I have some vinca minor (six plants) I planted two years ago under my ~50 year old Pin Oak's trunk. About a two foot wide mulch bed in a 6-8 foot circumference. It was doing fine...growing vigorously, filling in the area as I wanted it to. However, I was out doing some spring clean-up this after around my yard inspecting some of my plants. I noticed the leaves on the vinca minor weren't glossy and had what looked like burn marks (gray in the middle and black around the gray mark) on the leaves. Some of the 'shoots' or 'runners' appear to have lost all of their leaves. Not every shoot or runner has this problem, but enough to make it noticeable and, honestly, a little unsightly from a distance of a few feet away. Any suggestions what this may be and how I might be able to correct the problem...assuming it is a problem...??? thanks and happy easter everyone!