The farmer will substitute the production of wheat with the production of corn.
Step-by-step explanation:
The supply of a good is directly related to the price of that good. If the price of wheat is going to decrease, it will make the farmer produce more of corn so that he can earn more revenue.
If he continues to grow wheat, his revenue will decrease because of the decrease in the price of wheat. But growing more of corn instead of wheat will not decrease his revenue.