Just dusting

Last Friday I kept hearing very noisy planes taking off from the Jasper County Airport, so I meandered out there to see what was going on. There were four crop dusters on the ground, one of which was taking on a new load of whatever it was spreading on the fields. Not long after this picture was taken, it took off and headed north.

If they did not dust the crops, what would happen? Would they not grow as well because the dirt on the leaves would obstruct the sunlight? (I probably should leave humor to people who know how to do it right.)