I have been watching and waiting to see your reply David. I have never seen or heard of a growth on a turkey's foot like that. Good to know what would cause it, thanks for posting the answer and congrats on your harvest, nice shooting!

http://www.nwhc.usgs.gov/publications/field_manual/chapter_19.pdf

The game dept. here in Oregon felt it was something called the "avion pox"....

Thanks for the update

snag, they didn't think it was a form of pox because it was only traced on the knee joints of the legs. They named a couple other conditions that they eliminated because they were not found on the rest of the body but I can't remember the name of those.
Thanks, Dave
P.S. here is a closer pic of the knee Joints.

There are a number of nasty looking chronic conditions related to infections, look up madura foot, whick can happen in people.  This is probably something similar.

Good thing you put him out of his misery! He probably didn't get many dates with those legs...