Hi, I have an Apple espalier and the leaves on one of the grafts (it's 6 apple varieties) has nice leaves but they are curling. Not like the knarly leaf curl where the leaf bends and fold every which way.... more like the leaves are folding in half along the leaf stem. Kind of like a book closing with the leaf stem the book bind.

Any ideas of what might be causing this? Not all of the grafts are doing this.