INSECTS AND DISEASES 59 



of the fungous diseases, are carried from one season 

 to another in the soil, and in a certain sense have 

 come to be considered as soil diseases. TJiis name 

 is applied to them primarily because the infection 

 to plants invariably comes through the soil, and the 

 trouble is carried over from season to season within 

 the soil rather than within the plant tissue. The 

 wilts of cotton, tobacco, melon, flax, the club root 

 of the cabbage, smut of the onion, potato scab and 

 rosette and numerous others belong to this group. 

 Their particular destructiveness is due to the fact 

 that they not only kill or injure the crop, but pro- 

 hibit the successful culture of similar crops in the 

 following years. It is not known how long these 

 organisms can remain in the soil, but it is certain that 

 they easily live from season to season and become more 

 and more serious as susceptible crops are repeated upon 

 the same land. Many cases are known where rests 

 of six and eight years have not materially lessened 

 the trouble. Over large areas direct treatment is 

 practically impossible. The application of chem- 

 icals or of heat under these conditions is out of the 

 question and even if it were not, the success of the 

 treatment would be doubtful. A long system of rotation 

 is the only practical method of eliminating these 

 organisms from the soil and making it possible 

 again to grow the susceptible plants. In view of 

 these facts it is especially important that emphasis 

 be laid upon the protection of healthy soils from 

 infection and to understand the ordinary methods 

 of soil sanitation by means of which this can be 

 done. Under greenhouse, hotbed and cold frame 

 conditions where the areas involved are relatively 

 small and the facilities for treatment are ready at 

 hand, the soil may be effectively disinfected and 



