Agreed.
To me it looks like a possible healed burning injury from a too hot/too close overhead light.
Treating it now, might just slow down the healing process
The wood itself will quickly swell, rot and fall apart from the moisture.
The mold will be growing before that.
Any wooden enclosure will need a liner of some sort to protect it. Even then it'll be inadequate for most species and also too small.
I realize it's already too late to help. But...
Shell"rot"as we see it in the hobby is almost always 100% fungal. Mostly seen in tropical tortoises. Like Redfoot. Because of their high humidity requirements. It's quite common.
But technically there is a more advanced form that is ACTUALLY ROT. And it requires antibiotics. And sometimes...
Get a tube of Athletes foot cream from anywhere
I use the stuff the dollar store sells.
Apply a little smear on the affected areas once every other day for 10 days.
Done.
Include a photo so we can verify that it is a fungus issue. (Unless it's a Redfoot tortoise, it might be something else)
Not from that photo.
And it's the only photo.
We can rule out Russian. Redfoot, Yellowfoot and Sulcata.
I wonder if it's a desert tortoise or another type of Gopherus
I'm very sorry that your tortoise died.
It's extremely difficult to have success once a very small tortoise gets ill. They're not as robust at that age.
I have to agree with @Tom.
Redfoot are more of a south Florida or Hawaii tortoise. Not that it can't be done. But you'll need a very large indoor closed chamber enclosure or something like a greenhouse.
Correct diet and a correct type of uvb light will provide the vitamin D.
Does your tortoise get outdoors sunlight?
Do you have a T5 HO linear strip florescent UVB tube?
What is the diet?
We usually only use calcium/mineral supplements for baby tortoises. Adults generally get what they need from...
My south Florida yard is full of rocks that are almost entirely made of coral. If I get sloppy and not make sure that there are a few cuttlebones laying around in my enclosures, my tortoises will begin eating rocks.
Their bodies tell them when they're lacking in minerals and they'll try to...