26o Journal of Agricultural Research voi. xxii, no. $ 



of 20° and 25°, but none at 30°, showing that high air temperature 

 alone is insufficient to check the development of the disease. It appears 

 probable that the failure of the fungus to complete its development in 

 the case described above (where the plants after infection were exposed 

 to an air and soil temperature of 30° to 33°) was brought about at least 

 in part by some marked disturbance of the metabolism of the host and 

 not simply by the direct effect of the high air temperature upon the 

 fungus in the aerial parts of the seedling. 



Comparison between the development of the disease in plants grown 

 at 15° to 20° and at 24° to 28° C. (air and soil) was made. A high per- 

 centage of cotyledon infection occurred in both cases. At the lower 

 temperature the disease proceeded as usual to the infection of the true 

 leaves. At the higher temperature, however, the plants tended to out- 

 grow the disease, this being associated with a more rapid rate of top 

 development which apparently enabled the plants to slough off the 

 smutted cotyledons before infection of the first true leaf occurred. 



The foregoing conclusions as to the dominant influence of soil tem- 

 perature upon onion smut infection, while primarily based on greenhouse 

 experiments, have been found to accord well with field developments. 



Successive out-of-door plantings at Madison, Wis., made in inoculated 

 soil during the growing season, resulted in a gradual reduction of infection 

 as the season advanced and the soil temperature rose. Complete freedom 

 from smut was attained when the daily mean soil temperature at i to 2 

 inches depth remained at or slightly above 29° C. for two to three weeks. 

 There was also a tendency, as the temperature rose, for the seedlings to 

 outgrow the disease by the sloughing off of the diseased cotyledons before 

 infection of the first leaf occurred. 



An examination of records from one of the southern onion sections 

 (Laredo, Tex.) shows that during a good share of the critical period for 

 onion smut infection (August and September) the mean air temperature 

 is above that at which complete inhibition of infection was attained in 

 our experiments (29° C. or about 84° F.). If we assume, as observed 

 in northern sections, that the mean temperature for the upper layer of 

 soil is several degrees higher than that of the air at this time of the year, 

 it is reasonable to conclude that even though the smut organism were 

 introduced into southern onion sections, its development would be pre- 

 vented or greatly minimized, first, by the prevention of infection due to 

 high temperatures, and, secondly, by the rapidly developing tops out- 

 growing the disease, should occasional infections occur. 



In general we believe, therefore, that the regional distribution of onion 

 smut in the United States is conditioned upon the soil temperature 

 during the seedling stage of the plant's growth, the infection and develop- 

 ment of smut being favored by the relatively low temperatures and 

 inhibited by the high temperatures, with approximately 29° C. as the 

 critical point. 



