82 



Yearbook of the Department of Agriculture, 1921. 



Pig. 29. — The Corn and Winter Wheat Belt included 42 per cent of the Nations 

 acreage of winter wheat in 1919, and 3<i per cent more was located in the southern and 

 eastern portion of the Corn Belt. The southern boundary of this wmter wheat belt fol- 

 lows the isotherm of 72° during the month preceding harvest (June 15) ; and although 

 BOine wheat is grown south of this line, it frequently suffers severe damage trom rust. 

 The northern frontier of winter wheat follows, in a general way, the mean winter tem- 

 perature line of 20°, which extends in a northwesterly direction from northern Illinois 

 and Iowa diagonally across South Dakota and Montana. 



