Pin dots/cancer on ribbons?

[Strawberry Fair]

I came across a few more ribbons yesterday and several have brown spots/pin dots/cancer (never sure of the right term). I've seen other ribbons like this in the past. Since they aren't made of plastic (obviously) they can't really "rot" like ponies do. Any explanations? :?:

 

[Karasu]

They COULD technically be plastic.

If they are made of polyester or some other similar man mad product?

*has NO ideer*

 

[babysnuzzle]

probably mucky stain or oil or something.

 

[starship]

i'm not completely sure, but depending on the material chances are there are plastic fibres. it would certainly explain why they seem to rot when a pony does, anyway