Posted: 31/08/2016 at 11:46
Sarah, cut it back NOW if you are convinced it has mildew. Are the stems and leaves covered in a grey white "powder" ? If so, cut it back NOW. (If not it is not mildew). Clear away debris, spray with a fungicide or choose something organic like neem oil, and apply a mulch. It will regrow soon enough...several weeks yet for this.
however, on the edge of a plum tree canopy is far from perfect.......leaf marks, spots can be for a whole host of reasons and it could even be because of the leaves on the tree. where it is the same problems will occur again.
if there are clean, fresh shoots you could use them as cuttings but, really I would dig it up.
a sound principle in gardening is to grow a plant where conditions suit it. Otherwise it will fail or look awful. Grown in the right place and the plant will "smile" and make you smile. treat yourself to another penstemon...colours from white,,pink, blue, red to dark purple...and plant it in the sun. Take cuttings from it next year too. Grow a hardy geranium in place of your poorly penstemon.