30 Diseases of Economic Plants 



branch, and leaf. Prompt action here may materially 

 lessen the rapidity of spread of the disease in the field. 

 Every particle of the sick j)lant burned means the destruc- 

 tion of millions of the causal organisms. 



A long rotation of crops, one that will bring the suscep- 

 tible plant l:)ack upon the affected field only after an inter- 

 vening period of several years, perhaps after a period of 

 eight or ten years, is useful in some cases, notably with 

 tobacco wilt and onion smut, though in other cases, notably 

 watermelon wilt, such rotation is of little avail. 



The one means of overcoming these soil diseases, which 

 is most promising to farmers, lies in the discovery of a variety 

 of the crop plant that will not succumb to attack even when 

 planted upon infected soil. Varieties of cotton, tobacco, 

 tomatoes, cabbage, flax, cowpeas, and watermelon that are 

 resistant to their respective soil diseases have been dis- 

 covered or developed by lireeding. There is similar hope in 

 regard to other crops. 



