Colonel is a homophone of kernel. A colonel is an army officer of high rank. A kernel is a seed, often the seed of corn. The joke is talking about a man rising through the ranks in the Corn Growers association. He became a "kernel" which can be understood as the man literally turning into a seed, or can be understood as "colonel" since he rose in rank. 
He rose through the ranks of the International Corn Growers association , eventually becoming a kernel .