May 15, 1920 Halo-Blight of Oats 1 63 



Reisolation from the first halos gave typical white colonies which on sub- 

 culture and reinoculation produced halos on barley and oats. 



No halos were obtained on corn in four experiments, and no halos were 

 obtained on broom com in later experiments. Oat plants inoculated at 

 the same time always showed abundant infection. It is evident that the 

 halo-blight organism may attack wheat, rye, and barley to a slight 

 extent; but in Wisconsin, at least, halo lesions in the field rarely, if ever, 

 appear on anything but oats. 



VARIETAL SUSCEPTIBILITY 



All observed varieties of cultivated oats are attacked by the halo-blight 

 to some extent. Wisconsin No. 14, both in the field and in the greenhouse, 

 is more susceptible than any other variety and shows more lesions in 

 later stages of development, especially on the flag leaf, rachis, and spike- 

 lets. Two varieties, Wisconsin No. 13 and Wisconsin No. 15, grown in the 

 fields on either side of Wisconsin No. 14 during 191 7, showed considerable 

 resistance. Although leaves of Wisconsin No. 14 were badly spotted, the 

 leaves of Wisconsin No. 13 and 15, which came in contact with them, 

 showed little spotting. In the first plot (from 191 6 untreated seed), 

 described above, Wisconsin No. 128 showed only six primary infections, 

 while Wisconsin No. 124 showed 169. In the second plot (from 191 7 

 untreated seed) some varieties showed only slight secondary infections, 

 others moderate, and some heavy infection. 



Inoculation experiments in the greenhouse also brought out differences 

 in varietal resistance. Wisconsin No. 1,5, and 14 were used for several 

 experiments, Wisconsin No. 14 always showing so much heavier infection 

 than either of the other two that Wisconsin No. i and 5 were no longer 

 used. Wisconsin No. i showed more resistance to infection than Wis- 

 consin No. 5. 



While certain varieties are more susceptible than others under ordinary 

 conditions and show fewer primary lesions at the beginning of the season, 

 as above indicated, the differences are not marked in severe blight years 

 as the season advances. 



RELATION OF ORGANISM TO HALOED TISSUE 



The oval outline of the halo, its rapid spread from the point of infec- 

 tion, and the fact that the haloed tissue remains normal, apparently, 

 except for loss of color, have led to the conclusion that the discoloration 

 is probably due to some diffusible substance produced by the bacteria 

 rather than to their immediate presence. To determine whether or not 

 the bacteria were equally distributed throughout the lesions, isolations 

 were made from pieces of tissue cut from the centers of lesions and from 

 points at varying distances from the center as shown in the following 

 diagrams. Isolations were made after treatment with mercuric chlorid 

 as described above. The distribution of bacteria throughout the halo 

 lesions is shown in Plate 33 and Table III. 



