I have a crape myrtle that is about 5 years old and that was growing very well. It had four main stems as of last fall. One did not wake, and I noticed there was rot around the base of that branch. Now I suddenly have another that's dying, but has no major signs of rot issues, and now a third "healthy" stem has rot around its base as well. I took a picture of those two branches. The one to the left has a couple cracks but nothing that would cause alarm but for it dying, and the third branch with the major wound to the right looks like it's in big trouble. That wound was after I took my finger and with no effort flicked off the rotted bark. There were a couple ants in it, but I'm assuming this is fungal, particularly since I've had similar problems with the wax myrtles and azaleas over the past couple years. Is there any particular product I can use to treat this?

As an FYI, I am very careful not to mound mulch around them and only water them when I water the surrounding lawn during a drought, and only water deeply and once a week. I've occasionally seen one of the neighborhood dogs by it, but it seems rare.

I'd really hate to lose this crepe myrtle.

Thanks for any help you can provide.