 

 young leaves and 

 they begin to spot 

 (Fig. 89), and fi- 

 nally the parasite 

 invades all the 

 leaves and the 

 young stem, and 

 the plant dies. 

 The disease is 

 so very epidemic 

 that if it enters 

 ^v-.-,- -*>* a potato field, it 



^fH Ifc B f^ life. sweeps through it 



H W* w ^ great rapid- 



^^^SBB mr ' ity. If the attack 



is early in the sea- 



fj^i 5 ^ son, the formation 



% of tubers may be 



^j| . stopped; if it is 



later, the tubers 

 may have begun 

 to develop, and 

 in this case the 

 tubers also are in- 

 vaded. Tubers in this condition are said to have the 

 " potato rot" (Fig. 90), but the rotting is not caused by 

 the destructive parasite ; it is merely the natural rotting of a 

 plant structure that has been killed. This is one of the 

 hardest diseases to prevent, for since potatoes are propa- 

 gated by tubers, the danger of infected tubers is very great. 



FIG. 89. Potato disease on the leaves. After JONES. 



