We have 7 - 8 ft. 15 year old Burford hollies in our backyard serving as a hedge. We just noticed that the bottom leaves are covered in black and the back of them has small black spots. I can scrape the black off with my fingernail. Do you know what this is and how we might get rid of this? We have had a very wet and snowy winter here. Might this contribute? Thank you.