452 ELEMENTARY STUDIES IN BOTANY 



any disease, it is peculiarly hard to control. Once the use 

 of soil fungicides was recommended, but since we have 

 learned something about the soil, this has been shown to 

 be a very dangerous proceeding. The soil is swarming with 

 bacteria and other fungi, many of which are extremely im- 

 portant, and fungicides cannot pick out one organism for 

 destruction and leave the others alive. Such a treatment is 

 much like annihilating the population of a city to get at 

 one criminal. There is no evidence, as yet, that any so- 

 called soil fungicide does any good. 



If fungicides are not available for soil infections, such as 

 occur in numerous wilt diseases, what can be done? In the 

 case of garden crops, as cabbage, infected plants can be re- 

 moved or destroyed, but in the case of field crops this is 

 impracticable. The only known method of controlling soil 

 infection is to stop planting the susceptible crop on the 

 infected area, and to plant some other crop. This rotation 

 generally eliminates the infection or weakens it. 



149. Uninfected stock. In all cases of infection by 

 parasites living from one season to the next in a plant, the 

 only safe thing to do is to see to it that seeds or tubers or 

 cuttings used in propagation are obtained from absolutely 

 uninfected stock. This has been tried with the potato dis- 

 ease and found to be most effective. 



150. Resistant races. The breeding of races of plants 

 resistant (" immune ") to the different diseases is the final 

 resort in the matter of control. When nothing else avails, 

 the cultivation of immune races must be resorted to. Prob- 

 ably this will be the final remedy for all our plant diseases, 

 but those that can be controlled can afford to wait. For 

 this reason, the work on resistant races as yet has had to do 

 chiefly with diseases that arise from soil infections. The 

 following illustrations indicate that some progress has been 

 made. 



In the case of the potato disease it was shown how a 



