Squirrel fibromatosis may look terrifying, but it's a common skin disease among gray squirrels in the US. It's caused by the leporipoxvirus and is spread through direct contact between healthy squirrels and the lesions or saliva of infected squirrels.
To really maximize the effectiveness in warding off squirrels, use two pie tins. Put one on the top for the impact of the shiny metal and noise and one under the feeder to literally barricade climbing critters.