DISEASES OF PLANTS— SPRAYING 231 



been harvested recently will probably cause the next crop 

 to be diseased in like manner. Rotation of crops is, therefore, 

 advisable. Do not use beets and other root crops in this 

 rotation, as the scab disease also attacks them. 



4. Seed ^potatoes are usually more or less scabby or have 

 the germs of the disease upon them. If these are planted 

 without treatment the disease will likely be introduced into 

 the soil. 



The treatment of seed potatoes to prevent the planting of 

 scab disease is now a very cheap and simple process. The 



( 



Fig. 147. — Apple rust on leaf and twig. 



seed potatoes are soaked for two hours in a solution of forma- 

 lin. This solution is made by mixing one pint of the strong- 

 est formalin in thirty-two gallons of water. This kills the 

 disease germs but does no harm to the potatoes. The liquid 

 may be used for several sacks of potatoes. 



Exercise. — Showing Potato Scab. — Specimens of Irish 

 potatoes having the scab disease should be brought to school 

 and shown to all the students. 



Grain Smut. — In fields of grain, when it is in head, may 

 be often seen a black powder which clings to the husks of 

 the grain (Fig. 148). This black powder is the fruit of a 

 disease growing inside of the grain stalks. The disease is 



