32 USTILAGO AVENAE AND USTILAGO LEVIS 



glumeless varieties have presented greater percentages of both kinds of 

 smut but with the covered smut predominating. 



Since the inoculation of glumed varieties of oats must, for the most 

 part, take place during the period of pollination, it would seem that a 

 failure of the blossoms to open for pollination would be a most impor- 

 tant controlling factor in the amount of smut in the succeeding crop. 

 This must very often take place, because either cool or damp weather at 

 blossoming time prevents opening of the flowers. 



In regard to other phases of environmental influence, further emphasis 

 may be placed on the fact that temperature or moisture cannot be con- 

 sidered alone in an attempt to explain varying amounts of smutted 

 plants in the crop. As is necessary in many other physiological studies, 

 all factors which may influence the process must be taken into considera- 

 tion before the effect of any one may be designated as limiting. 



Of special importance is the fact that smutted panicles cannot be con- 

 sidered as an infallible criterion for infection, unless tests are made un- 

 der conditions where absolute control of environmental factors is main- 

 tained throughout the entire development of the suscept. This, it would 

 seem, is impossible under field conditions. 



Considerable doubt necessarily arises as to the value of certain exist- 

 ing data on varietal susceptibility of oats to smuts. All work along this 

 line should be checked by inoculating oats in accordance with the man- 

 ner in which it happens in nature. If the artificial way of dusting seed 

 with spores is continued, modifications should be introduced. The 

 glumes should always be removed prior to dusting, and the inoculated 

 seed exposed to conditions which will bring about that which would exist 

 in nature, namely, infection of the pericarp before the oat starts to ger- 

 minate. Comparisons of percentages of infection of naked and glumed 

 varieties are worthless if spores are applied to the latter without first 

 removing the glumes. Since there certainly must be differences in va- 

 rieties which would influence the possibility of spores entering their 

 blossoms, as observed by Diehl and Roesch, methods of inoculation ap- 

 proximating that in nature might change considerably the present status 

 of our knowledge of varietal susceptibility. 



SUMMARY OF CORNELL INVESTIGATIONS 



1. Spores which reached the stigma of the oat flower at pollination 

 time germinate at once or within a few days. This happens in glumed 

 and glumeless varieties of oats, with both Ustilago avenae and U. levis 

 spores. 



2. Spores lodging on the ovary walls also germinate in a very short 

 time. 



3. With glumed oats the inoculation period is rather short, being re- 

 stricted to the time during which the glumes are open for pollination. 



