General Diseases 29 



the soil, where they are able to live for considerable time, 

 frequently for 3^ears. The immensity of their number in a 

 diseased plant is inconceivable. Even a few diseased stems, 

 roots, or leaves in the field will stock the soil thoroughly. In- 

 fected plants, or soil in which infected plants have grown, or 

 which bears parts of infected plants can therefore convey 

 such contagion to new fields. 



Instances may be cited where the contagion has spread 

 by washing from higher to lower land; moreover contagion 

 may be carried by any means which can convey soil from 

 one field to another, notably through tools which have 

 been used on infested soil. Though apparently clean, such 

 tools, if they bear even minute particles of soil, may convey 

 hundreds of germs and thus start an epidemic in a field. 

 The hoofs of animals or the feet of laborers may in a similar 

 way bear disease-laden soil. Wind passing over an infested 

 field may pick up broken bits of sick plants or germ-laden 

 soil, and convey these to other fields. Infestation by wind, 

 however, seems to be rare, possibly because of the ger- 

 micidal 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 organisms may be spread widely through 

 manure which has here become infested from the feed 

 (cf. watermelon, cabbage). 



A field may be protected from higher land that is in- 

 fested by proper arrangement of dykes. In some cases a 

 thorough cleansing of tools so that there is no possibility 

 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 fonnalin or 5 per 

 cent carbolic acid. It is difficult to insure complete protec- 

 tion against disease dissemination by the feet of animals and 

 man, but if uninfested land remains to be protected, every 

 precaution should be exercised 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. 



