Before genetic modification was invented, Man used selective breeding to modify crops. For example, if a certain type of crop with resistance to a certain type of fungus is needed, you would plant some corn and see how each individual plant fare against the fungus. Then the seeds of those that fare well would be taken, planted and observed. Over the years, a strain of corn with very high resistance to the fungus would be produced. Selective breeding techniques have enabled us to create giant pumpkins, high yield wheat strains and other plants with high resistance to diseases.
Genetic engineering is a far more advanced form of crop modification technique.
Scientists insert genes into plants without having to go through the long process of selective breeding. It is therefore very rapid to produce crops of the desired type. Cross species can be done easily using genetic engineering. There are a number of ways scientists modify plants with genetic engineering. A company called Mosanto manufactured a herbicide called Roundup.