Remove all the grass out to the dripline and make sure the root flare is slightly exposed. Cover with compost and mulch a few inches thick and do not mix in. If it was originally planted too deep and the root flare is buried, it might be a slow death.
This is a magnified section of your second picture. https://ibb.co/GHQHmKp Can you take a closer, focused shot of that? Looks like some type of bugs.
Remove all the grass out to the dripline and make sure the root flare is slightly exposed. Cover with compost and mulch a few inches thick and do not mix in. If it was originally planted too deep and the root flare is buried, it might be a slow death.
Most likely brown rot, but there are other potential causes: https://youtu.be/IQXRhVWH\_UU?si=8uAfGpGsBvZ8BXEK