You can't deliver heavily tainted crops into CBOT/KE/MWE contracts.....4 parts per mil vomitoxin. So the supply of good crop remaining would be under heavier demand causing price action to be bullish, right? Reference to CBOT deliverable grade: http://www.cmegroup.com/rulebook/CBOT/II/14/14.pdf
Ha-ha-hee-hee-ho-ho! Not necessarily. The fear of acquiring tainted product still removes buying-pressure from the market. There can still a temptation to blend bad product with good product in order to sell it away. People can look for other substitutes elsewhere instead.
We are leveraged out enough with the amount of people in the world that if there was a major crop issue (corn blight, ug99) it would more than likely send prices of everything skyrocketing. Sure you can substitute certain things but that is a temporary fix at best.
Adding to the potential tightness is the developing world's increasing appetite for diets rich in animal protein. Wheat, bean meal, and corn going towards animal feed above and beyond human consumption leads to even more tightness of supply.