Here’s some red rot on the leadpipe of a Bach Stradivarius trumpet:
Bach trumpets are great horns, the standard, but they are known for their rotting lead pipes. If you clean your horn regularly you won’t have a problem but if you don’t, well, you get what you see in those two pictures. Once it starts, the lead pipe is finished and you need to replace it. You can have it patched, and it might last for a while, but you’ll have to bite the bullet in the end.