PLANT DISEASES 



387 



very young seedling. The fungus grows with the growth of 

 the host, forming its spores in the young oat kernels ; finally 

 the kernels and the chaffy scales enclosing them are largely 

 replaced by masses of the winter spores. The oat smut can 

 be controlled by treating the seed grain in such a way (for 

 example, with formaldehyde or hot water) as to kill the 

 spores that adhere to the grain, without killing the grain 

 itself. The stinking smut of wheat, so called because of 

 its disagreeable odor, hke 

 the oat smut forms its 

 spores in the kernel, but 

 it does not entirely de- 

 stroy the kernel or the 

 surrounding chaff. 



The damage done by the 

 corn smut in the United 

 States has been estimated 

 at $50,000,000 per year. 

 The oat smut causes an 

 estimated yearly loss in 

 the United States of 

 about $35,000,000. The 

 stinking smut formerly 

 destroyed from one-half 

 to three-fourths of the 

 wheat crop. The loss 

 has been greatly decreased by treating the seed grain, but is 

 still considerable in parts of the United States and Canada. 



391. Common Scab of Potato. — This disease is found 

 wherever the potato is grown. It produces rough, scabby 

 spots on the surface of the potato, which are often deepened 

 by larvse and insects. The fungus may live in the soU 

 for years, and the tubers may become diseased at any time 

 during the growing season. For these reasons it is neces- 

 sary to practice long rotations and to plow under green 



A B 



Fig. 221. — A, the head of a healthy 



oat plant, B, the head of an oat plant 

 attacked by the loose smut. 



