PLANT DISEASES 



295 



seed and does not cling to the outside, it is very difficult to pre- 

 vent loose smuts. It may be done, however, by soaking the seed 



in cold water for five 

 hours and then im- 

 mersing it in hot water 

 at a temperature of 1 29 

 degrees F. for wheat 

 and 126 degrees F. 

 for barley, for ten and 

 thirteen minutes respec- 

 tively. The smut germ 

 is more delicate than the 

 seed itself and is killed by 

 the rise in temperature, 

 while the seed remains 



FIG. 144. Potato scab 



The eruptions upon the surface of the potato are 

 due to the disease. (After Corbett) 



practically uninjured. 



For corn smut (Fig. 143) no effective prevention is known. 

 Potato scab (Fig. 144), also caused by fungous growth, is common 

 to potatoes all over the 

 United States. This disease 

 can be controlled by planting 

 clean seed in clean soil or by 

 treating the uncut seed tubers 

 for two hours or more in a 

 formalin solution composed of 

 one pint of formalin to thirty 

 gallons of water. 



384. Diseases controlled in 

 part by growing resistant 

 varieties. Rusts in grains are 

 common in almost all grain- 

 producing regions, but they 

 are most abundant in humid FIG. 145. Stems of wheat with rust 



regions. They are Caused by The parasite lives within the stem, and 



fungOUS growth and are indi- when spores are formed, the surface is rup- 



j i i tured, producing the reddish or brownish 

 Gated by the yellowish-brown, patches. (After Freeman and Johnson) 



