In my area, that is usually winter damage--or, more precisely, late winter/early spring freeze-thaw-freeze type damage--particularly if you are talking about hybrid teas. Every spring my HTs have to be trimmed back to healthy pith--and some years that has meant needing to prune them back to a few inches from the ground.

Have no idea if that applies to your roses or not. Pictures would help us diagnose the problem.

Kate

And this year was bad here (Michigan) since it was so dang warm in March that the roses started putting out all kinds of growth only to have a cold spell in April. I ended up having to prune a lot of my roses twice; once in March when we had 2-3 weeks of 80+ temps, and then in late April after countless freezes.

This was a new dormant bareroot planted this May,it had a healthy first flowering last month. All the growth affected is new growth and the worst affected is a new basal shoot. All the shoots had buds on them and none have been previously pruned to allow entry to anything. I'm going to fish the cuttings out of the trash and take pictures, I'll see if one of my computer savvy children can help me figure out how to post them later.

It's possible that the "rotted pith" was frass, or feces of a burrowing worm. These sometimes begin with eggs laid on the soft tissue at the top of a growing cane. If so, the stems feel hollow when squeezed and there's a larva is the section between your last and next-to-last snips. I find these around midsummer, occasionally. Rose Stem Sawfly is one such pest.

Bacterial blight of rose produces black streaks and wilting of top growth. There are oozing ulcers on the bark of lower canes.

Verticilium wilt has leaves yellowing and dying from the bottom of the cane up, with wilting and streaking of soft growth. It's possible for new bareroots to come with the infection. Canes die one at a time. Plants may eventually fight it off. The soil is infected.