258 Journal of Agricultural Research Voi. xxu. No. s 



of 4 feet, while the minimum temperatm-e of all the soils used, except 

 peat, was 0.5° to 1.0° C. higher, as a monthly average, than that of the 

 air. Our own observations in Wisconsin are in general accord with 

 these Michigan records. Assuming that the temperature of the surface 

 layer of soil under Texas conditions likewise averages several degrees 

 higher than the air, it is evident that the mean never went below the 

 point where infection is entirely inhibited (29° C, or 84° F.) during 

 August and seldom below it during September. Continuing up to 

 October 8 there were only a few days when the air temperature fell 

 below 27.5° C. (81.5° F.), the point at which our experiments have shown 

 smut infection to be markedly reduced. It seems probable, therefore, 

 that even if onion smut were introduced into this Laredo soil, it would stand 

 small chance of infecting onion seedlings to the extent of establishing 

 the disease as a permanent factor. The data available are not sufficient 

 to justify the attempt at more detailed geographic correlation of onion 

 smut occurrence with the temperature factor. We believe, however, 

 that the conclusion is justified that soil temperature during the early 

 seedling stage must be considered as a limiting factor in determining the 

 occurrence of the disease in any locality. It must be left with local 

 observers to make use of this fact in interpreting conditions as they 

 occur in any particular region. 



SUMMARY 



Onion smut was first noted in the Connecticut River Valley in 1869. 

 Since then it has successively appeared and become an economic factor 

 in nearly all of the northern onion-growing sections from New York to 

 Oregon. This has probably resulted from chance introduction of the 

 organism with seed or bottom sets, followed by its accumulation in the 

 soil where continuous cropping with onions is practiced. The disease 

 has not appeared in the southern onion-growing sections of Texas 

 and Louisiana, although they are exposed to similar chance introduction 

 of the parasite and the continuous cropping method is common. 



These facts have raised the question as to wherein lies the explanation 

 of the regional limitation of the disease. The southern method of cul- 

 ture, characterized by special seed bed and transplantation of seedlings, 

 does not offer sufficient explanation for the absence of smut. No differ- 

 ence in susceptibility between northern and southern varieties has been 

 found. Is regional limitation explained by differences in environ- 

 mental factors in the North and the South at the time when the seedling 

 is susceptible to infection, that is, during the first two or three weeks after 

 germination? An analysis of certain of these factors in relation to 

 infection has been the object of the present investigation. 



The cotyledon of the onion is susceptible to attack by the smut organ- 

 ism up to the time it attains full growth, a period of about three weeks. 



