WWII
As most American men left for war in the 1940's the importance of farming was left behind to fewer people; and combined with industrialization this led to a lesser importance on farming because of the large subtraction of people who were working on farms. The men who replaced them were foreigners, namely Mexican, who Americans felt they owed little to, which began the beginning of the end for farming influence in America.
Braceros - Braceros were encouraged Mexican immigrant to come and work in the United States to replace those who had left the farms to fight in the war. However, this influx of immigrants, culturally different than Americans was the big change from farming influence. As soon as people came back from the war they went to the suburbs leaving the immigrants, or small families, to fend for themselves as farmers.