It's way too premature to consider euthanasia. If this were to appear in my flock, the first think I would try is painting Blu-kote on the lesions. It can't hurt, and it may just clear it up as Blu-kote is both an anti-fungal and antibiotic.
If lesions show up inside the mouth, Blu-kote must not be used on those lesions.
If lesions show up inside the mouth, Blu-kote must not be used on those lesions.