All Landscape Splines having now the same material -.- Any idea how to fix / reset this?

Hello, everybody,
I accidentally drew a different material on a “Landscape Spline” in my level. The Problem is that now every landscape spline has the same material on it and i cant undo it -.-
Who, please, adds such a possibility that you can change all Landscape Splines materials at the same time?!
I hope someone can tell me a solution, i dont wanna add thousands of landscape splines agian …

Here you can see it, fences, railroads and parts of my street are completly white now -.-