My vet appears to have been mistaken.

Below are a couple of links where biologist answer the question definitively.

Appears to be from a lack of pigment and/or a nutritional deficiency.

Lovett Williams (biologist that has studied turkeys his whole life) says the discoleration is caused by a pigment deficiency. The like to the Mississippi Department of Wildlife states it is due to a nutritional deficiency and definitely not mites, fungus, or freezing weather.

http://www.carellc.us/beardrot.asp

http://www.mdwfp.com/forums/topic.asp?TOPIC_ID=45634

http://turkeyandturkeyhunting.coverl.../200902/?pg=48

http://www.gonewildoutdoors.com/tech3.html