Just wondering if you've had unusually wet summers it could be a mild form of crown rot. Since you say it only affects one stem I wouldn't worry about it too much. It will probably right itself when the wet weather subsides. Just remove the affected stem and check regularly for any recurrence.

I also checked the internet and most sites say goatsbeard is not usually susceptible to pests and diseases. One site mentioned fireblight as sometimes a problem since goatsbeard is a member of the rose family. But if fireblight were the cause there would be other infected areas, ie leaves. Hope that helps.

Thanks for responding, CD.
I also thought that goatsbeard was trouble free and that's why it's such a mystery. The first time it happened I didn't think much of it, but this time it's happened so early in the season, I fear that other stems will become affected.
The area where it is has well drained sandy soil. In fact, if anything, it may be too dry for it.

Hmm, that is a mystery, as they do seem to be trouble free for the most part. How's it doing now? This is just a thought, but is there a chance it could be happening from urine, ie a dog or cat?

Laurie, so far it is blooming and hasn't had any more stems collapse. But I think last year it happened much later in the season, so I'll have to wait and see. No, there's no chance of cat or dog urine where it is.