George Raymond Gage 31 



Aside from growth relations of pathogene and suscept, emphasis 

 should 1)!' placed on another factor which has perhaps been overlooked 

 or at least underestimated. Iu the ease of glumed oats and U. avenae, it 

 has Long been recognized that only those spores which enter between the 

 plumes and the ovary play a significant role as inoculum. Very fre- 

 quently, in damp, cool weather, oat blossoms fail to open for pollination. 

 The writer considers this to be a factor of great importance in the expla- 

 nation of seasonal differences in amounts of smut. 



GENERAL DISCUSSION AND CONCLUSIONS 



To a certain extent, conclusions have already been indicated from the 

 various data presented, but there are certain outstanding generalizations 

 which should be more or less emphasized. 



In the light of the discovery of spore germination at pollination time 

 by Zade and his students, and the investigations of the writer, it is evi- 

 dent that the current conception of the life histories of Ustilago avenae 

 and U. levis will have to be altered in several important respects. 



In the first place, it will be necessary to remove the oat-smut patho- 1 

 genes from the commonly called "seedling infection" group and place 

 them in the "flower infection" group. Since they differ from the patho- 

 genes in the latter group, however, in that the embryo is not invaded, 

 the mycelium being restricted to the pericarp, it would seem that some 

 distinction should be made. If a continuation of such groupings is ad- 

 visable, the writer suggests that the "flower infection" group could be 

 divided into an "embryo infection" subgroup and a "pericarp infec- 

 tion" subgroup. The oat-smut pathogenes could then be placed in the 

 latter subgroup. 



In addition, it must be recognized that infection of the pericarp is not 

 restricted to the blossoming period of the oat. With V. avenae and 

 glumed varieties of oats, this is largely the case. With U. avenae and 

 glumeless varieties, infection may take place in the field at any time af- 

 ter pollination and even during storage of the oat. Since most of the 

 spores are disseminated in the field, most of this infection would proba- 

 bly take place prior to harvest. With U. levis and glumed oats, the peri- 

 carp infection must be confined largely to the blossoming period. With 

 glumeless varieties, it doubtless takes place also after the grain has been 

 threshed since most of the spores reach the oats at threshing time, and 

 therefore it is highly probable that much of the infection ensues in 

 storage. In general, the length of the period during which inoculation 

 is possible is without doubt the most important factor concerned with 

 the resultant amounts of smut. All these points are in keeping with the 

 writer's observations on the relative amounts of loose and covered smut 

 during the years 1924 and 1925. Loose smut seems to be Car more prev- 

 alent than covered smut among the plumed varieties of oats, while 



