Q. I have three dwarf mock orange plants near each other growing in rock mulch. Two are a beautiful dark green. One has brown leaves on half the plant. My landscaper says the cause is sunburn. If so, do I pick off the brown leaves, let them stay on until they fall off, prune the whole plant or what?
A. Yes, rock mulch is a frequent culprit in poor growth of mock orange. Its poor performance is due to sunburn and sun damage but this type of damage is accentuated by the rock mulch.
Organic mulches like wood chips and the addition of compost to the top of the soil improves the health of these plants. Improved health helps these plants handle tough locations. When plant health declines they can't handle these types of locations very well and thus they burn and scorch.
The reason for it being on one plant in three is probably because the others time is coming. It is just beginning.
Yes, you can cut them off or remove them but wait until spring unless it is really ugly now. If you remove leaves now you're going to see a bunch of bare branches. Do you want to see bare branches or ugly leaves? Your choice.