The placebo effect offers evidence to the contrary https://…

The apple seed will always grow into an apple tree regardless of what you believe.

True... if it grows, and experiences no significant mutations. But that does not exclude the phenomena observable in the placebo effect, for instance.

It depends on how the placebo is administered.
As for the tree, the only subjective reality is where the seed is planted and your horticulture methods.
I have seen roses grow in concrete btw.

And if you graft in other species. Belief itself doesn't affect the outcome, just knowledge (or luck in the case of ignorance) and action.