KUI.ICS OF I'ROOF. 



Diri'XT-Infi'X'Tiox ExpKRnn';NT.s. 



Direct-infectiuii experiiuciits will freciucntlv separate out a parasite wliicli is 

 overwhelmed l)y some saprophyte and thus furnish better material for plate-cultures, 

 and they are also sometimes very useful when one is remote from laboratories and 

 so situated that it is impossible to obtain i)ure cultures. It is, however, a crude 

 method and onl)- to l)e emplo\ ed when more exact methods can not be tised or 

 would not serve as well. ]!y "direct" infection is meant the transfer of fluids or 

 solids from the diseased plant directly into the tissues of the healthy plant, an effort 

 beiuL; made to include some of the sujiposed parasites in this transfer. It is a cou- 

 \'euient expression and will be used often in this book. 



THE ORGANISM. 



This may be considered under three heads — its abilit\- to jiroduce disease, its 

 form, and its pln'siohjs^ical pecidiaritics. Many of the latter nii.i^ht ecpially well be 

 denominated cultural characters, and the pathogenic properties realh- belong under 

 ph}'siolog'y, but are kept distinct for sake of convenience and because the\' constitute 

 not only the most important attributes of the organism, economicalh' speaking, but 

 also a distinct and peculiar phase of the investigation. 



Pathoc;kni-;,si,s. 



What constitutes proof of the ]iathogenic nature of any organism ? Upon the 

 ability of the student to give a proper answer to this questi<jn depends \'er\- largely 

 his success or failure as an in\'estigator. Henle perceived clearh' what was neces- 

 sary as long ago as 1S40, and Koch's rules are still fresh in the minds of all. There 

 is consequenth' now so good an understanding of this subject among animal patholo- 

 gists and professional bacteriologists that if this book were designed principally for 

 such pei'sons no comment would be necessary. A glance, however, at the literature 

 of plant diseases shows that many of the writers on bacterial diseases of plants have 

 not had this professional training. The four cardinal requirements, as understood 

 by the writer, are as follows : 



RULES OF PROOF. 



(a) Constant association of the organism with the disease. 



(J/) Isolation of the organism from the diseased tissues and careful stud)- of the 

 same in pure cultures on various media. 



(c) Production of the characteristic signs and lesions of the disease by inocu- 



lations from pure cultures into healthy plants. 



(d) Discovery of the organism in the inoculated, diseased plants, re-isolation of 



the same, and groAvth on various media until it is determined be}ond 

 doubt that the bacteria in question are identical with the organism 

 which was inoculated. 



