Could be fungal, bacterial, viral and nematodal causes and each one of these is hard to control.

I have a couple of suggestions that don't include diseases:
1 could it have been wee from a dog/cat burning the leaf where it splashed?
2. they could be the oldest leaves from this years growth and are just dying back

I'd be tempted to just cut out the dead foliage. if you are really worried I'd be tempted to lift it and check that the roots are sound, split it into healthy sections replant and keep them well watered.
How long have you had it? Is it the first year it has done this?

it is a very nice variety and I hope you can save it..

Thank you for you comments. It may very well have been from cat wee since we have several ferral cats in the area. I've had this one for 5 years and this is the first time it's done this. However I transplanted it to this location in early spring before it leafed out. It has already spawned several babies. The last thing I did to it was cut off the flower spikes.

Although it's in a shade garden, my first thought would be a bit of sun scorch on wet leaves. Just cut out the damaged leaves - there are plenty of others to help the plant survive. I'm impressed that you have no slug/snail damage. I've used (Neudorf) Sluggo in the past when I lived next to school playing fields. I avoided using it this year as I want there to be food for the frogs if they ever decide to grow!

Thank you