Is one allowed to use non kosher (without certification on packaging) precooked rice , as a means to grow gourmet mushrooms? Although from what I understand non kosher fertilizer is not a problem since it is זה וזה גורם. Since the Mushrooms grow entirely on the rice without anything else is it different? Does the fact that it is only bishul akum make a difference?
It is not an issue at all. It is permitted to benefit from bishul akum, and it is not considered as if you are eating the rice itself, therefore it is fine.