240 



Journal of Agricultural Research voi.xxii.no.s 



basal sheath of the cotyledon thus protects the embryonic region of the 

 younger leaves from infection. That portion of the first leaf which 

 emerges from the cotyledon is susceptible to infection for some little 

 time after emergence, but since it ordinarily is not actually in contact 

 with infested soil, this fact is probably not of practical significance. 



Table II. 



-Relation of stage of development of the option seedling to infection by Uro- 

 cystis cepulae 



RELATION OF SOIL MOISTURE TO INFECTION 



Since there are these well-defined limits to the time of smut infection, 

 the possibility becomes clearly evident that variable environmental 

 factors during this limited period may exercise a controlling influence on 

 the occurrence of the disease. As already stated, the problem seems to 

 resolve itself primarily into the question of the relations of soil tempera- 

 ture and soil moisture to infection. The results of several workers 

 upon the grain smuts, as summarized by Jones (5), have shown that soil 

 temperature may influence infection. Hungerford and Wade (4) have 

 published evidence that high soil moisture, also, may favor infection of 

 wheat by the smut fimgus Tilleiia tritici (Beij.) Wint. Variations in 

 the moisture content of the surface layers of soil are likely to be wide, 

 especially during the late summer planting season in the southern States 

 when high temperatures and low humidity may cause rapid desiccation. 



