METHODS OF INVESTIGATING PLANT DISEASES. 177 



2. The isolation of the organism from the deseased plant 

 and its growth in pure culture whenever that is possible. 



3. The reproduction of the characteristic symptoms of the 

 disease by infecting the healthy plant with organisms taken 

 from pure cultures, or in case pure cultures are not possible 

 the direct infection may be substituted, using parts of the 

 diseased plant that contain the germs or spores of the disease 

 organism. 



4. The rediscovery of the organism in the diseased tissues 

 by microscopic examinations, and growth again in pure cultures. 



After the cause of the disease has been determined the path- 

 ologist must endeavor to find out the complete life history of 

 the disease producing organism. Many fungi produce several 

 kinds of reproductive bodies, and in order that proper treat- 

 ment and remedies may be discovered it is necessary to know 

 when and where and under what conditions these reproductive 

 bodies are produced. Accurate laboratory methods similar to 

 those outlined, have yielded many important additions to path- 

 ology in recent years. Clinton, in Illinois completed our know- 

 ledge of the organism of apple scab; Spaulding and VonSchrenk 

 have demonstrated the relation of canker and bitter rot in ap- 

 ples; Norton has followed the complete life history of the brown- 

 rot of the plum, and I might enumerate others but time will not 

 permit. There are many diseases which have been only 

 imperfectly investigated, and we may reasonably expect fruit- 

 ful results in the near future. Even in our own state there is 

 one disease that has caused the loss of hundreds of thousands 

 of dollars to fruit growers and still our knowled^^e in regard to 

 the nature of the disease is too imperfect to suggest efficient 

 treatment or preventive measures. 



The study of plant diseases thoroughly and effectively means 

 a matter of dollars and cents to the agricultural interests of a 

 community and consequently the pathologist must not stop 

 with the study of the disease producing organism. The extent 

 of the devastations due to plant diseases can best be appreciated 

 from a number of specific examples. It has been estimated that 

 the brown-rot of peaches and plums cost the state of Georgia 

 $500,000-$700,000 in the year 1900. The president of the 



