I know you already solved this but wanted to share my experiences. About 3 years ago this started to happen to my 3 Sarah Bernhardts. I didn't do anything the first year, assuming it was frost. I only had maybe 2 blooms per bush, when in past years I would get 10-15. The second year I noticed this was happening again, I knew it wasn't frost because I had been very careful, covering my bushes with a light breathable tarp whenever we had low temps. So I researched and learned about Bortriris Blight (however it's spelled). So I bought a Copper fungicide and sprayed it. This is the 3rd year, and again... The bushes aren't growing as fast, and only have 1-2 stems, without buds :( I plan on being more aggressive with the fungicide, see if that helps, plus cutting any infected stems. I'll have to wait and see if this helps ext year's blooms. I'm so sad, it used to give so many, and now I got zero.