Your Lagerfeld most likely has "friends", they are insects called thrips. They are usually seasonal, but the attached threads have more information.
How do you deal with thrips?
Thrips Have Destroyed My Roses!!

You could also have a fungal disease called botrytis, for that you would need a fungicide spray (which, if you are growing Lagerfeld, you must have around someplace anyway).

My Lagerfeld does the same thing, and it is usually accompanied with balling of the bloom as well.

My thrip infestation this year on it and others isn't too bad really as I did spray Merit twice this year already and

I have also sprayed for botrytis with captan and daconil both at different times. I think that combination worked pretty well as I don't see the botrytis on my other white and light pink roses like I did last year--just a little bit especially after a long rainy spell.
John