66 DISEASES OF ECONOMIC PLANTS 



bits of sick plants or germ-laden soil, and convey these to 

 healthy fields. Infection by wind, however, seems to be 

 rare, possibly because of the germicidal action of the sun's 

 rays upon the surface layer of soil upon which the wind 

 must act. If the crop be one which is used for stock feed 

 or one which may become mixed with stock feed, the dis- 

 ease may be spread widely through manure which has here 

 become infected from the feed (cf. watermelon, cabbage). 



A field now healthy may be protected from higher land 

 that is infected by proper arrangement of dykes. In some 

 cases a thorough cleansing of tools so that there is no pos- 

 sibility of conveying the germs will aid in repression. 

 The dirt may be knocked off, then wiped off, and the 

 implement finally disinfected with a solution of 2 per cent 

 formalin or 5 per cent carbolic acid. It is difficult to 

 insure complete protection against disease dissemination 

 by the feet of animals and man, but if uninfected land 

 remains to be protected, every precaution should be exer- 

 cised in this particular. 



Where but a few plants in a field are affected, they^should 

 be removed from the soil and destroyed by fire, — root, 

 branch, and leaf. Prompt action here may materially 

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

 Every particle of the sick plant burned means the destruc- 

 tion of millions of the causal organisms. 



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

 tible plant back 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 

 potato wilt, though in other cases, notably watermelon 

 wilt, such rotation is of little avail. 



