Yes, you can cut off the infected leaves of your peonies removing the stem and leaf but do not remove all the foliage. The peony needs foliage to make next year's flowers. Try to leave at least two thirds of the leaves. If your peony does not outgrow the black spot soon you can spray with a fungicide such as funginex following label instructions. Good air circulation, good drainage and sun will help prevent fungal infections. A fresh mulch on the soil will help contain any fungal spores. Keep growing, Marianne Binetti