146 Journal of Agricultural Research voi. xix.No. 4 



to yellow. Numerous confluent lesions quickly killed the leaf tips and 

 margins, which shriveled, turned brown, and died. Isolated lesions de- 

 veloped in the same way into distinct oval spots of yellow tissue i cm. 

 or more in diameter with small dead centers. Infection was always 

 abundant on inoculated oat plants. (See PI. 27.) 



Cultures proved by inoculation experiments to be pathogenic were 

 kept as stock cultures. In this way 21 such cultures were obtained. 



2. Since both yellow and white colonies were isolated from leaf sec- 

 tions showing halo lesions, inoculations were made with pure cultures 

 of each and also with mixed cultures of yellow and white colonies for 

 comparison with inoculation work done by Thomas F. Manns (j, p. loj, 

 PL I). In 25 inoculation experiments pure cultures of the white halo 

 organisms produced abundant and typical infections. In 13 tests, pure 

 cultures of the yellow organisms produced no lesions whatsoever. 

 Twelve sets of inoculations were made with mixed cultures by com- 

 bining the 2 white halo organisms. No. 36 and stock, with 4 different 

 isolations of yellow organisms. Isolation 39a from a leaf lesion from 

 Urbana, 111., was the yellow organism most often used. Separate pure 

 cultures of yellow and white organisms were used for control inocula- 

 tions. The cultures were mixed just before the inoculations were made 

 for the reason that long-continued attempts to grow mixed cultures in 

 broth or on various agars were not successful.* In the 12 inoculation 

 tests with the yellow and white mixed cultures typical halo infections 

 were produced, but the lesions were only one-half to three-fourths as 

 abundant as on plants inoculated with pure cultures of the white 

 organisms. The development of lesions from mixed cultures was also 

 somewhat retarded, the infections being evident from one to two days 

 later than those obtained from the pure white cultures. These inoculation 

 experiments showed plainly that the white organism alone is responsible 

 for the production of the halo lesions while the yellow organisms used 

 are neither parasites nor favorable to parasitism. 



3. In June, 191 8, field inoculations were made on the following 13 

 Wisconsin varieties: Wisconsin No. i, 3, 4, 5, 7, 13, 14, 15, 22, 25, 49, 

 52, and 62. The plants were just beginning to head out, and the experi- 

 ment was carried on to test the pathogenicity of the white organisms on 

 mature leaves and on panicles, and the effects of possible lesions upon 

 the development of the panicles, spikelets, and kernels. Water suspen- 

 sions of the halo organisms were sprayed into unopened sheaths upon 

 uninjured bundles of plants, the tops of which were drawn together and 

 tied close so as to be covered with bags, and upon bundles of plants 



' For two months mixed cultures of white and yellow organisms were grown on potato agar and in + lo 

 beef-peptone broth. Plates poured from these cultures when they were 5 days old showed a few white 

 and many yellow organisms in the broth cultures and about equal numbers of yellow and white on agar. 

 Plates poured from these cultures il/i weeks later showed no growth of either white or yellow colonies from 

 the agar and showed pure cultures of the yellow organisms from the broth. On the contrary, separate 

 pure cultures of the same organisms held for the same time in these media and under the same conditions 

 gave abundant and characteristic colonies on the plates poured. 



