Charlie,

Unfortunately I am unable to open your images. If you want to try to reply and send them as a .jpg, that might help.

In general, pawpaw trees are not known to have a lot of pest issues in our native region. And, what is common is usually damage to fruit or foliage, rather than the bark. Often times when we see bark loss at the bottom of the tree it is due to wildlife pressure or mechanical injury - like from a weedwhacker, string trimmer, or mower. 

https://ohioline.osu.edu/factsheet/anr-0187 - This is a fact sheet on pawpaw in general which might be helpful. It does mention some common issues in Ohio for pawpaw. One is Pythium infection, which can cause root rot, which might lead to effects seen on the trunk. This is usually caused by poor drainage or over watering. There may not be much to do to treat if it is a root rot issue but future damage can be minimized by ensuring proper watering and eliminating standing water around the trunk. 

If you do want to try sending those pictures again, I'm happy to take a further look. Thanks!

Charlie,

Unfortunately this looks like rot on the trunk. My guess would be that it is planted a little too deep, as I am unable to see the root flare in the pictures. This could have been fungus related or simply due to mulch and soil being up to high on the tree and retaining the moisture. You can try moving soil away from the bottom and allowing the root flare to be slightly exposed to see if the tree will heal over that area.  I think it will just take some time to see if it can make it through that damage, but trees surprise us all the time!

This resource has lots of pictures based on the type of tree that is purchased to show proper planting. When the tree is placed in the ground, you want to see that slight flare out from trunk to root right at the soil line. https://extension.umn.edu/how/planting-and-transplanting-trees-and-shrubs#container-grown-and-containerized-stock-1400311

I hope that helps and good luck with the tree!