Oct. 29, 1921 Relation of Soil Temperature to Onion Smut 



257 



CORRELATION OF EXPERIMENTAL RESULTS WITH FIELD CONDI- 

 TIONS OF THE SOUTHERN STATES 



These results obtained in both greenhouse and field experiments justify 

 the question as to the part played by soil temperature in determining 

 smut infection in the onion fields of the more southern States. As 

 noted at the beginning of this article, a recent survey of southern onion 

 sections indicates that smut is not prevalent in the southern fields — for 

 example, in Texas — as it is in the northern onion sections. As was 

 earlier explained, it is the practice in these southern fields to plant the 

 onion seed in late summer or early autumn. It is thus quite possible 

 that the mean temperature for the surface inch of soil in southern onion 

 sections is considerably above the maximum for onion smut infection 

 during and immediately following the sowing of seed. According to 

 Mally (6), onion seed is sown in the Laredo district of southern Texas 

 as early as August i , while most of the seed is planted about September 

 10 to 25. The mean air temperature as recorded at Laredo, Tex., by 

 the United States Weather Bureau for August, September, and October, 

 1917, is given in Table XIIL 



Table XIII. — Mean air temperatures for August, September, and October, iQiy, at 



Laredo, Tex.''' 



° Obtained by averaging the daily maximum and minimum temperatures. 



Table XIII shows that the air temperature ranged very high during 

 August and September, the onion-planting period. In this connection 

 it is to be noted, moreover, that the records of Bouyoucos (i) in Michigan 

 indicate that surface soil temperatures may considerably exceed air 

 temperatures. Thus, his observations showed that the maximum tem- 

 perature for the upper quarter inch of all the soils he studied was about 

 16° C. higher during hot, clear days than that of the air at an elevation 



