General Diseases 27 



and scabs, beet scab, lettuce drop, and numerous others 

 mentioned on following pages as well as the general diseases 

 discussed aljove. Since these diseases are similar in their 

 mode of multiplication and dissemination they are given 

 general consideration here. 



The peculiar destructiveness of these diseases is due 

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

 prohibit successful culture of susceptil)le crops in succeeding 

 years. This crop limitation, if the crop in question be an 

 important one, perhaps the only really profital^le one suited 

 to the soil affected, may result in large depreciation in land 

 values. 



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

 without susceptible host plants. That they can live 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 infested, 

 and the trouble will grow so long as such 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 normal condition. 



The application of chemicals to the soil is of value only 

 in rare instances, and even then is questionable. No means 

 of soil disinfection applicable to fields is known. Land once 

 infested, therefore, can become again usable only by eliminat- 

 ing the causal organisms by a long system of rotation which 

 is usually only partially effective, or by the use of resistant 

 varieties of plants, and these in most instances are not as yet 

 known or do not as yet exist. 



In view of these facts it is especially necessary to stress 

 the importance of protection of soils against infestation. 

 To understand the means of protection the modes of dis- 

 semination must be known. 



All plants affected with these diseases harbor immense 

 numbers of the reproductive parts of the causal organisms, 

 bacteria, spores, mycelium, sclerotia, etc. Upon the death 

 and disintegration of the host plant they are liberated in 



