256 Journal of Agricultural Research voi. xxii. no. s 



Considering first the amount of original infection, it will be seen that a 

 high percentage of disease resulted in all the plantings of June 18 and 

 June 26. The somewhat lower infection in that of June 18 may be ex- 

 plained in part at least by the fact that a smaller quantity of inoculum 

 was used than in subsequent plantings. The next two plantings (July 

 10 and 19) were so made that the resulting seedlings were exposed during 

 early growth to the maximum soil temperature of the season. By refer- 

 ring to Table XII it will be seen that associated with this higher tem- 

 perature there was a decided reduction in the amount of infection in 

 even the uncovered plots, while in the covered plots, where the mean 

 temperature remained continuously above 29° C, no infection whatever 

 occurred. 



Considering, secondly, the extent to which the disease became sys- 

 temic or was outgrown, it will be seen that in the planting of June 18 a 

 majority of the infected plants showed systemic invasion at the second 

 observation (thirty-ninth day). In the next planting (June 26), how- 

 ever, by the time of the second observation most of the external signs of 

 the disease were confined to the dead cotyledons. The amount of 

 systemic infection increased somewhat, however, at the third observation 

 (39 per cent). 



In the third planting (July 10) it is interesting to note first that the 

 plants in the covered plot remained entirely free from infection. In the 

 uncovered plot, although some cotyledon infection was noted at the sec- 

 ond observation, no disease whatever was found at the third observa- 

 tion. This indicates that the time when the temperature was at its highest 

 point the infected plants succeeded best in outgrowing the disease. 



The field data secured in the foregoing experiments at Madison are 

 thus in general accord with the experiments performed under controlled 

 conditions. In such controlled experiments the amount of smut infec- 

 tion falls as the soil temperature rises toward 29° C. and is totally inhib- 

 ited above this temperature. Likewise in the field trials with successive 

 plantings there was a gradual reduction in the amount of infection fol- 

 lowing the rise in the mean soil temperature, with omplete inhibition of 

 infection where the mean was kept above 29° for two or three weeks after 

 planting. Complete freedom from infection under these Wisconsin field 

 conditions was secured only by growing the plants under artificial condi- 

 tions in which by covering the plants with glass the temperature was 

 raised several degrees above the normal. It is, however, to be noted that 

 the summer of 1920, when the foregoing results were secured, was some- 

 what cooler than the average. The weather records of other years indi- 

 cate that in a hot summer complete inhibition of smut infection would 

 be secured by such summer plantings. 



