Do you know what variety of apple this is?* In looking at your picture I'm thinking the problem could be one of several different diseases.* This site has some good pics and info.
I'm thinking maybe nectrotic leaf blotch aka golden delicious necrotic leaf blotch.
Here's the overall symptoms on a Goldend Delicious tree.
Here's the info on this disease.
Here's the list of symptoms for apple.
Any of that look like what you see?
When weeding, the best way to make sure you are removing a weed and not a valuable plant is to pull on it. If it comes out of the ground easily, it is a valuable plant.