I am making trees to put onto a cake, the trees will be 3D and I was going to use pretzel rods for the trunk. I saw in the Wilton POP book that when they made the leaves they added gum-tex to the fondant. I never used gum-tex before and from what I have seen I can't figure out what the difference is if I were to use the gum-tex and fondant mixture or if I use gum paste. Can someone help me out?

I use fondant with gum tragacanth mixed in for all modelling, EXCEPT for things like trees, or buildings that need to be really strong. For trees/walls/roofs etc. I use mexican paste mixed with a bit of fondant (use about two thirds mexican paste). Mexican paste dries very hard/strong and the addition of a little fondant makes it less brittle.