"We have been battling for three years and did everything that Harrogate Borough Council asked of us," managing director John Smith told Grower.
"However, we knew that the development was contentious so we have a contingency plan to build in Market Weighton, 40 miles away.
"The irony is that the mushrooms will still have to be brought back to Wath for grading, packing and distribution. We thought that the traffic issue would swing the decision for us, but the situation will now get worse."
Planning inspector Graham Dudley dismissed the appeal by Greystairs over its plans to build a £4.7m growing shed following an inquiry earlier this year.
He ruled that the 100m by 70m building "would have a substantial adverse impact on the surrounding countryside causing unacceptable harm, particularly to its character, but also its appearance".
Greyfriars has been growing for 25 years in the area and is a major supplier to Morrisons supermarkets.