Don't know but mine do and the bush doesn't seem to mind. You could remove the diseased ones so they don't stay in the soil.

Mine did a bit of that last year. I did nothing and the shrub continued OK and this year it has been very healthy and vigorous. So perhaps its just one of those things that they are prone to but in the end the plant will be fine.

I also experienced this last year. My garden centre advised lots of water. This year it has come back to life and I have had the best show of flowers.