shell rot is usually a round, local spot. It will actually develop into a pit into the shell. Your turtle really looks like the scutes are discolored. Either by some mineral in the water, or like they are about to shed.

But it is best to take it to a vet, as one can't be sure from a picture!
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0.1.0 ornate box turtle
1.0.0 eastern box turtle
1.0.0 Yellow belly slider
0.1.0 Red belly cooter
0.1.0 Australian shepard
1.11.0 chickens
1.0.0 Dutch(rabbit)
3.2.0 children (do I still count the married ones?)
1.0.0 husband