Mayi5. I920 Halo-BUght of Oats 165 



The first two lesions used were produced by artificial inoculation. The 

 last three were natural infections from the experimental plots. In all 

 of the five isolations only the plates from the centers of the lesions 

 showed any growth at all, and these plates were heavily seeded with 

 typical white colonies. The only exception is the one colony on a plate 

 from isolation IV, section 2. In isolation I the broth cultures from sec- 

 tions outside the center did not even cloud. In other isolations where 

 broth cultures from sections outside the center clouded, subsequent 

 plates showed that the clouding was sometimes due to the growth of the 

 halo organism and sometimes to contamination. 



The bacteria are evidently abundant only in the centers of the lesions, 

 and if any do occur outside in the halo they are very few in number. 

 This indicates that the discoloration of the halo-tissue is due only indi- 

 rectly to the presence of bacteria, and that some enzym or toxic by- 

 product destroys the chlorophyl. A suggestion of what this by-product 

 might be was obtained from some plates of potato-dextrose agar on 

 which colonies of the blight organism were growing. When colonies 

 of the stock organism were 3 days old distinct halos appeared in the agar 

 about the colonies as illustrated in Plate 32, B, The agar around these 

 colonies was less translucent than that outside the halos and was dis- 

 tinct in outline. These halos in the agar increased in diameter from day 

 to day, showing the concentric circles of growth illustrated in the plate 

 and characteristic of the lesions on oat leaves. Acetic acid dropped on 

 the agar-plate halos cleared them in a minute or two. Drops of am- 

 monium carbonate [(NH4)2C03] and ammonium chlorid [NH^Cl] on 

 sterile plates of the same potato agar produced in a few minutes halos 

 similar in size and appearance to those produced by the colonies of 

 bacteria. Acetic acid also cleared these halos. Litmus was added to 

 melted potato agar at the rate of one drop of a saturated solution to 10 

 cc. of the agar, and plates were poured. Streaks of stock were made 

 across the agar as soon as it had hardened. Similar streaks on plain 

 potato agar produced distinct halos about them in two days. In the 

 same time the litmus potato agar had turned a distinct blue for i cm. 

 or more on all sides of the growth. It seems probable, therefore, that 

 the ammonia produced by the blight organism is responsible for the 

 destruction of the chlorophyl and for the halolike lesions produced in 

 the oat plants. 



Stained sections of haloed leaf tissue also show bacteria only in the 

 center of the lesion. The bacteria at first are intercellular, but later 

 they destroy the cell walls and cause the collapse of the tissue. The 

 collapsed tissue is evident as the dead brown centers of lesions. (See 

 PI. 35. C.) 



