THE MULBERRY BLIGHT: CAUSE 351 



Bacterium mori does not lose virulence readily. Cultures 

 of the Georgia organism (Berckmans I and II) carried along 

 on culture media in my laboratory for 12 years (1908-1920) 

 were still infectious. Contrast with Nos. IV and XIV. 



Technic. — At first* the writer isolated the wrong organism, 

 owing to misplaced confidence in European statements re- 

 specting its color. All the various types of yellow colonies 

 appearing on the agar-poured plates were subcultured and 

 inoculated into growing mulberry shoots, but to no purpose. 

 The yellow bacteria would not produce the disease. In this 

 way a whole summer was wasted. The following year, however, 

 no difficulty was experienced in plating out an actively patho- 

 genic white schizomycete. 



For the isolations the student should select, from stem or 

 leaf, clean parts recently diseased and swarming with the 

 bacteria as determined by a microscopic examination. The 

 surface of the part selected should be flamed lightly, if stem; 

 or plunged for a minute or two into 1 : 1000 mercuric chlorid 

 water, if part of a leaf, and then rinsed lightly in sterile water 

 (long soaking in water removes this poison and may resuscitate 

 the surface bacteria which one desires to kill or to keep dormant 

 for a day or two; on the contrary a little of the surface poison 

 carried over into the bouillon does no harm). If it is a leaf-spot, 

 the piece may now be thrown into a tube of bouillon and crushed 

 with a sterile glass rod. If stem, the diseased part should be 

 scraped out with a cold, sterile instrument and thrown into 

 bouillon. In either case the tissue should be allowed to soak 

 for an hour before the dilutions are made and the plates poured. 

 Plates should be sown both from the original tube and from the 

 dilutions, seeding some heavily and others lightly. All yellow 

 colonies appearing on the plates should be rejected unless it is 

 desired to study also the saprophytic followers of the parasite. 

 For this reason it is best to keep the agar-poured plates under 

 observation several days before making transfers, since the 

 yellow colonies are frequently quite pale at first and if picked 

 off when small or from plates sown too thickly might be mis- 

 taken for white colonies. 



For inoculation, select young growing leaves and shoots, 



