I had three rescues from the same owner, one had what appeared to be a dime-sized spot of yellow fungus on his underside and a little on his spiky side. One I kept and is brumating. One seemed fine in the beginning, then quickly went downhill to look like that photo, and died. The third (with the fungus), was treated, cured, and placed with a knowledgeable rescue home.
Here's what I did:
*As for diet, calcium rich greens (history of MBD, no proper lighting) and veggies, insects. No fruit or high sugar veggies that can feed gut fungus such as candida. Added probiotics to diet.
*Natural sunlight for MBD and to improve skin health
*washed with chlorhexiderm shampoo and alternately applied miconazole and tioconazole twice daily.
I think the secret to this guy's success was that it was external and seemed to be in the initial stages. He ate like a beast, was strong, and the lesions were smooth, not raised up and advanced. During treatment, he lost a lot of scales in the area, and the skin turned black. Then he shed and the skin looked clear again. I kept up with weekly bathing just to make sure, and I kept his cage very clean, with newspaper substrate and no furnishings. I was not able to keep the humidity low since he lived in my garage, the quarantine area.
Good luck with yours.