As JUnit is interacting with a test class by reflection anyway, there's absolutely no need to insist on @Rule fields to be declared as public. Neither that's the case for setup methods or test methods in general.

A framework interacting with the developer's types by reflection shouldn't impose visibility rules to standard type components like fields or methods unless absolutely necessary. Right now I find myself having to add public virtually everywhere with the only effect of losing control of visibilities in my code.