GENERAL DISEASES 63 



Soil diseases. — A type of plant disease of special de- 

 structiveness comprises those of which the causal organism 

 resides in the soil from year to year ready to infect any 

 susceptible crop planted thereon. 



Notorious among such are the wilts of the cotton, tobacco, 

 cowpea, watermelon, flax, and potato ; the club root of 

 cabbage and other crucifers; black rot of similar plants; 

 smut of onion; Texas root rot. Other soil diseases are 

 wilt of tomato, cucumber, cantaloupe, eggplant, potato 

 rosette and scab, beet scab, lettuce drop, and numerous 

 others mentioned on following pages. Since all of these 

 are to large extent similar in propagation and dissemination, 

 they are given general treatment here. 



The particular destructiveness of these diseases is due 

 to the fact that they not only kill the crop, but they 

 also prohibit successful culture of susceptible crops in suc- 

 ceeding years. This crop limitation, if the crop in ques- 

 tion be an important one, perhaps the only really profitable 

 one suited to the soil affected, may result in large depre- 

 ciation in land values. 



It is unknown how long the germs can live in the soil 

 without their favorite host plant. That they can Uve from 

 one season to the second season following is certain. A 

 field slightly affected one year, if put to a susceptible crop 

 the second year after will be even more seriously diseased, 

 and the trouble will grow so long as susceptible crops are 

 cultivated with but short intervening periods. Many 

 cases are known where a rest of- five and even eight years 

 does not materially restore the soil to health. 



The application of chemicals to the soil is of value only 

 in rare instances, and is even then questionable. No 



