It sounds like it could be Fowl Pox. Fowl Pox is caused by a virus, and it causes whitish-yellow lesions, scabs, and blisters on the comb and wattles of a bird. It is spread by mosquitoes, flies, and other flying insects. There is no treatment for Fowl Pox; it runs its course in three weeks or so. Birds infected will be immune to further infection, but can be carriers. Fowl Pox does not usually cause much mortality, unless it is wet pox, which can clog up the windpipe of a bird.
There is no treatment for Fowl Pox, but there is a vaccine. The vaccine is administered in the wing web of a bird, and can reduce the spread of the disease. Fowl Pox isn't usually dangerous in itself, but it can cause a secondary infection. To prevent secondary infection, give the bird Oxytetracycline (sold under names like Duramycin, Terramycin, LA200, or Tetrxoy HCA-280) for five days.
Sometimes, scabs will obscure the bird's eyes, and it will not be able to see to eat and drink. To help, put petroleum jelly on the scabs. This will soften them, and you can remove them.
If your birds are extremely valuable, you may want to cull him. However, the other birds are probably infected already, if they have been kept in the same place with the same insect conditions.
Hope I've helped!
